Honda-SOHC

Ok, an old chesnut, got good compression and valve timing now as per manual ( last owner had it out by a lot but valves survived somehow.) I've set the points as per factory manual and carbs came off a running engine on the bench. New air filter fitted, fresh fuel, good spark. tried re setting points several times. However when engine turns over it won't fire and there's air/fuel blowing back into the airbox... the compression tester shows normal compression on all 4 cylinders. Is there normally air movement in the 'wrong' direction?
this bike has it in for me at the moment...... I've run my 500/4 for years as a commuter but recent gearbox problems meant switching to a spare 550 lump from a breakers until time allows a rebuild of the original motor.