New engine - intermittent backfiring - stuck valve?

[SteveR.]

Y'all fixed it! Carb float level was off by 1/4" or more. Set it to 1" from the gasket surface and she runs nice and strong now, which is good for a new $4,000 engine. Thank you all for the help...the carb was running fine on the old engine 6 months ago...not sure how it was so far off. Thank you Sourdough.

Ya gotta love a car so simple you can remove the carb, clean it, check/set the float level, re-assemble and re-install all in 30 minutes or less. I think I was born a century too late.

[Ak Sourdough]

Glad you got it working. Have fun.

[Big hammer]

Thanks you for posting your fix! Now go for a drive, break in the motor, Enjoy!

[wwirz]

Great work. Glad you found out the cause and shared with the rest of us

[Gene F]

My most likely things: condensor, fuel line, try backing out the cable that goes into the distributor about 2/3 of a turn. Sometimes too much spring pressure makes that lower plate JUST BARELY touch. Especially when the engine heats up. I have had this happen.

[DHZIEMAN]

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Originally Posted by SteveR.

I have a new engine with maybe 80 miles on it, built by a reputable manufacturer. It has started intermittent backfiring "fits" and corresponding power loss after it gets warm. I don't think it is the distributor or ignition, timing is correct, point gap is .020. Wondering if it is a stuck valve, is that common or likely for a new engine? What is the easiest way to check?

I checked between the upper and lower distributor plates, don't think there is a short there. Distributor worked fine with the prior engine. Low RPM & low load it runs fine, but when hot, it will intermittently run OK, then go into a fit of lost power & backfiring. Enrichening the GAV might help slightly, but the problem remains.

It seems on our vintage A's, when something goes amuck, it just drives us nuts! I see U have tried some things. To me in reading your post, I would be taking a good look at gas flow from the fuel tank and then a vacuum leak at the intake manifold someplace. It is an easy thing to check the tank and fuel flow, and either it is the problem or it is not and you can move on to the carb and gasket leaks. Disconnect the gas line at the carb, turn on the petcock and see what you get, should be good flow. if the flow is bad, hopefully then blow air back in the gas line and see if flow imporves! Make sure you have a way to capture the gas in a can or something. Enriching the GAV gets you to the carb, and/or mainly the intake manifold, and the seal of the gasket also the one on the carb! Air (Vacuum leaks) can cause confusing havoc and backfiring. I use a spray can of starting fluid as a spray test to determine these kinds of leaks. If you spray someplace on a suspect gasket or hose and engine speed picks, you are zeroing in on the problem. Next, if all goes well with the starting fluid spray (no engine speed up), move on to a vacuum gauge. If you have access to a vacuum gauge, connect at the windshield wiper port on the intake manifold. At Idle you should see a STEADY, depending on your altitude, between 18 and 22 in Hg. The higher the altitude the lower the reading. There are google searches that will tell you what various vacuum reading indicate. Hope you find the problem!