VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1929-1932 Backfiring through carb

Hello Everyone,Well I finally got the new fuel line,fuel filter,cleaned out tank and rebuilt carb all in and car is finally running!!
Now the newly rebuilt carb is in perfect shape and working beautifully thanks to JYD!! I am still having a problem with backfiring through the carb which I have never had in the 5 years I have the car.Carb seems to be functioning perfectly,so,can anyone steer me to what the problem might be? I cannot detect any vacuum leaks,have installed new intake/exh gaskets,have not changed the timing,dwell,point or plug gap.Could it be as simple as adjustment in the air/fuel mixture or should I look elsewhere---Thank you all in advance!! A special hats off to JYD--carb is a showstopper Skip...

Maybe a valve clearance problem perhaps?

More info needed. Do you mean backfiring on rapid accel from idle or accel from cruising? J.D. is going to come back to you on this and he will want details.
In general the backfire problem is lean mixture, over advanced timing, or an intake valve holding open from way tight adjustment/ incomplete closing(broken valve spring,tight guides,carbon buildup on valve seat or?)
You might have been right on the edge of this problem before carb rebuild and J.D. adjusted to somewhat leaner- enough to take you over the edge.
I have experienced all of the above and face the problem again on the latest project.Diligent sleuthing is necessary - maybe no easy answer. Good luck! Dan

For the rest of the story, Jay has tried two different rebuilt carburetors and the backfiring problem persists on both, so the problem is definitely elsewhere.

Glad to hear that the carburetor is working beautifully.

Since the only other thing that changed other than the two other carbs was the exhaust & intake manifold gaskets.I am going to do as the Big dog suggested a couple weeks ago and cheak more carefully for a vacuum leak---then move on.
Thanks everyone

Did you check the distributor cap for cracks?

My 1931 had a back firing through the carb problem last year after it sat for the winter.
On the first start up I had the back firing. It was a stuck exhaust valve. A little oil and it was fixed. How long did the car sit while you were working on it?

exhaust valve will backfire through the tailpipe. intake valve will backfire through the carb.

Backfiring through the carb. is caused by firing when the intake valve is open. Could be caused by a broken valve spring , burnt valve or sticking valve - or distributor timing way off.

Agreement with the intake valve still open when ignition occurs. Might have jumped a tooth or a few on the cam gear or have a weak valve spring. My 32 engine did this after it was sitting for over 20+ years and had a sticking valve. Used some MMO in the oil and in the gas and ran it at idle each day for a few days and it cleared up and hasn't done it in 8 years.

Working on an AMC 258 ci six cylinder with back-firing through carb, although not consistent, sometimes it seemed like every revolution and than again it would go some time before a huge backfire from carb. Ending up pulling valve cover and low and behold, pushrod had jumped out on no 6 cylinder on exhaust side. Once reinstalled after making sure push rod was not bent, the back fire was completely gone. I agree with you that if an exhaust valve is closed completely and after a few rounds, there is enough of a gas situation that it will escape through intake vale at the right time, following intake back into carb and thence back fire. Been there and done that.

Jim.