Backfire and gas in oil

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old46ford

Senior Member
What should I be looking for... after my car warms up, the engine begins to back fire. I drained the oil and it came with at least a quart extra, that being gas, not oil.

I am thinking fuel pump diaphragm? The fuel pump is original and the car has sat for at least 50 years.

Would a fuel issue also lead to backfire once warmed up?
 
Fuel pump diaphragm. The issue with the diaphragm could be the fuel you are using, ethanol in gas with eat up a lot of parts.
 
>>>Would a fuel issue also lead to backfire once warmed up?>>>

Define what you mean by backfire? Out the tailpipe or out the carb? Jack E/NJ
 
Sometimes the fuelpump diphragm works cold but once they heat up the cracks opens up leading to lean mixture and backfire.
And since you got a fair amount of gas in the oil it would be my prime suspect.
 
Another cause for "backfire" is a vacuum leak at or around the intake manifold. Check all connections, such as the wiper motor hose or the vacuum connection to the distributor. If it is a "pop-pop" sound, it could also be an valve that is sticky. A compression test will give you an answer, or a vacuum gauge is also a valuable tool. I agree with the others regarding the probability of failing fuel pump diaphragm and the effect of alcohol in the gas. A quart of dilution to the oil pan is serious if it is coolant. A quick check is to hold the dipstick above a flame. If the substance on the dipstick crackles, it is probably not gas, but coolant.
 
Thank you for all the advice. I definitely feel it's gas, due to the gas smell. I have just put new water pumps in and replaced the coolant. The coolant levels have never gone lower than the day I filled it and there is not sign of oil mixing.

It is a pop-pop sound that then quickly goes to a large back fire noise ... the engine has been sitting for some time, could be a valve sticking. However, I have been fooling with the getting the wipers working again. I have the hoses all hooked up. Before that I had a plug on the wiper hose connection at the carb. I didn't have this pop/backfire when the plug was on the carb. I do now with the wiper hoses installed. Hmm...
 
Terminology can get easily mixed up on the terms after fire and back fire. Back fire is when the engine hickups back through the carburetor and is generally caused by an overly lean fuel air mixture. After fire is when raw fuel gets into the exhaust system either from an overly rich mixture due to a fuel delivery problem or from a stuck/burned exhaust valve or valves.
 
It's definitely coming out exhaust and pretty loud once warmed up. What could cause this only once warm? Oh and took fuel pump apart. Diaphragm seems untorn. Need too look for another fuel in oil reason now.

I can get the popping to stop some by closing the choke plate more.

I am leaning towards the additional fuel from somewhere being the issue. Unburned fuel in exhaust?
 
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Take the carb off. Then take the airhorn off and fill the bowl with fuel. Let it sit for a time and see if gas leaks out. If it does, the power valve is not properly seated. I'm assuming you have a Holley carb.
 
I'm working on a 41 Ford for a customer, he had a Pertronix system and hadn't been able to start it for a long time. Dumped the Pertronix and got the engine running. When I checked the oil, it had gas in it. Put new oil in thinking it was probably from all the cranking he had done with out starting. Worked on several other problems. Checked the oil again and more gas. My first though was the fuel pump, but it was just fine. Did the test with the carb and found the power valve was leaking.
 
I installed a new fuel pump, and took the carburetor apart. Power valve was replaced about 2 years ago and seemed perfectly fine. I unscrewed it and reseated it... no leaks.

I started the car up and it ran beautifully...but I haven't run it long enough to see if there is any gas in the oil.

This brings me to my current problem... when adjusting the idle mixture screws. I turn them all the way in and there is no difference whatsoever in how the engine runs. It's like they aren't making any changes to the carb.

Any advice?
 
That would be a leak most likely. It is still getting fuel from somewhere with the idle screws turned in. Power valve, float set to high, could be several things. Might be worth letting one of the carb guys look it over.
 
Had the same problem recently on the driver side of the primary 97; idle jet adjustment had no effect. Against all the wisdom of this board, I had just installed adjustable main jets. I was so careful in opening the adjustable jets on both sides the exact same amount as I sought to find the optimum main jet setting. Never got it to run right and idle was a mess. So I took out the adjustable main jets and replaced with .045. Still running a bit rich, but idle is back to normal.

What I found was the adjustable jet on the driver side was way more open than the other side. I thought I started out with both bottomed out, but maybe be left one was misaligned with the main jet and felt bottomed out when it wasn't. Adjustable jets sounded like a great idea to avoid yanking the center carb umpteen times to get the optimum jet size. Got my money back on the adjustable jets.

None of this story helps you, except to confirm that if one barrel is getting too much fuel, the idle jet adjustment is swamped. Others have mentioned float setting and power valve. Ditto, but check your fuel pressure. Even if you have the float set perfectly, the float bowl can get too high if the fuel pressure overpowers the float needle valve. Anything over 3 PSI can cause this problem.
 
Seams like i have the same problem.
I've narrowed it down to cylinders 6 & 7
I have gas in the oil too but may have to do with all the testing I've done with bad distributors. (Ill change oil and keep an eye on that)
And I get it out of the carb if i get back on the gas after maintaining speed.
 
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