classic motorcycle forum

My father has recently purchased a BSA A7 which has undergone a complete rebuild including engine rebore. He got it off a guy who got it off the guy who did all the work.
The mag was shot so it has been rebuilt by Dave Lindsley. The valve clearances where quite big so these have been reset. New NGK plugs B6HS which are the right value. Mag installed with pretty much bang on timing. The carb has been stripped and seems to have had an overhaul new needle jets screws gaskets etc, all to right value Now the problem. when the throttle is openend you apear to get resinence similar to a high speed misfire. I took the bike for an MOT no problem on the way back it started to misfire badly and cut a couple of times. At home the plugs where examined. they were very sooty. Once cleaned the bike started ok but after a mile or so run it starts to misfire and plugs are very sootty again.
Any suggestions please. Obviously a rich mixture but what cause, Carb good. Have installed a new brass needle and float ok no flooding there. All carb settinge correct for the bike. Timing corrct both plugs firing. I can only change the plugs now but I did have one spare and this one behaves the same. Could I try a hoter plug B5HS. but that doesn't seem right. It could be badly seating valves. But before going to that much trouble I am looking for other suggestions.