backfire at 3000-4000rpm

Hi. Im having problems with my 1.3 driver. Its just had a weber 34ich fitted proffesionaly and idles perfect. Its fine when im in lower revs but as soon as i open it up a bit it backfires and just splutters, come back down to around 3000 and its fine. Any idea what this could be? Ive just had a new condenser fitted on the distributer. I was having problems with the distrib before cos the condensor housing was broken n it was backfiring and going nowhere but now it only does this between 3000-4000rpm. somtimes it does let me ease it up past but still stutters n backfires past 4000

Hi,

Check the vac advance is working - you can do this just by sucking on it and see of the arm moves
Other things I'd check:
Coil
Carb shut off valve is working correctly and opening properly
Fuel pump is delivering sufficient fuel - could be the pump or the pick up in the tank.  Also have seen perished rubber hoses at the tank sucking some air

You could also try spraying in some easy start when its missing to proove if it's fuel or an ignition issue.

Cheers

Rich