EV005 VM ENGINE

At first I had the problem of stalling when taking off at the lights. But as my confidence in the bike started to grow and it got hot, a gentle opening of the throttle in first to get the bike away, then with some momentum changing up and applying more throttle. This reads as pretty obvious but it did require patience to get it correct. I suspect something needs adjustment. Restarting the bike was a problem. It seemed to respond to a prod on the carburettor float pin to encourage some petrol flow. I made sure the tank was full, but that did not seem to be a factor.
The second major issue was pulling up hills, by this stage the bike had been running for over an hour. It did not seem to like to pull through the revs in lower gears. The bike in fact seemed to be heading to a stall. Easing the throttle even changing up a gear seemed to help. At one point I was worried that the bike was seizing, but by easing off it kept going. We got up some very steep climbs (slowly) but the bike was not happy and it seems out of character with what you would expect from the bike (and have expected in all my vehicles). I wonder if the timing is off? - David Royston 11/11/2000 

It shouldn't be as difficult as it sounds to pull away or climb hills. A Viper should have no problems with either, other than its bottom gear is a little on the high side, so when pulling away it can bog down if you feed the clutch in too early - but your symptoms are different.
My first thought would be mid-range carburettion. If, once you've nursed it through the mid-range it seems OK and runs along at over 70mph and responds to the final bit of throttle, then the ignition timing and fuel feed etc are OK. If it did seem strangled at the top-end then I might suspect either of these, but if it's the mid-range throttle openings where power disappears and even stalling seems likely, it usually means the mixture is too weak. At mid-range this is controlled by the position of the needle in the slide, which is determined by which groove its spring clip is in. Check which, and if it is in a groove near the top of the needle (say the first or second of, probably, five), raise the needle two grooves to richen the mid-range and see if that makes a difference. It does sound to me as if your carb needs adjusting - if the bike starts reasonably and goes OK above 4000 revs & 3/4 throttle, the ignition is probably sound. 
Tuning the carb is an art rather than a science, but the best approach is to check all the jets and other settings are as in the service manual and that there is no serious wear or deep scratches on the slide or the surface of the body it rubs on. Then go through all the steps in the service manual to check and finalise. It's all part of the fun, and rewarding when you develop the skill, but it does require a methodical and precise approach. All Velos can still be set up to start easily, run smoothly and strongly and rev freely - as they did in their first lifetime; and when you ride a good one you realise why they are rated a "quality" bike.
Re-starting after a stop of anything over just a few minutes usually requires a light flood by depressing the "tickler" for a few seconds. Some Velos nowadays refuse to start when hot, which they didn't when new, and this can be for one or more reasons, but by far the most likely is a weak spark from a tired magneto. One diagnostic check is to close up the plug gap to under 15 thou (it should be 18) and see if that makes a difference, and try new versus old plugs etc. If a new 15 thou starts more reliably than an older 18 thou then the mag is border-line and needs attention. - TW 12/11/2000

My Viper still runs out of puff at around 60mph (on the clock - which appears to be OK) and does not want to pull up steep hills. Following your advice, this weekend I took off the carburettor to have a good look at it. It is an AMAL Monobloc (from looking at the instruction manual, it is the type with a hinged float) with a body marked 376 inside the float chamber and a 3 on the inlet. The throttle slide has 376/3 on it. As it happens the bore is oval. It measures 25.5mm,1 inch, in a horizontal plane (through the mounting bolt holes) and 27mm, 1+1/16 in a vertical plane. The carburettor appears to be in reasonable shape, nothing appears missing, broken, battered or bent. The float appears sound (no fuel in it). It just shows its age but in a well cared for way. Nothing seems blocked. The main jet is numbered 260. The previous owner said that he had fitted that jet, it was a guess on his part as he did not know what type of carburettor had been fitted.
From the Velocetteowners.com data sheets for the 1956/60 Viper the Carburettor was an AMAL Monoblock type 376/61 with a mixing chamber body number of 376/003 and 1+1/16 bore. The data sheets say a 270 main jet is required, as does the Velo. Service Manual for the 376/61. [The Service Manual also lists a 376/56 for the Viper - a 1 inch carburettor with a 230 main jet.]
The needle was in the middle position, but I am concerned at its condition. Apart from around 15mm at the very (tapered) end, the rest of the needle has a rough surface to it, almost as though it has been rubbed with a very coarse "sand" paper. It is very hard to understand why. Maybe the needle jet has been affected - the mounting bolt is proving difficult to unscrew - so as yet I do not know.
I have no idea of the significance of a 260 vs 270 main jet, but at 3/4 to full throttle I understand it is this jet that determines the fuel flow to the engine. At lower throttle openings the needle position and needle jet have the major "say".
I'll work away at that bolt to have a look at the needle jet - DR

I don't think you would notice the difference between a 260 and 270 mainjet at anything other than top speed - it might make a 2 or 3 mph difference, and a 270 would run cooler.
One other thing to inspect is the fit of the slide in its bore - particularly around half way up. This is quite important to good carburretion, though not as crucial as the fit when just opening, when wear can really mess up your mixture. A well used monobloc could have appreciable wear around the slide - you can see it getting thinner on one axis - and also the bore wears. So air goes round the slide rather than through it, giving lower depression on the jets, giving weaker mixture. On my TT carb, when replacement slides weren't available (let alone carb bodies), I had some slides resleeved and the body bored out. A difficult and expensive job, and I still had quite a lot of work to do on 'high spots' to stop it sticking open. But I believe parts are readily available from re-manufacturers for the monobloc type carbs.
I would try a new needle in a new needle jet, position 3 may then work or if the slide is just a little worn then position 4 or 5; and so on. 
From your other comments about the needle looking as though it had been modified with emery paper, it does look as though someone has tried to tune the carb out of this problem before
I still think the carb mid-range is the most likely problem area, and the best test for this is if the top-speed and 'feel' is OK at around full-throttle. If you take the speed up through the problem area to 75-80 mph and it is going well then (and if it starts OK) then I think it must be the carb's mid-range.
If the top-end is also poor - difficult to get over 70, seems to go a bit quicker as you close the throttle slightly, then one other possibility is that the silencer is not a genuine Velo design. Pattern silencers just don't work on Velos unless they copy the Velo baffle tube design - see the other Q&A on this subject.- TW

I'd read the story on the silencer earlier in Q&A and it certainly reflects my problem. But the problem that I am having seems much bigger than that described in Q&A. The silencer on the bike certainly looks OK, and make a very "singular" sound, but I can only see the outside!
When I have the new carb. parts I'll reassemble and refit the carburettor and give the bike a run and check that mid range.
I will also try your ideas on the spark gap to find out if the magneto is giving a good spark. Just cranking the engine over by hand (i.e. reasonably slowly) with the plug removed, shows a spark across the plug gap each time, not a fat zap of a spark, but a spark each time nonetheless.
A question concerning the magneto. I have had problems many years ago in old bikes that used flywheel magnetos. They would start, but behaved as though the spark was not strong enough and got "quenched" when you wanted to use full throttle and power. Is this a characteristic of Velo. magnetos?
This has been passing my mind as an aspect of this problem as well as could the timing be out?
A question concerning timing. The approach in the Illustrated Service Manual to set the timing looks daunting. Is there an "easier" way?
It has been raining here (pretty heavily at times) this past 10 days so bike riding has not been a priority. We now have flood stories that match those from England, but it happens every 2-3 years here, thankfully in country areas away from major populations. For these people it is very serious with whole towns cut off for days. - DR

Yes, it is quite possible to "blow the spark out" if your magneto is borderline. And this will happen when there is maximum compression in the combustion chamber, which is not at highest revs but when breathing is most efficient, i.e. lower down the power curve.
But such borderline mags don't usually start too well cold and very badly when hot.
If yours is a manual advance & retard magneto you can test the quality of the spark by putting it on full retard at tickover. Manual mags (that haven't been re-assembled with incompatible parts) give their highest voltage at full advance, and it can be halved at full retard. So if you open the throttle a fraction and then apply full retard the revs will fall because of the retard but if it still ticks over reliably there isn't much wrong with the voltage. You can still have faults that cause mis-fires at higher revs, but unlikely the spark will get blown out on full advance.
Also if you have a manual mag you can check the timing is about right by measuring through the plug-hole how far down the piston is when the points separate. First make sure you are on full-advance, then carefully put a rod (with a 'top' wider than 14mm in case you lose hold!) down the plug hole. Rotate until the points just separate, and mark the rod. Carry on up to tdc and mark the rod - if the difference is 9mm +/- 1mm (on a Venom or Viper) the timing will be near enough to be not causing mid-range problems.
(If you have an auto-advance mag you can only reliably check the full retard position and since that is around tdc it is much more difficult to judge but still worth doing in case it is wildly out. Of course the auto advance could be sticking etc, so it is not a fully reliable test.)
One more thing - check that you don't have a "suppressor" type plug cap or lead. Magnetos hate either. The resistance from the pick-up on the mag to the plug points should be almost zero, a few ohms at most. - TW

Thank you for your observations on magnetos and timing. I though you might like some feedback from my applying your ideas this Saturday afternoon..
First of all I did not get the carburettor parts in time for this weekend - so I had to make do with what I had. I polished the needle using some coarse valve grinding paste to try to get the roughness down - I managed to get it to the point where it felt smooth to the touch, apart from a length of around 10mm around 5 mm up from where the sharp taper begins, that was still badly marked and definitely reduced in diameter. As I now suspect that I have a significantly smaller (in diameter) needle than standard, I refitted it at the highest notch - I'll start the notch experiments once I have a new needle and needle valve (by the way I have just discovered that there are two types of needle a "c" and a "d", mine is a "c"). I also thoroughly cleaned the carburettor, drained the petrol tank removed the taps and washed it out with petrol. Nothing particularly suspicious in all this except the float valve was prone to stick for reasons that baffle me - anyhow I removed the float valve seat and cleaned it out with Jiff (a mildly abrasive household cleaner) then washed it out.
I summary, one clean carburettor back in service with a very worn needle in the topmost notch.
I also checked the ignition. Mine is an automatic advance and retard magneto with points that are attached to a central brass stand that has to be removed and an "anvil" moved to adjust the points gap. Using your idea of a rod to detect the piston, I got the piston on the compression stroke, then engaged low gear and by moving the back wheel (bike on the central stand) very carefully found TDC by feel. Then I checked the points, they were opening at this point. Just a little wheel movement would release a 5 thou shim. So it seems that the basic timing on full retard is around OK. I then rotated to maximum points opening and found the gap was quite small <10thou. So I reset it at 12 thou easy, 15 thou tight. Then I moved the piston to 10mm before TDC, as you suggested for the advanced position, hand-rotated the magneto advance mechanism and re-applied the shim test. It indicated that the points were just opening - so I think that I have the timing just about correct by the book. I checked the HT pick-up and it looked to be in good order; the HT cable looks ordinary and runs to a KLG spark plug cap. AND I fitted that new NGK B8ES plug.
In summary, timing appears OK, the points are in good condition and set by the book, new plug.
SO the test run. The bike fired up first try after following "the knack". The bike pulled away without stalling or any sign of stalling. It ran up to road speed in no time and felt "eager". We live on a steep hill (I'm told that is a normal thing for Velo. owners - joke?) actually its a ridge with steep road up to the top and a relatively flat road along the ridge and in the valley below. So I was able to give it a good blast up hills and it pulled really well, but if the throttle was over-opened power fell away closing it a little brought an immediate improvement - but there is a LOT more power available that on my first long run. Here's hoping the weather holds out so that I can repeat my favourite run in the local hills and get a comparison with its previous performance. Especially that mid range effect, I can't test that riding on the local roads. - DR

I'm glad you are getting somewhere.
No point doing much else until you have the 'right' needle and jet. (Newly manufactured ones aren't necessarily always 'right'...)
Sounds like you are well on your way to sorting your Velo - maybe 80% there...but the next 15% requires a great deal of attention to detail and checking every factory setting is right, and testing on the road. And the last few % only come with real experience. But when you get a Velo just right it is smooth, strong - and you've already used the right word - eager.
Velo engines were developed, evolved and made to a higher standard than anything except a Vincent, is my belief. - TW

I did fit in my 'favourite' run through the hills, a nice run but with both good news and bad news in terms of the Viper.
The bike is much improved but the essential problem is still there; though perhaps now more clearly defined. The bike starts easily and runs flexibly in ordinary 30-40 mph traffic (with no stalling on take-off as it was doing) and appears to have good torque up to suburban traffic speeds.
On any sort of a climb however, the bike would not rev out in the lower gears (the engine would reach a point where the power would just fall away and the bike would seem to be stalling). On the really steep steady climb (1:15 over 2-3 miles) it just plodded away in second gear at around 20-25 mph, when the climb eased it pulled just as well in 3rd at the same basic speed - though at this point the bike speed did start to pick up a little as the hill slope eased.
Down hills the bike appeared to stall no matter how much throttle, and started to misfire if pushed (i.e. opening the throttle more), at around 50mph in 3rd and 60-65mph in 4th. In essence this has been the problem from the beginning but now it is more clearly defined as a misfire at those sorts of revs. / speeds. I calculate this as occurring at around the same engine revs - 4200rpm in either 3rd or 4th, probably around the max engine torque rev band. I simply could not force it through this rev band even down hills - it was a though something and the misfiring was holding it back. This reflects the problems in the lower gears climbing hills, i.e. problems around 4 - 4500 rpm.
Could this be the automatic advance and retard not working I wonder? or a tired magneto? As I note below the fixed timing settings seem about correct - certainly not out by the amounts that should cause this level of problem I should think?
I attach a picture of the NGK B8ES plug - after around 40 miles from new - it shows a dark brown coloration. Not what I would anticipate were the bike running weak (eg the needle too low), rather the opposite. Of course I have that suspect very worn needle in the carburettor this weekend and set as low as possible (the clip is in the top notch). When I get the new parts in next week the results may be more useful. - DR

You'd have been very lucky to fix such a problem the first time around.
There isn't much point in testing further until you have the new needle and jet. Having the needle in its top groove might well be giving just too weak a mid-range mixture which would match the symptoms. The plug might not show weakness if you'd run at lower throttle openings before you got home and checked it.
It is most unlikely to be a faulty auto-advance - if you could move it by hand as you did it is going to work fine - I believe it fully advances by about 3,000 revs or less. And the timing is near enough, and would affect the top-end rather than mid-range if a bit out.
It still sounds like mid-range mixture to me, but if by 'misfire' you mean really momentary misfiring rather than 'staggers' or 'choking' then it could be a magneto problem. Normally, such problems show up as poor starting (particularly getting worse when hot) or misfiring at higher speeds, rather than mid-range.
Anyway, definitely only change one thing at a time once you have the new needle - starting with it in groove 3.
If symptoms unchanged, I would do a test and find a road with 'repeatable' symptoms, then quickly change the plug for a new one set at 15 thou gap and repeat the test. If the problem is borderline mag performance the repeat test should show a noticeable improvement. If no difference it's back to the carb, trying bottom groove and then top groove, and if these make no difference then something is really wrong!
And do check that the vent hole in the petrol filler cap is clear.... - TW

As part of the check-over from today's run, I took off, and had a close look at, the silencer. Physically it is in very good condition and looks to be made from chromium plated steel.
By comparison with photos of the Viper in the 1956 MotorCycling road tests book and the original catalogue photos from that time, EXTERNALLY it looks OK, including the size and number of holes on the side and in the end. Internally I have my doubts.
My silencer has a baffle pipe (welded into the outer cover) of around 44 mm ID and has 4 rows of holes set at each quarter (these two aspects look correct). However, it is only 250mm long and has only 6 holes per row, 5 are punched (maybe square 4mm side) and 1 is round around 100mm diameter (very hard to see all this inside the pipe).
If I scale the Illustrated Spare Parts Manual (ISPM) drawing , the pipe should be around 390mm long with 9 holes per row. In other words it should run along the inside of the bottom of the outer cover until the "up-turn" for the fishtail. A Thruxton cutaway drawing shows a baffle of around this length (ie to the "up-turn") with 5 punched and around 6 drilled holes per row. Mine in effect runs only half way.
As the inlet pipe length to baffle probably has a great deal to do with the "stopped pipe" resonance in the exhaust hence exhaust breathing, mine might have some problems delivering full power. The inlet pipe openings open area is not that bad by my best estimates (around 5-10% less than the ISPM and much less that the Thruxton but it would have to pass up to twice the gas flow). The area is not as we would wish them to be (from the above sources) but not that bad. I still find it hard to believe is it the only source of my current problems. - DR

Your silencer sounds different....and you don't mention the baffle tube, unless that is what you mean by "inlet pipe"? (The Velo parts manual illustrates what we call a baffle tube as part KA142/2 "Silencer baffle" but the one in the illustration is just a repeat of an ancient drawing of an early, much shorter, baffle tube..... probably a KA142!)
I have attached a scan of what I believe to be a new (original?) KA142/2 baffle-tube. I don't think they ever changed the internal design, from the '54 MAC and then MSS to the Thruxton, but I could be wrong.
The slots you can see in it are in the final 250 mm of the tube, then there is the 130mm of plain tube (split for the last 60mm on the side opposite to the scan) to extend to and go over the exhaust pipe.
So a Velo silencer has a separate baffle tube with semi-circular indented slots that fits on the end of the exhaust pipe and, when the exhaust pipe is inserted into the silencer, this baffle tube goes into your "baffle pipe", which is of the length you have described, but the two I have just examined have quite different holes from yours.
The holes in my "baffle pipes" (the tube welded into the silencer) are as follows: 8 rows in 4 pairs, each pair at a quadrant. the 2 lower pairs are rows of 4 holes the 2 upper pairs are rows of 6 holes. The rows start at the ex-pipe end and only go back for about 1/3 the length of the "baffle pipe". The holes are about 6mm. At the end of the "baffle pipe" is a cap, with just a 6mm hole in its centre.
There is a 60mm gap between the end of the "baffle pipe" and the neck of the silencer where the exhaust pipe goes in, which is bridged by the plain part of the baffle tube.
If you have this gap but no baffle-tube you get less silencing and peculiar power characteristics - mainly reduced top-speed on a 500, I'm not sure what on a 350.
If you have a full length "baffle pipe" and no baffle tube it might not sound so loud but performance will still be hit - Vipers and Venoms have quite sporting cams which have evolved to closely match the silencer's extraction effect from the gas reversal by the serrated slots.
How does yours compare with the above.....? - TW

I think I now understand the mysteries of the Velo Silencer. My bike is on the noisy side.
When I look at the "Thruxton" cutaway drawing, I can now recognise this "baffle pipe" and the "baffle tube" arrangements. Exhaust gases in this arrangement pass from the exhaust pipe down the "baffle tube" then through its openings and (open end?) into the "baffle pipe", then reverse flow out of the "baffle pipe", via the open leading end and openings of the "baffle pipe", into the silencer "can", then out via the fishtail There appears to be no restrictions to flow in terms of open area - just baffling edges and reverse-flows.
My silencer is not designed as you describe. I do not have a separate "baffle tube" i.e. KA142/2.
I thought that this "baffle tube" KA142/2 had been welded inside my silencer - hence the confusion in my terminology.
My silencer can has just a welded-in inlet pipe, which is 250mm long. Its open end provides the inlet to the silencer. This inlet fits over the end 25mm of the exhaust pipe and is attached to it with a clamping ring. The other end inlet pipe (inside the silencer can) has an end cap (with no hole in it) and, in each quadrant, a series of 5 holes of around 4mm punched in its far (internal) end together with one drilled holes of around 8-10mm. In my silencer, exhaust from the exhaust pipe travels down the inlet pipe and has to exhaust through the holes into the can. The can has no other silencing baffles.
In my design appears to present more restriction and far less baffling to reduce sound that the standard design.
My calculations indicate that the open area of the fishtail outlets (the four openings at the end and the 40+smaller holes in the side) is a little higher than the cross section of the ID incoming exhaust pipe. This aspect appears to apply both to the "standard" and to my design.
In the "standard" design the baffling appears not to restrict flow.
However, In my case the holes inside that inlet pipe are around 50-60% of the exhaust pipe ID cross section. Not a good design feature!
Could this be the essential cause of the "can't get the revs / speed" problem - excessive backpressure caused by poor silencer design? - DR

I would say you almost certainly have a typical pattern silencer. (From the image you sent me, even the seam welds look rougher than the standard silencer, though they have never been smooth)
From your description of it I would expect it to produce a severe mid-range 'flatness' as well as taking 10-15 mph off the top speed of a Venom.
I think a Viper is even more sensitive.
Last year I rebuilt a MAC (not as sensitive as a Viper) and put on a pattern silencer because that's all I had and I thought a MAC might not care. But it ruined the mid to high range. I then tried to make a replica of the Velo baffling system, but couldn't get it right. Putting on a real Fishtail was like throwing a switch.
So, if the new needle & jet doesn't make a big difference, as we expect it won't, the next step must be to fit a standard Fishtail, and baffle tube. The club doesn't handle those since the dealers have always maintained a supply. Maybe another club member in your area can lend you (an old, discarded, rusty) silencer set to carry out a test.....? Look forward to hearing the results. - TW

Maybe I need a quiet piece of road and just slip the silencer off the exhaust and see if that allows the engine to spin. If that works, it looks like knocking the end out of the fixed inlet tube would be something to try - I am not sure about the noise as a result. Then something of a retrofit "baffle tube" might be in order.
What is the diameter of the new "baffle tube" you scanned, is it less than 44mm? My silencer inlet pipe ID is around 44mm.
It is looking like buying a proper "baffle tube" to fit inside my can is the way forward. I could cut-out the end cap on the existing pipe so that the baffle tube can fit in and pass through the end. It should protrude beyond the end of the existing pipe far enough for the openings in the baffle tube to be exposed. If doesn't, it would be a straightforward task to shorten (or open up) the existing pipe.
Out of curiosity I hit the end cap of that internal pipe using a long rod and hammer. Two? tack welds came away easily so I have the end cap at an angle acting as a final "baffle" directing gases downwards and outwards against the outside of the can. This re-enforces the notion of using a proper KA142/2 "baffle tube" through the end as it looks as though it would be no trouble to remove the end cap altogether. I any event we will find out if simple back-pressure is the issue this weekend! I wonder how noisy it will be? Maybe not so bad as a Harley.
I got the AMAL parts today. The needle is significantly larger (than the current one) in diameter towards the tapered end, it was this part that somebody had vigorously "emery-papered". - DR

I think your only option is to get a genuine Velocette (design) fishtail silencer. And your friends! It makes me sad to think how all your Velocettes (and many others in the world) are not performing as beautifully as they were designed to, all because someone saved £50 in the past by putting on a pattern silencer rather than the real thing. I say that you'd be wise to buy the real thing for two reasons....
1. The construction isn't as simple as you have envisaged (see below) 
2. I have tried it myself and given up. I tried to convert a pattern one similar to the one you described, which had a short pipe welded inside the can, as yours did. But it was a smaller diameter pipe than standard, so I had to reduce a standard baffle-tube in both length and diameter. Hard work, and it still didn't perform anything like as well as the standard one. If you look at my ex-racer on my 'members machine' page you'll see a fishtail under the seat which I had to put a severe 'waist' in to wrap round the suspension unit. I tried that without a baffle-tube but it wasn't right and so I just had to make a baffle tube to fit. I also raced that bike with a straight-pipe and a megaphone - both were worse at first and took a lot of sorting.
The baffle-tube doesn't have a blanked off end, the pipe in the can has that (with a small hole in its centre).
The baffle tube is a 1.75" tube for all its length apart from 2.5" at the end where the exhaust pipe goes in. For this 2.5" it is split and opened up to an ID of just over 1.75" to go over the exhaust pipe. The tube wall thickness appears to be 3/64" so the OD is about 1.85. The neck of the standard fishtail has this ID, but pattern silencers usually have a neck ID of 1.75 Any Velo owner will tell you what a sweat it is to fit the standard baffle tube into the silencer and around the exhaust - impossible with a standard baffle tube and pattern silencer. And the performance depends crucially on the reversing of the gas flow out of the slots of the baffle-tube into the gap between it and the baffle pipe in the silencer, then out of the end and sides of that pipe.
If I can quote the factory Red Book (with which I completely agree...) 
" Do not in any circumstances alter the silencer by modifying the baffle or shroud tubes. The design is carefully worked out to give the best power output consistent with reasonable silencing. From experience we know that any alterations always reduce the power and speed - we have never known the contrary to result. For races where open exhausts can be used the choice lies between a constant diameter pipe or a megaphone. The former is better for standing start events, and sprints, but the megaphone will give a better top end power output at the expense of the output at the lower end. The length of pipe is important and the measurements quoted in the appended tables of carburetter settings are all as measured down the centre line of the pipe. Changes in the exhaust system make resetting of the carburetter essential. Whilst settings are quoted these must be considered as a guide only. Final setting must be done to suit the course, the altitude, and the prevailing weather conditions. Always err on the slightly rich side, even at the expense of ultimate maximum speed, to avoid risk of overheating, particularly in lengthy races where heat ' builds up ' and may not become excessive until the major part of the distance has been covered."
I would certainly try an experiment with an open pipe, but you should really extend its overall length by between 7" and 11" for it to work properly. (You vary the length to adjust the balance between maximum power and torque) I do this by using the chromed tail-pipe extender accessories you can find at auto parts shops - next to the furry dice and air fresheners. - TW

During the week I fitted the new carburettor parts:
* new "c" needle - the main difference being in the area above the tapered end the new needle is around 2 thou bigger in diameter.
* Needle set on the 3rd notch - how can you be sure? I did it several times to convince myself largely by estimating how much of the needle should be showing out of the top of the hole.
* new needle jet - a 106 replacing a scored 106A - I wonder what the difference is between an 106A and 106?
* new main jet a 270 in place of the 260.
The "pattern silencer" had part of the blocking end plate bent away from the end.
Result?
Significant improvement - now the bike seems to want to rev. freely I took it on the "standard" run and on the 1 in 6 to 1 in 10 hills where I had been plodding at 20mph it now pulled strongly in the 35-40 bracket in 2nd. The bike was particularly lively and flexible in 3rd, the best I got in 3rd was around 65 mph - but this is through bends and hills with serious trees and steep falls off the edge of the road - there is not much chance of building speed at my level of courage and confidence. The bike handles all this very well - pity the brakes are limited - but after a while I was really riding the bike using the gears and "engine-braking". The best in 2nd was 45mph but that was only because the vibration suggested that changing up would be prudent. So we now have around 1300 more useful revs i.e. around 5700 (top in 3rd) up from 4400. One irritation that spoiled the tests was that the choke was closing with the vibration. The choke lever currently fouls the chromed inner end of the throttle and has been set too loose.
On return, I took off the silencer and around 3/4 of the pipe end-plate had "fallen off" and was removed via the end openings in the fishtail. In effect the bike had been running on a straight pipe of 250mm inside the can - I think the baffling can be assumed non-existent - compared with the 380 mm length of the "baffle tube" in your illustration earlier in the week.
I took out the plug and it was dark grey to black - dry soot black. I had cleaned it after last weeks run (you will recall it was dark brown after that run). This was not a good indicative test owing to that choke lever closing issue. I cleaned the plug again and replaced it.
So we are now into the territory of finding the next 1300-1500 revs to the engine's peak at around 7200 rpm if the Motorcycling 1956 test is correct. This is where a "proper" silencer would come in handy, as well as a vibration-free choke lever! - DR

You'll need a long straight road to do the final sorting out.
The Viper really had exceptional performance for a 350 (with good manners, as compared with a 350 Gold Star) and it requires that everything is right to deliver it. I think you'll find that the correct silencer will give you as much again as you have got from removing the baffle; and you'll get the distinctive and much loved Velo exhaust note.
I don't know much about Monobloc needles (on the GP you do have to choose between a 'standard' and a 'weak' needle, but I don't believe this was the case with monoblocs). It could be that there wasn't room to stamp the part number (376/063) so they used a letter instead. I've just looked at such a needle (supposedly) and it has a "C" stamped on it, so you probably have the right needle now.
I've looked in the Amal parts book for the monobloc to '63 and they list one alternative needle (376/116) and jet (376/117) - but they are just for Alcohol - with a much bigger gap between the needle and jet but how much this was through a larger jet or smaller needle or both I don't know. It would seem quite likely that they would mark such a jet with an "A". But not with 106 if, as would be expected, it would be much larger than that...
According to my parts book Amal didn't mark the 106 jet at all....because it was "the standard". Very British! Amal would certainly have marked an Alcohol jet, and a remanufactured 106 jet would probably be marked nowadays.
The standard air lever is adjustable for friction by the screw, or rather coin, slot in the dome-headed screw that holds it all together. (So also is a typical period twistgrip, by a small screw, I adjust both so they just hold position.)
The Viper peaks at 7,000 and Velocette strongly advised that these revs weren't exceeded (M/Cing were a bit silly in that era in the way they always went on until the valves floated) and you'll certainly need a Velocette design silencer to get there. - TW

Prior to today's run I inserted a imitation baffle tube into the silencer can via the pipe. The baffle tube was 380mm long and I did my best to imitate the original as a trial. The bike appeared better a little more responsive and a little quieter - the exhaust note changing to a more muffled bark.
It climbed the hills readily over 40mph in second on pulling climbs.
I tried to open the bike out in 3rd on a fast section and it got up to 65mph with very severe vibration through the RHS foot peg - I could not keep my foot on the peg. Then something gave way and the mechanical noises changed as though something had come loose or broken.
When I stopped the bike the decompressor would not work (I use it to stop the engine). The cable moves the lever but nothing happens to "decompress". I stopped the engine by winding back the carb throttle stop.
Nothing else seemed to be broken and nothing seemed loose. That vibration I think is because the RHS footpeg mount is used to attach the exhaust pipe bracket.
I rode the bike home (around 30 minutes) carefully but it seemed to run OK just with some new rattles. Some more investigations tomorrow.
I am puzzled by this unwillingness to rev. out 65mph in third should be well inside the engine's capabilities. Using the MotorCycling test data this is around 5700 rpm.
So I have been looking over things more carefully. First: the gearbox has a 14 prefix: its number is 14-3605. Vipers should have a 12 prefix I think Second the gearbox output sprocket has 19 teeth - I think Vipers were supposed to have 21. (The rear wheel sprocket has 55 teeth). This difference may have had the bike up near 6500 revs when whatever broke. Third: there appears to be a compression plate under the cylinder - it is around 1 mm thick (45 thou). My calculations indicate that would result in a compression ratio of around 7.8 vs 8.5 nominal for the Viper, ie more like a MAC compression. Maybe I simply don't have the power!?
I've looked at the data on the gearboxes from the Velo. Web Site Technical data sheets (a very thoroughly prepared resource).
I am assuming I do have a #14 gearbox with #14 internals.
The critical difference appears to be with top gear, the #14 has a gearshaft pinion with 16 teeth vs the #12 with 17. Comparing the (MSS) #14 gearbox overall gearing data with the Venom (both using the same gearbox sprocket) the effect appears to be that the same top gear is achieved but lower gear ratios are lower geared with the #14 (i.e. the MSS engine would rev higher in lower gears for the same road speed).
I am assuming that I have a Viper engine sprocket 21 T and clutch chainwheel 44 T ( I have not checked). I have the correct Viper rear wheel with 55 teeth. It looks like with the 19T gearbox sprocket on the #14 box, the aim was to get a similar top gear to that of the original Viper box, but all the lower gears rev around 10% more for the same road speed as a "normal" box. So at 65+ mph in 3rd I could have had ~ 6500 revs up and not realised it - in other words the engine may have been trying its best!
Nothing in the papers that came with the bike state that a gearbox was bought for the restoration - but the information is patchy. The original registration paper shows the engine and frame numbers to be original, but there is no record of a gearbox number..
Today I went looking for the problem from the last run. The exhaust valve tappet had loosened. The nut had worked off and the tappet was resting on top of the push rod. No apparent damage. The nut was recovered from the trough under the exhaust valve springs. It was in average shape - a bit of tidying up with a file and it is back in use. So I have learnt how to take off and replace the tappet cover, the tappet adjusting cover and the timing gear case cover.
The inlet valve tappet clearance looked fine, but I adjusted both according to the book to be sure; 6 thou for the inlet and 8 thou for the exhaust.
I am left to wonder if the exhaust valve tappet had been working loose for a while and adding to the lack of performance? We shall see next weekend. Meanwhile I have noticed that the engine mounting bolt near the RHS footpeg had loosened adding to the vibration in that area no doubt. - DR

Having read your note about the exhaust valve lifter not working I was going to say that the tappet had worked loose (if you were lucky, and the pushrod had bent, or worse, if you were not). By the time I checked my mail I see you have confirmed both that it was the reason and worked out for yourself why you might have been over-rev'ing.
Very Good! And all part of the fun and interest of owning and RIDING a Velo or any other old bike. After 40 years I still discover new intrigues, and still find it satisfying when I work something out or make something better.
I wouldn't worry about the gearbox being a type 14. Work out the peak revs in the gears, and try not to exceed 6,500 on a Viper. Your engine will last much longer if you use this as a limit rather than 7,000. Rev'ing until the valves bounce is very bad practice - you'll wear the cams & followers and valve seats, etc prematurely. HOWEVER, I'm not sure about your conclusions about the effect of your setup. There seems to be a 'non sequitor' in what you say.... 
"Comparing the (MSS) #14 gearbox overall gearing data with the Venom (both using the same gearbox sprocket) the effect appears to be that the same top gear is achieved but lower gear ratios are lower geared with the #14 (i.e. the MSS engine would rev higher in lower gears for the same road speed)." - I agree. 
"It looks like with the 19T gearbox sprocket on the #14 box, the aim was to get a similar top gear to that of the original Viper box, but all the lower gears rev around 10% more for the same road speed as a "normal" box. So at 65+ mph in 3rd I could have had ~ 6500 revs up and not realised it - in other words the engine may have been trying its best!"
 - I think it is the smaller (21 versus 23) engine sprocket and 21/55 final sprockets that give the lower gearing for the Viper:
The type 14 and 12 had the same internal top-gear ratio of 1:1 (The lower gears were wider spaced on the type 14 (MSS) than on type 12 (Viper, Venom) Personally I prefer the wider ratios for today's road use.) The sprocket ratios then gave an 18T Venom and an 18T MSS a 4.9:1 top. The Venom engine rev'd higher so was faster on the same gearing. Conversely, the Viper rev'd higher still but with less power than the Venom so was geared lower, by the sprocket combinations, to 5.5:1. So a Viper with a 55T rear wheel sprocket and either a type 12 or a type 14 gearbox should still have a 21T gearbox sprocket. If you have a 19T gearbox sprocket (& 21 engine & 55T r/w) then you will over-rev 10% in top and your third gear being internally lower by another 10% means you will wreck the engine if you try get the standard speed in 3rd.....
I think you were lucky the tappet nut came loose.... (They always get battered and rounded, because of the difficulty of getting a decent spanner in to loosen or tighten them when adjusting the valve-clearances.)
The compression plate may well be original, as Velocette fitted either 10 thou or 31 thou plates on occasions to get the CR right; and many of us have fitted them when 101 octane fuel disappeared from the pumps. As I get less interested in top-speed I fit more plates for a smoother, longer- lasting engine with easier starting! - TW

Yes I agree with your analysis of the gearing (20% more revs in 1,2,3 and 10% in 4th) - assuming I have the same primary side sprockets as a Viper should. I was also puzzled by the bike's behaviour in top (before the tappet event) on the last trip. It seemed overgeared in top despite that 19T gearbox sprocket or maybe top was always that way for a Viper. It seems to be very much an "overdrive" gear - It just pulled the bike along in the speed zone speed (60 kph ~ 37.5 mph) and plodded up small rises at the same speed. I put it into 3rd for any type of hill. Anyhow your analysis indicates - once I had fixed that back-pressure - I could have been approaching peak revs in 2nd and 3rd without realising it (i.e. thinking I had 5400 in 2nd 3rd when in fact it was more like 6500 and that is my suspicion; but being new to "bigger singles" I did not realise it). Roll-on next weekend when hopefully I will have most of the current issues checked-out.
This evening I spent a little time checking over the engine mounting bolts. Several could be tightened (which no doubt was adding to the vibrations at high revs).
But one was fully loose and exactly why has me puzzled because I can't locate the fixing nut. The bolt in question holds the base of the engine plates in place and (I think) runs between the two lugs on the base of the frame.
The bolt is item 49 on the illustrated diagram of the gear box - part number SL11/2. This has a spacer (FK221/3) between the frame lug and the engine plate {and no doubt I will find it has a spacer (F305) between the plates}.
What I am puzzled about is what the bolt screws into as it is hidden behind the primary chaincase. I can rotate the bolt head (on the kickstart side of the bike) but it does not tighten up.
Should I find a nut between the lug and the inner side of the primary chain case? If so non appears specified in the illustration. - DR

You are right to check the engine mounting bolts are tight. And that the head steady is both tight and not cracked! And that the holes in the plates and frame are not oval! Purists have been known to bore out then put bushes in any worn holes and ream them out for a perfect fit (Not me!)
Bolt SL11/2 doesn't have a nut, it goes into the lug which is threaded. It sounds like yours has stripped, and usually it is the bolt not the lug that wears and strips (thankfully) There is the outside distance piece FK221/3 but I thought on all Vipers there is not one between the plates - the crankcase goes between them and is the spacer - let me know if yours has a distance piece between the plates on this bolt. - TW

During the past week I have re-set the tappets on the Viper and fitted a new bolt between the frames at the base-rear of the engine. I had to use a UNF 4inch 3/8" bolt. This has 24T vs 26T in the original. With a washer to shorten the length, some loctite on the threads, it bolted up well and firmly. If this fails I think the next move would be to make a M10, I mm pitch bolt which has 25.4T and around 0.5mm larger in diameter so would allow the lug to be threaded out almost to the same pitch. I am not looking to that prospect but it could be done.
I gave the Viper a run along my favourite route this morning - this 1.5 hour run is always interesting with lots of variety and I can make progress comparisons. I think I can say with confidence that the bike is now performing to around specification, certainly as I would have expected it to. I did not rev. it out in the gears. Since the gearbox ratio exercise of this past week, I have been clearly running the bike to its limits. But the bike performed willingly with the sense that there was something left. The transformation in top gear is very significant. I mentioned before that it seemed to have no pull and I was always changing down for significant hills. Now is runs very flexibly in top and runs up most main road hills with nothing more than more throttle in the 50-60mph range. I can now sense the big drop from top (4th) down to 3rd as would be expected from the #14 gearbox. But I found I could now get into 3rd on hills that required 2nd to be revved out before. I also got it up to 70mph in top without too much trouble (and without revving out in 3rd) on the short stretches that allow some safe speed, top speed I calculate at around 82mph with the current gearbox sprocket. There seems to be a bit of tappet noise but maybe there should be. But the vibration was much less, I think the tightening up has helped a lot. The plug was a dark brown - I noticed during the week that the piston top has changed to brown from black as the engine breathing has been improved. The next project will be to look at that front brake. I am sure it must be able to do better than it is. It really needs to be pulled on to work. - DR

I think you would still notice a real improvement from a genuine fishtail.
If you start work on the front brake we'll definitely have to start a new thread - it could get equally long! - TW

Last night I re-tightened the last of the engine bolts - the ones at the front that I had looked at early in my ownership of the bike.
The bike is a delight to ride now, vibrations are so much lower except at very high revs. and I have built a high level of confidence in what it will do.
Overall, as I look back on the things I have done on the bike, I think these stand out:
a.. replacing the plug at the beginning to regain immediately reliable low speed running
b.. removing the old restricting pattern silencer baffles and replacing with a mimic baffle tube which immediately gave me the revs and power at the top end (OK I should have the real thing, but this fix is doing fine).
c.. getting the front brake drum skimmed with new linings to fit, now I have a decent front anchor.
d.. putting on the 21 tooth (standard) sprocket (in place of 19 tooth) with immediate gain in engine-gearbox range and response
e.. getting all the engine / gearbox mounting bolts, especially the head stay, really tight resulting in an immediate significant reduction in vibration and with it a much more "ride-able" smoother bike.
There were other things along the way such as the fixing the carburettor and re-setting the tappet clearances, but those were more part of the TLC along with all the checks and general care. - DR 3/3/2001

The bike is running well but there are a couple of issues arising:

STARTING
I've found something that helps starting. It is a commercial pressure can "starting spray" with something like lighter fluid in it. A very short sprayinto the carburettor intake with the throttle open (before cold starts after a couple of weeks non-use) works miracles.
No longer do I have to rely on living at the top of a steep hill!
SMOKE
Today after start-up, the bike was blowing smoke from the exhaust for quite some time say 10-15 minutes of running through traffic. I use a 50 SAE monograde oil (Pennzoil), temperatures here are in the mid 20s to low 30s Celsius at present. I may have put too much oil in the tank; I filled it to the oil tank level without accounting for anything in the crankcase. Anyhow
once the bike was really warm after the run coming back into town, there was no smoke even revving the motor did not produce any. The Viper runs with an Alfin cylinder. I seem to have quite a lot of piston noise until the engine is really working hard and hot. I have tried to follow oil consumption, I am adding around 1 cm before each run to the tank. I wonder how much I am "blowing away". It does leak and I know I am losing oil to a catch pan under the bike, but not much.
PLUGS & LRP
I am still satisfied with my current choice of plug - it is doing fine; no mis-firing and no pre-ignition. I'm still running on LRP at around 97 Octane. My 1984 SAAB 900 (we have had it since new) used to lead its plugs in the early days and liked the lower levels of lead put in "Super" in recent years. One we changed to LRP it started lose power (quite dramatically on 3 occasions - once due to clogged fuel filter), but a fuel injection specialist on the other 2 times could find nothing really wrong but somehow got most of the power back. After some months of this it refused to start at all. The garage finally discovered an intermittent fault in the fuel pump relay, replaced it and the car has been running well ever since.
But I now run it on 50/50 unleaded and premium unleaded as the engine is supposed to have hardened valve seats. The point of my story is that LRP additives work differently from leaded and LRP seems to require an engine to be in good shape to do its job. It seems to find out any weaknesses; I have heard many LRP stories similar to mine with the SAAB. - DR 28/1/2002

You must be clocking up some miles, that's a (minor) reason why it is beginning to rattle a bit, and use a bit of oil.
The main reason it is rattling is because it is an Alfin barrel and it is now a little worn. Velocette could only eliminate the rattle when cold with split skirt pistons and/or iron barrels. It is nothing to worry about, even if it rattles when hot, as long as oil consumption isn't too bad - and that has more to do with worn rings & valve guides. 1cm per ride is OK.
All the smoking sounds like what is referred to as "wet-sumping". Oil has drained down into the crankcase while standing - if it only happens now and again don't worry about it. It's explained in the service manual. But watch out for it - e.g start up to clear the crankcase before topping oil up or only top up after a run, otherwise you will overfill.
I know the Easy Start cans - they were much used for UK cars in the 60's in winter, but nowadays you can't get at the air intakes to the 'carbs'. The smell is ether, as used in model airplane engines, because it is highly volatile and 'diesels' so theoretically can start without a spark.
I agree there is something different about LRP, and that it shows up in engines which are a bit out of shape. Or perhaps engines that are out of shape blame the LRP! - TW