Are the secondaries coming open too quick creating a bog? Just thinking out loud. A lean bog will usually cause a backfire. Backing off the timing is worth a try. It's easy to bump it back up if need be.
Thats a good question on the secondaries, you wouldnt think so but I can certainly tie them shut. No backfire happening, but it does go off the AFR gauge lean, when I stomp on it.
