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Posted by David Sherman on Thursday, August 21, 2008 at 04:01:36 :
In Reply to: 251 cid motor with serious issues posted by Ron on Wednesday, August 20, 2008 at 07:09:24 :
I'm with those who suspect valve timing. You didn't say if you've verified that it's correct. If all else fails, there's no substitute for pulling the head and the distributor cap, turning the engine by hand, and looking to see that the valves on each cylinder open and close when they should and the rotor points to the right terminal at the same time. I don't know any sure way to do it on a flathead without pulling the head. You could check the valve positions through the tappet covers, but since the spark plug hole isn't above the piston, it's hard to know for sure when the corresponding piston is up.
Generally, backfiring through the carb means that there's fire in a cylinder while its intake valve is open. This means either the spark or the intake valve is timed wrong, or the intake valve is stuck or is very leaky.
One experiment you could do prior to pulling the head again is disconnect spark plug wires one at a time and see if there's only one cylinder that causes a carb backfire, or if many of them do. If it's only one, I'd suspect a stuck or leaky valve on that cylinder. If spark in any cylinder will cause a carb backfire, it's more likely the timing chain jumped or the distributor is way off.