I'm getting a lot of visable petrol vapour coming out of the mouth of the carb when I run my Matchless G3, which is ruining the paint on the oil tank!! What could be causing this? I've tried adjusting the settings, but it seems happiest as it is at the moment.

Bill, this is a problem that I'm suffering on the Norton. Valve timing could obviously cause it (but mine's OK).

Float height could be a factor. I assume that the G3 has the long mixing chamber nut and that you have one of those fitted ?

Float needle seating, especially if a newly painted tank is releasing fine dirt will cause over-fuelling - It is a weak point of this set-up that rubbish isn't washed away but rather builds up round the needle seating.

And finally - Ian will hate me for saying this - but is the thin, light ethanol-based fuel more inclined to blow back as a vapour when the thick old heavy stuff that we used to have would just form a brown stain around the inlet ?

I don't hate you Rik...Blowback in the carb is usually caused by an over rich mixture..so try weakening the mixture at whatever point in the range it is doing it by making the appropriate carb adjustments.(assuming you have eliminated any basic carb faults which may cause richness)..Ian

email (option): ian@wright52.plus.com

Thanks for all that chaps, I'll have a tinker at the weekend and see if I can fix it. It's in for it's first MOT since 1985 on Thursday morning, so I won't start tinkering now or I'm bound to ruin something and make it immobile for the test.

Hey Rik, all of us Norfolk Norton owners suffer the same, after a good run we all have matching sooty black oil tanks! Nature of the beast I think.
Cheers, Mick.

email (option): mick@motorbikemike.org.uk

Maybe you're right Mick, perhaps I'm being too picky and it's just one of those things? It is a 70 year old military vehicle after all! On the other hand, you'd expect that sort of thing on a Norton, but surely not on a Matchless.

Well neither my G3 or G3L suffers from this, may be your valve timing is slightly out?
Cheers, Mick.

email (option): mick@motorbikemike.org.uk