Hi guys
I will try and explain my situation as best as i can as i am really stumped

My A12 is backfiring in my 120y it's not a fireworks type of backfire
it's more of a popping sound near the carburettor but i can definately say it is coming from the exhaust manifold
it sounds like a flopping and its only doing the flopping at idle and sounds clean and normal on load
kind of like a misfire
also i've found that richening out the mixture helps it alot for some reason which is wierd

what i've done so far
changed carburetors, from hitachi to weber
changed head gasket, reground the valve seats
adjusted valves to .025mm i think if i can remember correctly
changed leads, dizzy cap, rotor
pulled the dizzy out to check for worn shaft, put back in to play with ign timing
adv timing as far as i can then retarded when it sounded like a truck
drained fuel and put a fresh batch in

what i suspect is:
worn condensor and points as i havnt changed these before although im not 100% sure if its this or not
would anyone know the symptoms of these when they go bad?
fuel filter blocked? i doubt it as it's brand new
vacuum leak? i hope not, i've done everything to eliminate this and sprayed carb cleaner to see if engine revs change and nothing
does anyone have any other ideas?