1929 model a back fire

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ROBERT JAM

Member
I have a 1929 closed cab truck that has started back fireing thu the carb. Carb has been rebuilt and is in good condition. It seems to back fire after it warms up. Plugs are good, gas line is clear. Could it be a coil or coil wire? Any other trouble shooting sugestions would be helpful. Thank you.
 
check your points! proper setting gap
 

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I agree with Gary that you need to check the points gap. While you are at it check to see that they are not burnt or pitted. Also check to see that the flat spring on the move able arm has not gotten weak causing your points not to close as they should. It should take a bit of effort to open them with your finger.

To check for an intake manifold leak spray starting fluid around the manifold mating surface while it is running. If you have a change in engine speed while doing so you have a leak.
 
As mentioned above could be point gap has changed that is it has become smaller.

50 years ago I could put a set of points in and 4000 miles later point gap would be fairly close to what I set it at.

Nowadays the point gap can reduce to almost nothing in under 100 miles, to the point where you get problems like you see.

The solution is to check and adjust gap several times like at 100, 200 and again at 300 miles until the gap stops changing.

What happens is that the rubbing block "work hardens" as it wears. Finally it gets to a point where it will change very little when you check.

Then you can go for a long time without any wear.

I have no idea why they can no longer make a rubbing block on points which will last longer!

Maybe some Bean Counter can chime in and tell us how many millionths of a cent they save on each set of points by using cheap materials!!
 
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I have new points and they are gapped correctly. That was my initial thought. Thanks for your response though.Any other ideas?
 
Cam lobe going flat, weak or broken valve springs, cross-fire in cap or body, Plug insulator going to ground intermittently, Bad intake valve guide or burned seat. Weak spring on ignition points.
 
ROBERT JAM said:
I have a 1929 closed cab truck that has started back fireing thu the carb. Carb has been rebuilt and is in good condition. It seems to back fire after it warms up. Plugs are good, gas line is clear. Could it be a coil or coil wire? Any other trouble shooting sugestions would be helpful. Thank you.
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You are running lean.

That can be the GAV or a combination of the GAV and Idle mixture adjust setting! Do some adjusting.

If no success, hook up a vacuum gauge to the intake manifold at the windshield wiper connection point. The vacuum gauge should hold steady with the engine idling and should be running around 18 to 23. ( a good engine) and these numbers depend on altitude. The key is steady reading. If it is bouncing around there is a valve issue.
 
It may be a dirt particle or a small flake of rust that has made its way through the fuel bowl and is partially blocking the bottom of the main jet causing the engine to run lean.
 
Do you have or can you get another good proven carb ? Lots of little things can be wrong with carb or dirty carb
 
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