I have just fettled my BSA A65 Spitfire for the summer (had her 13 years) she was running great but suddenly started backfiring like mad on one cylinder.
I don’t think the carbs are flooding - any ideas what to check???
A backfire in the exhaust is caused by unburned fuel igniting on the way out when the engine is running rich. This happens in exhausts normally but the effect is more noticable on a trick exhaust.
Actually, it’s a lean mixture that commonly provokes backfires, especially on the over-run
See here for some good information :
Thanks for the tips guys!
Does lean mean less air than needed or less fuel… I know the ratio is either 12.1:1 or 14.1:1 right?.. but when you say lean, which one is low air or fuel?
Lean means more air and less fuel. Either that or someone been trimming the fat from the bacon. 
Sorry Kevsta
Lean mix is too much air/not enough fuel.
(Rich mix is the opposite.)
isnt that what he said though ?
also possible is a leaking manifold gasket on exhaust header allowing it to suck air in causing backfire especially on overrun
That’s what I was thinking. 
running lean, maybe air getting in around carb rubber or gaps in the exhaust headers, silencer…
it may not be bad fueling, just air getting sucked into the exhaust therw gaps and going pop!
btw you say you’ve fettled her … what have you done exactly just in case anything has had a direct result and saves us make wild stabs in the dark