my B40 has started with the most awful backfire and or popping on overrun and between gear changes... Could it be fuel mixture or timing gone walk-a-bouts ? Electrex World system and new Concentric carb...
Normally, popping or backfiring on the overrun is due to a leak in the exhaust system that allows air in that causes unburnt fuel mixture to re-ignite. Unless the B40 exhaust system is rusting through in places, the only place air can get in is at the cylinder head exhaust port or the inlet flange for the silencer. Worth checking the downpipe is fully home into the cylinder head and is not a loose fit and that it is fully home in the silencer and the clamp is a good fit.
The only other time I had this problem was when an exhaust valve seat was starting to fail and so it is worth checking the compression is still good and you don't have this problem.
Had backfiring on engine braking the B40, previous owner had made a poor fitting bracket when fitting a high level exhaust. Pipe could be pushed 2 millimeters more into head. Made a new bracket and smeared silicone on the exhaust pipe end. Used both ordinary and high temperature silicone on some other bikes to seal exhaust leaks. Though I thought silicone should not work in this application, it actually has worked for years. Didn't make disassembly harder for me.
Just remembered my first motorcycle back in -63. A 1930 Harley flathead. Left hand twist grip controlled ignition timing. When at speed, twisting back both throttle and ignition made a lot of lovely backfiring.
Thanks lads will have a look at the system...