Troubleshooting my FE

[DanEC]

Backfiring through the carb can also be a lean fuel mixture. But from what’s been posted, I don’t know why the carb would suddenly go lean unless the rebuilder changed the jetting or you do somehow have a vacuum leak. Also FEs are known to occasionally develop an intake gasket leak by sucking part of the lower gasket area into the runner and then it’s sucking air from the crankcase. Those are hard to diagnose other than just pulling the intake to inspect. You might be able to snake an inspection camera down the intake runners to look for any gasket protrusions into the port. You may get an oily combustion chamber too if it’s sucking oil in.

I’m still suspicious of timing though. If you can find someone with a dial back timing light and have them power time it at high enough rpm so that all the centrifugal advance is spun in. Did you receive any tuning instructions with it? I would guess you are looking for around 35/36 total advance (base timing and centrifugal).

I would also see if the accelerator pumps are responding immediately when you yank the throttle and there is a strong jet of gas in both primaries and secondaries. But be careful as you don’t want to be looking directly down a carbs throat when it backfires out the carb.

Good luck.