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1940 Chrysler 8 cylinder backfire, drivability


kenp
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I have a 1940 Chrysler traveler with the 8 cylinder engine, I purchased the car last fall. I get a backfire like noise through the intake? under acceleration, mostly hot. The carb, fuel pump were rebuilt over the winter. Has a new set of plugs, plug wires and distributor cap. Seems better but still noticeable. I set the timing at TDC.The car was restored and engine rebuilt about 10 years ago but was never driven.

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Could be a sticky intake valve, flatheads are prone to this if they sit around and a fresh rebuilt with tight fitting parts even more so. A compression test will tell the tale. I could be all you need is some Seafoam, MMM or other upper cylinder lube in the gas and oil and a little driving but do a compression test first. If you have a stuck valve and the lube doesn't fix it there are other ways to get at it than taking off the head, like going in the valve cover on the side of the engine and prying it down.

Edited by Rusty_OToole (see edit history)
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A boom out the exhaust generally leads me to ignition timing or exhaust valves.  Popping through the carb leads me to intake valves.

 

If you take the air filter assembly off the cab, you can have a real good listen down in the carb. As Rusty mentioned a stuck valve. A bent valve. When the piston comes up on the compression stroke, air/fuel may be pushed back out the intake valve. 
 

Make sure the popping noise is not coming from an exhaust manifold gasket. They sound similar. Yet the exhaust manifold gasket noise is sharper, closer to a popping sound. The intake noise more muffled. 
 

Agreed, a compression test will reveal what is likely the culprit. 

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Another possibility is a couple of plug leads got swapped, in other words, firing order wrong. It seems like a dumb mistake but I have made it and so have a lot of other people. The way the wires are laid out makes it easy to do on a Chrysler.

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23 minutes ago, Rusty_OToole said:

Another possibility is a couple of plug leads got swapped, in other words, firing order wrong.

 

This is a good suggestion.

My first thought was valves, I would check the firing order and if correct do a leak down test. I am thinking maybe a burnt valve.

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