Originally Posted by carbking
I know this is a thread about a Rochester B, and I think everyone knows my opinion about them wink But not sure that is the culprit here. Wouldn't want the B to get unwarranted abuse wink

However, a backfire through the carburetor strong enough to blow off the air cleaner is virtually never the fault of the carburetor. Virtually always valve issues or ignition. Locate top dead center, then make your timing mark, and time according to specifications.

Unless you have done one recently, a compression test would be in order to check for a sticky or burned intake valve.

If you think about it; you get a backfire through the carburetor WHEN there is fire in a cylinder AND an intake valve is open.

Jon.
Agree completely. Check your firing order vs dizzy cap wiring. Check Timing as said, ASAP. I hope you don't have one of those organ grinder monkey hat air cleaners. If it idles well, I wonder if that means valves are OK? Not sure. Is there a potato or rat stuck in the exhaust pipe?
Had to close the choke..hmmmmmmmm? Running lean or it thinks it's running lean. Runs good, gets carb rebuilt, runs bad. Provided you are not forgetting something else you did, no matter how slight, it points to the carb. But symptoms aren't right, as Jon says.
I say you took the dizzy out and didn't put back correctly. smile

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn