Sudden onset backfiring thru carburetor...

[David Pollock]

further notes on addition of diesel fuel :   

I had a customer once who inadvertantly pumped five gallons of diesel into his 47 Dodge on which I had done a complete overhaul. Needless to say he was concerned.  It made lots of smoke at idle but not so much under load. Since it ran OK, I drove it 25 miles to my shop before doing anything about the problem.  I was surprised to find that the level of oil in the crankcase had risen by at least a quart which suggested to me that the elements of  diesel fuel which had not burned had washed down the cylinder walls into the crankcase. I reasoned further that the lubricating qualities of diesel fuel could well be applied to a gasoline engine which had top lube problems. Since then I have added small amounts of diesel to the fuel whenever a problem like a sticky valve arises, usually after a long period of idleness. It has worked every time without any ill effects.  Marvel Mystery Oil would probably work better I but I cannot buy it around here. I would not do this to an engine with a catalytic converter .

[John Hendricks]

Or dribble a little Marvel Mystery Oil down the carb intake as it is running at fast idle

[brucepine]

Is the addition of diesel fuel to gasoline in the gas tank recommended for older vehicles that are not fired-up for many months?   Or as a regular program for seldom driven vehicles?    (Perhaps this should be a new subject)

[David Pollock]

I think your problem is a sticking intake valve which not only disables its own cylinder but fouls up the mixture to every other cylinder but particularly the one next to it.  A quart of diesel fuel  added to about five gallons of gasoline in the tank makes an excellent top oil.  In the interim, hook a vacuum gauge onto your windshield wiper port at the engine and if a sticking intake valve is the problem, it will really make the gauge flutter. Then using the same vacuum hose, let the engine suck in some straight diesel. It will make lots of smoke but it should free up any sticky valves.

[12905]

Bryan:

In addition to what you have listed, backfiring can be timing - possible distributor drive problem, or timing gear/chain problem, or? lean mixture, or carbon track/crack in the distributor cap. A really remote possibility (with a 6 volt system) is cross-firing from one adjacent high tension lead to another.

Brian

[BryanD]

Well, I took my 31 PA out for a run today.? It hadn't been on the road for a few weeks.? It was hard to get started (not uncommon when its been sitting) but then warmed up and ran smoothly as usual.? I took it a few miles out and everything was fine, but then it suddenly lost power dramatically and started backfiring thru the carburetor.? I was able to turn around and limp home but it was running terribly? :(

I had planned to take the manifolds off for some long overdue repairs of the exhaust manifold.? I'm still going ahead with that, so troubleshooting the problem will need to wait until everything is back together (could be a month or so).

Any ideas on what could cause the sudden onset of backfiring?? I've always heard that backfiring thru the carb is invariably a timing problem.? Perhaps the mechanical advance in the distributor has failed?? Or perhaps I have a stuck intake valve?? Any suggestions appreciated.? Even though I won't get to setting things right for awhile, I'd like to know where to start? ;D