M38A1 Engine Troubleshooting

[whyndman5]

I have a 1955 M38A1 and I cannot get it to run right. I bought it 4 months ago and it ran decent for a month and then something happened. It runs rich and backfires with very limited power. It does start up easily and idles. When gas is applied, it spudders and back fires. Here is what I have done so far:

• rebuilt/tuned carburetor
• installed new spark plugs and wires
• installed new coil
• installed new distributor cap/rotor/condenser and points/adjusted points
• cleaned fuel filter in tank

Thanks for your help which is needed because I do not know what to try next.

 

[DeepSeaTadpole]

Maybe get a non-contact temperature guage and monitor the exhaust manifold at each piston.
See how much they vary and see if it's consistent.

 

[NDT]

What carb are you running? Check for stuck open float due to trash stuck under needle.

 

[Tracer]

What carb are you running? Check for stuck open float due to trash stuck under needle.

Whyndman5, NDT is right. Check for crud between the needle and seat. Also install a fuel filter on the fuel line, this will keep your needle seat area clean. Also take a plastic mallet and tap the carburetor float bowl just to make sure the float isn't stuck. The filter in the picture fits the Carter YS950S carburetor on my 1953 M38A1 perfect. The number is Gulf GF 408. Just looked and there are some on e-bay. Keep us posted and Good Luck. PS. What carburetor is on your M38A1?

 

[whyndman5]

I have a 1955 M38A1 and I cannot get it to run right. I bought it 4 months ago and it ran decent for a month and then something happened. It runs rich and backfires with very limited power. It does start up easily and idles. When gas is applied, it spudders and back fires. Here is what I have done so far:

• rebuilt/tuned carburetor
• installed new spark plugs and wires
• installed new coil
• installed new distributor cap/rotor/condenser and points/adjusted points
• cleaned fuel filter in tank

Thanks for your help which is needed because I do not know what to try next.

I think my carburetor is a Carter YS 950. Here is the picture. I will try to check the needle and seat and ensure the float is not stuck. Thanks for any other suggestions. Much appreciated.

All the way,
Bill

 

[Tracer]

Whyndman5, is this the type distributor that is in your M38A1 Jeep? Is your M38A1 still 24V?

 

[whyndman5]

Whyndman5, is this the type distributor that is in your M38A1 Jeep? Is your M38A1 still 24V? View attachment 823025

Yes, 24 volt.

 

[whyndman5]

Whyndman5, is this the type distributor that is in your M38A1 Jeep? Is your M38A1 still 24V? View attachment 823025

yes, this is my distributor, 24 volt.

 

[Tracer]

whyndman5, my M38A1 had the same problem. It would idle nice, but if you applied the throttle it would backfire, run rough, and had no power. The fix on my Jeep in the end was easy, so I hope this works on yours. I did the same things you did. I had the carburetor rebuilt, new points and condenser, plugs and so on. After all this it ran better, then poorly again. The fuel filter wasn't the problem, but it's good insurance, install one any way. Your points gap should be set at .020 and spark plugs at .030. Make sure your distributors mechanical advance is lubed and moves freely, and make sure your points contact surface is clean. To me it sounds as though your engine is running retarded. I don't have a timing light for the Jeep, so when it comes to setting the timing, I start the engine and let it idle. Now with a box end wrench, loosen the hold down bolt on the bottom side of the distributor. Not to loose, just loose enough to where you can turn the distributor with a firm hand. Now with the engine running, and the hold down bolt a little loose, slowly turn the distributor clock wise and the RPMs should increase. When the RPMs start to decrease, turn the distributor back to the fastest idle and tighten down the lock bolt. Now when your turning the distributor and the rpms increase, the the distributor may hit the side of the engine block, if it does turn the distributor back till you can see daylight between the distributor body & engine block. Maybe .040 of daylight. I did this and my Jeep ran great. But in about 30 to 40 miles it started running poorly again. Problem was the little rub block on the points bedded in and the points closed. (cheap points) So I reset them and in 40 to 50 miles it did it again. So one more adjustment and some light oil on the little felt oiler on the points, and she is running well and staying that way now for months. This will get you through till you can get a timing light. (I'm still looking for one) Hope this isn't to confusing, and I hope I have helped addressed your problem. Good Luck, and keep us posted.

 

[whyndman5]

whyndman5, my M38A1 had the same problem. It would idle nice, but if you applied the throttle it would backfire, run rough, and had no power. The fix on my Jeep in the end was easy, so I hope this works on yours. I did the same things you did. I had the carburetor rebuilt, new points and condenser, plugs and so on. After all this it ran better, then poorly again. The fuel filter wasn't the problem, but it's good insurance, install one any way. Your points gap should be set at .020 and spark plugs at .030. Make sure your distributors mechanical advance is lubed and moves freely, and make sure your points contact surface is clean. To me it sounds as though your engine is running retarded. I don't have a timing light for the Jeep, so when it comes to setting the timing, I start the engine and let it idle. Now with a box end wrench, loosen the hold down bolt on the bottom side of the distributor. Not to loose, just loose enough to where you can turn the distributor with a firm hand. Now with the engine running, and the hold down bolt a little loose, slowly turn the distributor clock wise and the RPMs should increase. When the RPMs start to decrease, turn the distributor back to the fastest idle and tighten down the lock bolt. Now when your turning the distributor and the rpms increase, the the distributor may hit the side of the engine block, if it does turn the distributor back till you can see daylight between the distributor body & engine block. Maybe .040 of daylight. I did this and my Jeep ran great. But in about 30 to 40 miles it started running poorly again. Problem was the little rub block on the points bedded in and the points closed. (cheap points) So I reset them and in 40 to 50 miles it did it again. So one more adjustment and some light oil on the little felt oiler on the points, and she is running well and staying that way now for months. This will get you through till you can get a timing light. (I'm still looking for one) Hope this isn't to confusing, and I hope I have helped addressed your problem. Good Luck, and keep us posted.

Awesome game plan and I will try next week. Thanks for taking the time to help me. Much appreciated.

 

[GopherHill]

That method of setting the timing will get you by just fine. If you get ping on acceleration, retard the timing by trial until ping goes away. For a timing light adapter, use some short solid core ignition wire and an alligator clip to work with the waterproof ignition system.

 

[chawgleg22]

I have a 1955 M38A1 and I cannot get it to run right. I bought it 4 months ago and it ran decent for a month and then something happened. It runs rich and backfires with very limited power. It does start up easily and idles. When gas is applied, it spudders and back fires. Here is what I have done so far:

• rebuilt/tuned carburetor
• installed new spark plugs and wires
• installed new coil
• installed new distributor cap/rotor/condenser and points/adjusted points
• cleaned fuel filter in tank

Thanks for your help which is needed because I do not know what to try next.

Do you have the original fuel-pump on it? If you have a newer after-market fuel pump, that may be the problem. My M38A1 had a new after-market fuel pump and ran ruff. I had to install a pressure regulator between it and the carb. The new fuel pump was running at 8psi. The engine and YS-950 Carb need to have fuel at 4psi. That, or install a factory Rebuilt Dual Action Fuel Pump. I did eventually put a factory rebuilt dual action pump on my M38A1.

 

[cobra5]

If you don't have the special timing light or adapter for the 24v system, you can still time your engine using the following method. Get a standard timing light and a normal sparkplug wire. Cut off the ends of the normal spark plug wire. Remove the waterproof wire at the #1 spark plug and distributor cap. Install the normal spark plug wire by just inserting into the cap and the spark plug. Take your normal timing light and connect to the spark plug wire. Take the timing light cables and connect them a 12v power source. It can be an extra car battery or your personal car, it doesn't matter. You can now check your timing and make adjustments. I have used this setup on my vehicles and it works perfectly. Just remember to swap the spark plug wires back when your finished. Hope this trick helps you out.

 

[exhogflyer]

Have you checked valve clearance? Common problem on these old engines is the valve seat wear due to unleaded fuel. Another good upgrade is the Pertronix pointless ignitor plate............does away with points and the constant adjustment from wear. It wouldn't hurt to do a compression test to eliminate a mechanical problem.