The Stovebolt Site Forums The Shop Area Engine Shop Backfire timing problem?

I wondered if someone could give me some advice on how to get rid of my backfire. On quick acceloration, I get backfire, sometimes from the carburator, and sometimes thru the exhaust pipe. Do I need to retard or advance it a little or is it a carb problem or other things? I have a 55.1 235 with oil pressure. I just had it rebuilt and put in a rebuilt distributor. I got it to line up the ball on the flywheel with the needle, and have been advancing it from there on the octane marks. Thanks - Scott

First suspect in backfire on throttle application is the accelerator pump in the carb. Did you rebuild the carb along with the engine rebuild? If not then the pump plunger may have dried out while you had it apart for the engine rebuild.

Old Blue,
Thanks for the tip. No, I did not rebuild the carb. Please explain how the dry plunger causes the backfire. Thanks - Scott

If the plunger dries out, then you don't get a good seal of the plunger in the bore nor the full shot of fuel into the venturi when you press on the accelerator. i.e. the fuel leaks back around the plunger instead of going into the venturi. Many of the plungers use what appears to be a piece of leather formed into a cup as the seal. When the leather dries out, it shrinks and never seems to go back to it's original size or flexibility.

Another problem could be a little sludge built up in the passage that feeds fuel from the bottom of the float bowl into the bottom of the accelerator pump well which could also be causing a fuel shortage.

When you quickly open the throttle, the intake manifold vacuum momentarily drops and the amount of fuel being pulled into the venturi drops correspondingly. The accelerator pump provides an extra shot of fuel to compensate for the loss of vacuum and prevents the mixture from leaning out. It's the lean mixture that causes the back fire.

(Sorry if the theory is old stuff to you!)

When I helped Teen48Flatbed overhaul his carb last spring, the little balls that go in several different places in the main body casting had been switched around. The ball that was supposed to be in the bottom of a small bore that fed fuel into the accelerator pump well (and serves as a check valve), had been switched with another one just slightly larger. End result was that the ball was jammed into the top of the bore rather than resting in the bottom and there was NO fuel getting to the accelerator pump. Ended up drilling a hole through the casting so I could drive ball out and then used JB Weld to fill the hole. The correct ball was made of aluminum instead of steel. That was just about the only way to tell the correct and incorrect balls apart. Only reference to the difference I found was in the factory service manual.

Olblue, Thanks This is a new thing to me. I am old enough that I should know this stuff, but only dabbled with the cars enough to get them running. I appreciate the step by step.

Metal Man, Thanks Yes I did disconnect the vac advance and plugged the line.

I have now tried both advancing and retarding. I think the retarding made it worse and the advance better, but nothing cured it completely.

I watch the vac advance when I stomp it, and it never pegs it out even nearly all the way, but it does move a little....less than 1/2 the way. I tested the diaphram in the vac advance for leaks, and it is OK. I have about 17 inches of vacuum at idle. The vacuum line is open. Thanks - Scott

The 261 in my 48 1.5 ton had a backfire problem. It was that the valves needed adjusted. they were to tight. Just a thought.... An old mechanic told me that was the" fix" They did years ago............