M38 back fire

[ROBCDN3]

Hi. All. I have a back fire problem with my buddies 1952 M38.
The jeep will start and idle fine but when you give it some gas and rev it up it back fires. The jeep ran fine last fall when he put it to bed. He runs Shell hi test fuel with no ethanol in it. He also installed a new condenser recently. The back fire was so bad it split the seam on his muffler(not a happy camper). Any ideas on what to look for to fix this muffler destroyer.
Thanks Rob.

[wesk]

Does the backfire occur while advancing the throttle or after you release the throttle and let here de-accelerate? Is there a single backfire each time you accelerate the engine or multiple backfires?

[4x4M38]

He's running premium gasoline 91 octane plus,
in an engine with 6.48 to 1 compression ratio?

Wow.

[wesk]

No wow at all. It works just fine. I mix 100 Low Lead Avgas with mogas and run it in my tractors and jeeps. Beats the $&% out of the corn oil gas crap. The 60 to 75 octane ratings on these old engines was not their maximum! It was their minimum ratings.

[4x4M38]

No, the wow is the price of premium gas !

Nice to be able to find no ethanol gas. When I finish this gas tank
I'm going in search for some. The ethanol not only results in lower
mileage, but causes much worse corrosion issues.

[eagle64]

Thats all I run in my mules, less carb and fuel pump problems. My step son is a pilot and instructor. those planes sit for months at a time with no ill effects from the fuel. Good to hear I can run it in my m38 (if I ever get that far in the restoration...... the only problem I have is when I buy it at the airport it ask you for your tail number and other aviation questions. and buying only 10 gallons at a time is a pretty small amount. I hate to see what it cost to fill even a small plane..
I'm interested in hearing what is causing the back fire. good luck figuring that out.

[oilleaker1]

A split muffler obviously indicates a buildup of combustable fuel in the muffler. So, if you lose ignition, fuel goes on by, and then a ignition source comes in at the wrong time and bang, or in this case, kaboom. Possible causes are late ignition, bad valve or stuck valve letting the ignition out of the cylinder into the exhaust. If it starts and idles fine, I'm a bit confused. So, where is this fuel coming from? John

[wesk]

That's why I asked when the back fire occurs.

[Bretto]

John, that reminds me of something we did when I was younger. My buddy and I would drive his 70's Nova around the streets making it backfire by turning the ignition on and off. Sometimes you would hit just right and POW! Scared the bajeebus out of people. We didn't have a clue what we might be doing to the exhaust system.

[ROBCDN3]

Hi. Rob here. He says that when he applies the gas he will get a hesitation and then the back fire. Thanks Rob.

[wesk]

Pull the distributor cap and check the rotor to make sure it rotates about 10 degrees in one direction and then springs back.

[ROBCDN3]

Thanks Wes. I will call him tomorrow and pass this on but it will be a week now before I get a reply. Rob.

[wesk]

A frozen or sticking centrifugal advance will cause the hesitation/stumble on acceleration resulting in an over rich condition. However I would still check the float setting and make sure you have no cracks in the throat of the carb. I am assuming the jeep is bone stock!

[Hawkshadow]

Following this thread with interest! What is the best way to lubricate those centrifugal weights in the distributor?

[wesk]

Manually during dis-assembly. Grease is too heavy, WD40 is too light. A heavier oil as used on the wick's in old distributors and mags is what to use.

This is what I use:


40 weight engine oil or heavier will work as well.