M38A1 hard o start when cold

[Ratdaddy]

My newly restored M38A1 now will not start (except with starter fluid) when cold or after it has been sitting for two or more hours. It starts fine if it has been run within the last hour or so and it runs great with plenty of power and acceleration once it does start. I checked the vacuum at the base of the carburetor and it reads 23. Timing is good. I thought maybe the fuel pump is weak and is not pumping fuel up to the carburetor after it sits a while but it runs good and smooth with plenty of acceleration and power. It has me really stumped. Any ideas out there?

 

[porkysplace]

Is the choke hooked up ?

 

[Ratdaddy]

Is the choke hooked up ?

Yes. Choke is hooked up and I try starting it with full choke.

 

[porkysplace]

Maybe a bad plunger in the accelerator pump .

 

[Ratdaddy]

Maybe a bad plunger in the accelerator pump .

Carburetor is a professionally rebuilt unit. Anyway, I took the accelerator pump apart and looked at it. Everything is brand new and the diaphragms, springs, needle valve, etc. are new.

 

[dmetalmiki]

Could be a weak spark, Those insulated leads tend to leak a bit when old. Check points gap and coil output via the king lead. (big fat spark? (jumps to ground?))' when craking each plug lead (off the plug) should have a good spark accross a 30 - 40 thou at least, 125 better!. to earth. check your plugs, are they new?
With full ckoke crank it Then pull the plugs to check for "wetness"). Our M38A1 was a pain to start untill we checked out and serviced the electrics.

 

[Ratdaddy]

Could be a weak spark, Those insulated leads tend to leak a bit when old. Check points gap and coil output via the king lead. (big fat spark? (jumps to ground?))' when craking each plug lead (off the plug) should have a good spark accross a 30 - 40 thou at least, 125 better!. to earth. check your plugs, are they new?
With full ckoke crank it Then pull the plugs to check for "wetness"). Our M38A1 was a pain to start untill we checked out and serviced the electrics.

All new plugs, new wires, new points, coil, condenser, rotor and cap. Everything on this Jeep has been rebuilt including engine. Only thing on it not new is the fuel pump which was a take-off.

 

[wilfreeman]

Mine does the same thing when it sits for weeks not being run. In pretty sure it is the fuel leaking back to the tank ( for mine anyway). I give it about 10 second with the choke on, push that in and then part the pedal until she starts. Have you tried shutting the fuel shut off valve before the fuel pump when you shut her down for the day? I'm too lazy (or forgetful - whichever) to do that on mine!

 

[RodUSMC1962]

We had an m38-a1 ambulance we had to choke the heck out of and pump the gas pedal lots, to get the thing to start when cold. The mites would need a good choke also, but not like the ambulance.

 

[Ratdaddy]

Mine does the same thing when it sits for weeks not being run. In pretty sure it is the fuel leaking back to the tank ( for mine anyway). I give it about 10 second with the choke on, push that in and then part the pedal until she starts. Have you tried shutting the fuel shut off valve before the fuel pump when you shut her down for the day? I'm too lazy (or forgetful - whichever) to do that on mine!

Yeah. I'm pretty sure it's because the gas runs back toward the gas tank. And I'm wondering if a weak fuel pump could be weak enough to not be able to pump the gas all the way back up to the carb but is strong enough when the engine is running to keep it running fine. Problem with that theory is that I would think if the fuel pump is weak, it would show up while driving it as it wouldn't have much power and acceleration would be slow. The fuel shutoff valve is frozen in the open position and won't turn. I've been totally unable to find a new replacement one anywhere. Even sites that advertise them do not have them.

 

[Ratdaddy]

We had an m38-a1 ambulance we had to choke the heck out of and pump the gas pedal lots, to get the thing to start when cold. The mites would need a good choke also, but not like the ambulance.

My M38A1 has the correct YS-950S carburetor on it which has a vacuum accelerator pump so pumping the gas pedal doesn't do anything. It also has all of the fording gear on it. Part of that fording system is a tube that runs from the breather crossover pipe to the top of the carburetor just behind the choke so I'm wondering if the fording valve on the crossover pipe is not working properly or leaking, then air could be going through that tube basically bypassing the choke. I think I'll try disconnecting that tube from the carb and temporarily install a plug and see if it starts that way. I'll let you know.

 

[wilfreeman]

If I recall correctly, the valve is just a tight tapered fit with packing at the top. You might try loosening the cap and tapping on it until it loosens.
The fuel pump cam lobe on mine is worn out, so even though I have a new civi pump, I think mine is not pumping enough to start quickly and limits my power. As you suggest, you might have a weak pump or worn lobe like mine. You can check fuel pressure at the carb inlet - should be 4.5 - 5#.

 

[Ratdaddy]

If I recall correctly, the valve is just a tight tapered fit with packing at the top. You might try loosening the cap and tapping on it until it loosens.
The fuel pump cam lobe on mine is worn out, so even though I have a new civi pump, I think mine is not pumping enough to start quickly and limits my power. As you suggest, you might have a weak pump or worn lobe like mine. You can check fuel pressure at the carb inlet - should be 4.5 - 5#.

Okay, thanks. I'll try that with the fuel shutoff valve. I'll also put a gage on the fuel pump and check that.

 

[John Smith]

I installed a primer pump to dump a little fuel into the top of the carburetor.....starts good now....

John Smith
Clyde, TX

 

[wilfreeman]

I tried to start mine yesterday (40°, after sitting for a month or so with the shutoff closed). Took a lot to get it started. So much for my 2¢ worth!

 

[John Smith]

My m38a1 had the fuel line from the tank to the pump replaced with a hose, It was 5/16 or maybe even larger, and the fuel pump took quite a while to suck up the fuel that had probably run back to the tank. After putting in a primer, as I said before, it would start right up cold. I intend to replace the fuel line to the original size and possibly remove the primer pump. Some early M37's had primers to assist starting....I used a primer similar to small aircraft primers, from a homebuilt aircraft....1/8 th inch lines from the fuel line to the pump to the upper intake of the carb.

John Smith
Clyde, TX