Quote:
Originally Posted by Kudzupatch
Backfire through the carb means fuel is igniting in the intake manifold. That typically means an intake valve was open during combustion.
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It could be this, or it could be the beginning of an intake valve and/or seat 'burning', and as the valve rotates (which is normal for a running engine) and the burned part of the valve matches up with the burned part of the seat, a pathway for ignited fuel/air mixture to enter the intake manifold explosively occurs.
If this happens too violently, or too many times, it's possible to bend the throttle shaft in the carburetor, causing even more problems.
Because your issue is sporadic, I'd put money on either a valve hanging up (i.e. getting stuck in the guide and not being able to close completely every time) or a burned valve and seat.
Nope, I highly doubt that a valve adjustment would do anything. If you have time, instead of running the valves I'd do a compression test and also a leak-down test.