CA004 CARBURETTOR

I have just taken my '58 Venom out for a trial run on which I fitted a new 389 monobloc, replacing a 1034 concentric that had an occasional habit of emitting a 'tick' followed by a drop in engine revs when stationary at lights etc. sometimes it cut out, other times it didn't. Apart from this it ran wonderfully.
I made a decision to 'correct' a few non standard fittings (for 1958) and amongst other things, bought a new 389 monobloc.
The 'tick' or 'spit' is now much worse. The tick over speed that is needed to stop the engine from stalling is far faster than I think it should be.
I also have a '61 Dominator (with single monobloc) that has never done this, Different machine, I know, but same principle?
The Venom is also fitted with manual advance/retard and when stationary with the engine retarded slightly, it seems to favour that moment to 'tick' - Apart from this, it runs beautifully. What do you think? - Steven Buck 24/6/2003

I think your spit back/poor idling/stalling problem may be a symptom of a weak spark rather than primarily a carb problem. However, it is also a normal characteristic of the Venom and Viper as opposed to the MAC or MSS, though not perhaps to the degree you are experiencing. (Even a perfect Venom will 'hiccup' and then sometimes die on a low tickover - I've often found it necessary to blip the throttle every 20 or 30 seconds while idling a Venom)
When the engine is idling there are two negative effects: the magneto produces a lower intensity spark (which gets lower still if you use a manual retard), and with the relatively sporting cams and largish bore carb of the Venom & Viper the mixture is pretty sensitive. With a weaker spark and the highish compression of a Venom and/or a slightly incorrect mixture and/or a little oil getting into combustion chamber the spark can get 'blown out'. Your Dominator probably has coil ignition, and a relatively smaller bore carb, so can idle more reliably.
Everything gets far worse if you have an air leak at the carb/head joint.
So I'm afraid all you can do is check the joint is good, then that the points and pickups in the mag are clean and then try a new plug with the gap at 18 thou and try setting the idle screws using no retard (to maximise the spark). If you can get a reasonably slow idle for more than 30 seconds then things aren't too bad. (If half-retarding the mag and/or using a 25+thou plug gap as an experiment makes it unreliable then your mag is probably a bit weak. If it just slows the idle revs by 20%ish then the mag seems OK.) If you can't get a decent idle at all then either the mag or carb is faulty (in an old carb it would most likely be a worn slide or body), or a valve is sticking or lots of oil is getting into the combustion chamber. - TW 24/6/2003

I think I have solved the problem - basically, me not setting up the carb correctly, when the engine is hot. 
I have a rev counter fitted, what is the ideal idling speed? - SB 28/6/2003

Glad you are back to an adequate tick-over
If you can get it a Venom to idle at 1000rpm such that when it 'ticks' it recovers 9 times out of 10 without you having to yank the throttle then I would settle for that.
Ideally you would get down to 800 rpm but in my experience (with mag ignition) only a MAC or MSS is reliable at that. Sometimes my Venom will tick over at about 500rpm for "minutes", but it is certain to die sooner or later.
(Few rev-counters are accurate below 1000rpm, but I'm sure your ear can detect "just a little faster than you'd ideally like!)
I always set up the pilot mixture for best idle then richen it by a click or two to improve starting and tickover reliability. - TW 28/6/2003