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Good Morning! I am presently on the 69th Glidden Tour in Ohio. Yesterday was the first day and part of the adventure was the hard starting of my 1940 120 when we stopped for more than 10 minutes. I've followed the stating directions in the manual, but that doesn't seem to solve the problem. Of course it happens when there are many people standing around commenting on how nice the car looks. Then, my embarrassment grows considerably with each revolution of the starter. One time it even backfired and blew the air cleaner off. I've got to be doing something wrong. Never had this problem with my other cars. Any help would be greatly appreciated. Thanks.

Carburetor backfire is usually caused by four possible problems:

1. The ignition timing fires the spark before the intake valve is closed. (The distributor timing is very far off or the wires to the spark plug are in the wrong order or arcing is occurring between wire or posts on the cap.) If the point gap closed up the ignition timing will be retarded.

I'd check the point gap first then ignition timing.

2. The cam timing is has the intake valve open at the wrong time. (The timing chain has slipped or jumped a tooth.)

3. The intake valve is leaking badly. (Cracked, burned, bent, or very far out of adjustment intake valve.)

4. The exhaust valve on one cylinder is not opening, (damaged valve train on the exhaust side with the intake working normally.)

Thanks for the quick comeback. These are things I'll have to check when I get home. At least it's running pretty well once it starts.

In addition to the list above, one more thing to easily check which caused a friend's car to also backfire andblow it's air filter off (and split it down the seam!) was a distributor rotor which was not a firm fit on the distributor shaft. NOS rotors usually had a small metal clip inside to insure a snug fit, recent NORS rotors that I've seen no longer have the clip and sometimes require a little shim to make them secure.

Thank you....I'll check it out tomorrow.

Too lean a fuel mixture will cause backfiring.

Maybe a vapro lock situation.

Check and see if there is fuel squirting out when you step on the gas.

I also never try to start a car with the air cleaner off as I have seen a backfire start a fire in the carb.

Jim

Most likely the fuel is evaporating from the carb or boiling over and flooding. This due to the modern fuel.

Next time it happensss try holding accelerator to the floor.

Block off any carb heat passages or restrict them.

Most likely the fuel is evaporating from the carb or boiling over and flooding. This due to the modern fuel.

That was my first thought as I had a Merc that did that constantly. Part of the problem with the Merc's carb was the needle didn't seat properly, so gas would drool into the float bowl, then overflow into the manifold. I've seen an engine so flooded that it backfired through the carb 3 or 4 times before the excess gas was blown out so that it could run.

From the pix I see on line, it looks like a 40 has a downdraft carb, so overflow would drool into the manifold, vs an updraft carb where overflow would drool onto the garage floor.