EV001 VM ENGINE(1) 

Why does it pump oil into the primary chaincase? - TW 10/6/2000

It is probably a breathing problem and I hope others will give their experiences and CURES(!?), but it may be similar to my strange problem....
My "life-time" Venom (1957) has always done it, but in the '80s I had a real problem with this.
I was racing the bike and Ralph Seymour and I were tuning it (all the skill was Ralph's).
After the first major rebuild, I ran it in on the road and in 250 miles it emptied the oil-tank into the primary chaincase and thence into the gearbox until the gearbox over-flowed.
Ralph and I then tried EVERYTHING to stop it. Every breathing mod - drains from timing chest to crankcase, even a breather in the primary chaincase(!!!). Ralph made and fitted robust scrapers under the flywheels to scrape the oil off them and throw it up the return feed!  
I have tried blocking-off every breather, including the hole in the drive-side mainshaft, in every combination, with no difference - not even to increase the flow! 
Nowadays it's a road bike again, with a discrete catch-tank under the gearbox and any more than the required 1/8th pint in the chaincase drains into it, at 1 pint every 180 miles in road use..... 
Ralph had never experienced anything like it, and I feel it may be related to the batch of crankcases mentioned briefly in Bob Burgess' book 'Always in the Picture' as exhibiting this characteristic for no known reason.- TW 10/6/2000

I took delivery of a Venom 4 months ago which had a slipping clutch because there was too much oil in the chaincase - despite a regular leak when the engine was running. Despite trying to make the primary chaincase virtually oiltight as discussed in "members questions" there was still a substantial leak which coated the rear tyre when on the move. On draining the chaincase I found about 200ml of oil still in there. I recently fitted the owners club oil filter modification and a new ball in the non-return valve. I took the outer cover off the chaincase and started the bike and found about 25ml of oil came through the engine shock absorber unit. This stopped after about 5 minutes of running. On starting the bike the next day no oil was seen coming out. After 4 -5days I tried to start the bike but found that even on kicking it over without the engine firing a regular stream of oil came past the shock absorber. It appears that the oil is draining into the crankcase and getting thrown out past the engine shock absorber as the crankcase pressure builds up. Should I look at the ball valve, the pump, the engine breathing (wherever that is) or is it oil draining back from the filter. I am not sure whether I am looking at the cause or the symptoms. - Dougal Burns, 1/12/2000

You've analysed the problem nicely.
The three main 'normal' reasons for the oil transfer from the crankcase to the chaincase are:
1. Crankcase breathing problems (complicated)
2. The scavenge side of the pump not clearing the crankcase (either because the pump is faulty (unlikely) or the suction filter plug isn't airtight, or the return pipe/filter is blocked)
3. The anti-drain valve is not working
and for less severe symptoms
4. The filter housing draining its contents back (remove the pipe below the filter after a run and see if more than a few cc comes out)
From your analysis, it is number 3. - you've observed it stops once the excess oil in the crankcase has been cleared (either by the pump back into the tank, or through the S/A)
After 1 day, the amount that has got past the anti-drain valve isn't a problem; after 4 days it is. I left a bike with this problem on its side-stand in a lock-up garage over winter, and came back to find four pints of thick black Castrol surrounding it...
No doubt you've seen the series of articles in FT about 'sumping', which is the name some people give to this.
One test is just to get the oil warm, let the engine stand for 15 mins, then gently detach the oil feed pipe union from the bottom of the ball valve assembly. Wait 5 mins and if there is a regular drip (though perhaps only one a minute), the valve needs attention. (This test isn't totally reliable because with the pipe reattached there is more 'suction' to cause the valve to leak, so you could pass this test but still suffer leaking.) You must bleed the pipe when you reattach it, and test for oil circulation in the standard way.
Changing the ball doesn't usually help - it normally stays perfectly spherical. It is a seat that has lost its perfect sealing edge and perhaps a tired spring that causes the leak. People say a LIGHT tap of the ball on its seat can restore the seal, but if it widens the seat edge it will then lack the contact pressure to seal. To my mind it will only work if there is a tiny burr or roughness, perhaps from original manufacture, that a tired spring no longer compensates for, and the tap removes it. There may just be a spec of dirt, or swarf, or a scratch on the seat, and you should check for this before any 'tapping'.
If your ball-valve remains a 'leaker' after you've done the above, I believe a batch of bodies was manufactured earlier this year and may still be available from one of the Velocette parts dealers.
One final thought - did you have this problem before you changed the ball and installed the filter kit, or only after....? - TW 2/12/2000

I assume from what you are saying that there is no oil seal to stop the oil coming through past the shock absorber? - DB 3/12/2000

No, there is no seal between the drive-side main bearing housing of the crankcase and the back of the sprocket. On a new engine there was minimal clearance between these items but still enough room for oil and air to pass. On a worn engine there is even more of a gap - but the official route for the air to get out (for crankcase breathing) is via the drilled hole in the drive-side mainshaft. How the air knows it supposed to go that way, and the oil isn't and neither is supposed to go out through the open bearing only Velocette knows...but it works 99% on new engines.
Some racers have fitted an oil seal around that shaft, to keep the dreaded slippy stuff off the rear tyre, others use lots of foam under the engine (e.g. my racer) and on my road Venom I have fitted an overflow in the chaincase that lets excess oil drain into a catch tank hidden under & behind the gearbox. - TW 3/12/2000

I think you are correct about the non return valve although when I disconnected the pipe it was not conclusive. I turned the engine over after it had stood for several days and oil came out of the engine shock absorber. I drained the crankcase at this stage and it contained about 560ml of clean oil. I added this back to the tank, started the engine and let it run for about 5 minutes. No oil came through the shockabsorber. During this time the level in the oil tank went down and it started to return into the tank. At this stage I drained the crankcase again and there was about 400ml of oil, which I presume is about normal although I thought I read that it should be about 2-300ml. Anyway, clearly on standing a further 160ml of oil is getting into the crankcase which must be enough to take the line of least resistance past the engine shock absorber. - DB 5/12/2000

It's good that you are measuring the amounts.
I too have seen different figures for how much oil gets left in the crankcase. I think mainly the differences are because there were different sized flywheels in different models at different times, but also engines seem to differ in how much they hold up in the various chambers that then drains down when stationary (and of course the later crankcases had a drain from the timing chest which would make a difference, as would the grade of oil). 400ml is on the higher side but still OK, though I would expect noticeable drag when kickstarting from cold - enough to make starting difficult with a magneto.
Did you stop the engine as soon as the oil started to return? If so, you would leave more in the crankcase because the scavenge side has to get primed by the oil rising towards the height of the return gears in the pump - once primed it keeps the bottom of the crankcase pretty clear because the return pump gears are wider than the feed, which is why you get air in the 'gulping' return flow from the filter chamber outlet.
It is still just possible that it is your oil-filter chamber draining back into the crankcase. This could allow around 200ml to drain back, but it would then stop, of course. From your earlier info it sounds as though you got large quantities into the primary chaincase when standing, however....
I would do one more test - run the engine for a couple of minutes until you see steady gulping in the return, and the oil is just warming, then immediately drain the sump. Leave the drain plug out, and put an empty container underneath. Check every 30 mins then every day - if up to 250ml comes out within an hour or so, then nothing, it's the filter chamber draining back (easy to fix), if the drain is slower but goes on day after day it's your ball valve leaking. - TW 5/12/2000

After a fair bit of trial and error it looks like the problem is the same as the one described in Fishtail 309, ie the oil leaking back from the filter. I started the engine, ran it for 5 minutes, stopped it and drained the oil. I immediately collected 400ml (as before). Over the next 4 days I collected 100ml, 85ml, 50ml and 15ml. At he end of this period I put the oil back in the oil tank to see if the spring was weak when the tank was full. Since then I have collected a maximum of 5ml. All I need to do now is ensure a good seal of the pipe in the filter assembly. Any recommendations on what to use? - DB

Provided the surfaces are clean and the pipe is a reasonably close fit over the boss, almost any locking product would do fine - screw lock, stud lock, or bearing fit. If the pipe is a pretty sloppy fit then Araldite, or a new pipe (or the other end of the same pipe with some work). But check for cracks or other mechanical reasons for the leak rather than just a loose fit of the tube over the feed boss. And do check that the hole in the feed boss is completely clear, and in particular that the threaded rod (that goes up the middle to the top-cap) has not been screwed in too far and started to obstruct the hole for the oil to come out of the feed boss into the tube - this has been known to happen, causing poor oil return....... - TW