Help please
 

1929 ford A pick up . mostly all stock (modern point plate) . driving along & big back fire . checked everything & can not find the problem . back fires occasionally thru carb , but has low power . things we have checked ….
1) points / timing
2) valves open / close
3) carb
4) distributor
5) moved spark lever different ways
6) it will run but not as before


checked these items several times . need new ideas please ……..

Re: Help please
 

The big bang could be a bad electrical connection, it will allow raw gas to exit cylinder and enter exhaust manifold then when connection comes back you get the bang, a weak engine could be a weak condenser or a weak coil,
might do a compression check

carry on nick c

Re: Help please
 

Check the little carbon dimple in the center of the distributor cap. I've personally seen two occasions were a car started running poorly, and after checking all the things you checked, it was found that the carbon dimple was cracked or missing. From now on, that will always be the first thing I check!

Re: Help please
 

Condenser can cause such things-----

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Also, could be fuel mixture too lean. ?

Re: Help please
 

It is running lean on gas, probably a blockage at the firewall or pencil filter in tank.. Start at the sediment bowl and work backwards to the gas tank. if you take a fuel line off you will most likely see the gas flow stop after a minute or so. You could verify by pulling out on the choke when the backfiring starts. Art

Re: Help please
 

good ideas ……….….. forgot compression was weird … #1/4 65# #2/3 50# thanks & I will peruse it .

Re: Help please
 

My father had a lot of old tractors. He would use an old riding lawn mower gas tank with a shutoff on it, disconnect the gas line at the carb, drain the carb , clean the carb filter if it had one, and substitute the mower gas tank. Run the tractor awhile, if issue was gone he knew he had a fuel issue.

Re: Help please
 

I'm guessing the head gasket is beginning to fail. When cyl 2 is on its power stroke cyl 3 is on its intake stroke and vice versa when cyl 2 is on intake cyl 3 is on power. If the hot gasses from either cylinder are crossing to the adjacent cyl the incoming air/fuel charge could be ignited. Resulting in a backfire through the carb.

Good luck,
Tom

Re: Help please
 

The head gaskets don't usually fail between 2 & 3, there's a lot of meat there.

Re: Help please
 

There was discussion in a thread here a while ago as to whether it is even possible for a backfire to happen through the carby. Some argued that a BACK fire would only be the explosion of unburnt gasses in the exhaust (ie, out of the back of the engine) and that a "cough" through the carby would be a FORWARD fire. I thought it an acedemic difference but it would at least be more accurate and avoid misunderstandings.
This post reminded me of it.

Re: Help please
 

Does it idle smooth or does it sound like one is missing ? Short one at a time to see if one is dead by listening to the RPM drop off. This is called cylinder balance test. Takes 1 ear or 2 and a screwdriver. Where it backfires is important: tailpipe or carb. If guessing is the preferred diagnostic path way based on stories of others who don't understand diagnostic procedures : go back to original points and plate in the dist. In the ideal world, Your good list would tell what test you did and what were the results and NOT just say Carb, Dist, Valves move,. By the way, even smelling the tailpipe is good information. What you learn along the way is more important than just getting it to run better. Good luck. There is pretty much a test for everything that is necessary to run.

Re: Help please
 

True that.

Interesting.
If we consider the fuel/air mix entering the cylinder as Forward flow wouldn't a reversal of that flow be considered a Back fire and an explosion in the exhaust, which flows the same direction as the exhaust, be considered a Forward fire? Or, in this discussion, were they looking at it from a perspective view? That is the intake (carb) is ahead of the exhaust and therefore Forward?
Personally, to avoid misunderstandings, when discussing these sorts of things I think the term backfire is appropriate but should be delineated as the OP said "back fires occasionally thru carb". No ambiguity there. At least in my mind.

Re: Help please
 

I will send a copy of these to my friend . thank you much …………. any new ideas appreciated

Re: Help please
 

Maybe we can call a firing through the carb a "front fire".


It would be rare, but a backfire could damage the exhaust or muffler baffle, and might cause an exhaust blockage. This will be noted by a smooth idle, but not revving up as it should, and the manifold vacuum will drop very low at open throttle. If you hold your hand at the tailpipe, you should feel a strong blast of exhaust when someone snaps the throttle open.


In the 70's and 80's I had to repair a few exhaust with double wall pipes, where the inner layer collapsed, and couldn't be seen blocking the exhaust flow.

Re: Help please
 

I would check for carbon tracks in the distributor cap. This has happened to me more than once. The tracks are hard to see, but will allow spark which is supposed to go to one cylinder to go to the wrong place.

Re: Help please
 

paul , we have checked & cleaned ……… thank you

Re: Help please
 

Certainly checking the conensor is a great idea, but I would certainly listen to Tom. His advise to me a while back was spot on - baffles had come loose in aftermarket muffler, causing blockage. Car ran great at idle, but no power and backfiring when under power.

Re: Help please
 

Might pull the instrument panel and check the tightness of the ignition switch wires.

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I would chande back to original points and condenser .