AJS & Matchless Owners Club

Hi I have been riding a g80s 1951 mod,witch I took ownership of a few months ago.iv done over 350 miles and have been taking it further and putting it through its paces on each outing. Apart from one problem the bike seems fine.i understand that a top end speed of around 70mph is not expecting to much from a g80 but when I get to around 60mph if I try for more it’s has if the bike seems to retard its self only to pick up again if I throttle back to under 60mph .plug looks good bike pulls and accelerates well,I’m thinking main jet .after that perhaps timing .just wondering if anybody has come across this problem,

shaun pearson wrote: ↑Thu Nov 10, 2022 11:01 pm if I try for more it’s has if the bike seems to retard its self only to pick up again if I throttle back to under 60mph

Do you think it's trying to tell you something?

But yes, if I was determined for more with those symptoms I'd be increasing main jet.

certainly worth increasing main jet , what colour is the plug does this give you a clue as to if the mixture is lean
10% Ethanol fuel has lower energy than regular petrol by about 3%
You should always check timing when investigating poor performance

better performance with slightly closed throttle i a classic indication that mixture is running lean at this condition

Forgive if this is an egg/grannie scenario but for anyone monitoring the chat who may be interested...If you want a definitive answer on mix when running on main jet, you could try the "kill" method. Ie. ride at least quarter mile at the relevant throttle setting and kill the engine without changing throttle. Using valve lifter/ignition off/magneto earth as relevant. Plugs should now tell you what's happening at that throttle setting.
Rolling off and pulling over before stopping engine may otherwise disguise the situation. Best done after cleaning plugs in a warmed engine.

Thanks Andy think I’ll try increasing main jet size and while I’m there I’ll give the carb aclean