Backfire with Weber Carb

[pbishop]

I installed a Weber carb on the Model A and am VERY pleased with the added performance. One thing though, the car will 'backfire' from time to time. It seems that I can stop it from happening 'sometimes' by letting the car idle for a minute after driving, but not always. This cannot be good for the exhaust manifold gaskets or the muffler (or am I just worrying to much?).

Seems like a carb adjustment (leaning out?) might help, but I do not know were to start.

[Jim/GA]

Very common.

Try lowering the idle RPM as much as you can.

Or you can turn the fuel off and starve it until it dies. Just remember to turn the key off after.

Yes, sounds like it's a bit rich. Especially idle mixture. But if you have idle RPM too high, the idle mixture no longer enters into it. You are running on main jets. Don't go too lean on them.

[MikeK]

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The fact that sometimes idling for a minute before shutdown prevents it is a significant clue.

You may be running with insufficient ignition advance while driving. That causes the exhaust manifold and pipe/muffler to get extremely hot. After the ignition is switched off the spinning engine pumps raw un-burned fuel/air mix down the overly hot exhaust and.. BANG!