Panhead backfiring / hard to start

Still wrestling with the pan. Early 60's pan in a rigid frame bobber. Stock distributer style single circuit breaker with manual advance. Recent build, 60 over, STD heads & valve seats recently dressed, don't know specs on the cam, Bendix / Zenith carb.
Hard to start cold but a short squirt of starting fluid usually does the trick. Very prone to backfiring through the carb. Initially the carb fires were exciting, now just irritating. Also VERY hard to start hot.

I'm trying to understand how this is happening. What is the combustion source for the backfire?

on the compression stroke and the power stroke the valves are closed.
with the spark retarded for starting, the spark should occur near TDC.
on the exhaust stroke the intake valve will be closed.
on the intake stroke the exhaust valve should be closed and the intake valve should be open.

I guess the question is: Is there sufficient overlap between exhaust closing and intake opening for the exhaust to ignite any fuel standing in the carb??

I'd appreciate any insight.

Thanks,

Dennis Mc.

Panhead backfiring / hard to start

Malx

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