M38 Carburetor Surging Issue

[KHH37435]

I have what I'm calling a surging / sputtering issue with my M38. When I bought the vehicle it had an older carburetor on it. When first starting the engine I found I needed to have the choke pulled out for quite a long period of time during warm-up, and it idled at a very high rpm to stay running smoothly. Seems reasonable for a "cold" engine. Once the engine was fully warmed up, I could push the choke back in and it would continue to idle, but only if I had the idle speed set very high. If I tried to set the idle speed to a more reasonable lower speed the engine would sputter, surge from about to die to a fast idle, then eventually just die.

Since I wasn't familiar with the history of the old carburetor, I replaced the old carburetor with a new (rebuilt) one, but the problem remained the same. The new carburetor did provide better transition from idle to acceleration, and pulled strong during acceleration and at higher speeds, but the idle sputtering / surging remained. It got me to thinking it felt like the engine wasn't getting adequate fuel at idle, like a potential vacuum issue (or obstruction like dirt) was keeping the idle circuit from getting adequate fuel to keep running unless the idle speed was set high enough to put the carburetor operating on the primary circuit. That evaluation was based on my experiences with 4-barrel carburetors, not the 1-barrel Carters on the M38.

The other day I decided to do some experimenting and removed the vent line from the carburetor to the intake tube. I did that to verify that the vent connection into the intake tube wasn't plugged somehow (debris, etc.); I verified it wasn't plugged. On a whim I started the engine with the vent tube still unconnected, and the engine runs very smooth with no sputtering or surging, and I lowered the idle speed very low and still remained steady and smooth. I took it for a drive and the engine operates perfectly, with a slow smooth idle and excellent transition from idle to acceleration. It also now starts instantly when warm.

Any ideas on why the engine operates perfectly with the vent tube disconnected but not when it's connected as originally designed? FYI, I haven't verified what the fuel pressure at the carburetor is. Thanks.

Kenny

[wesk]

First a couple of assumptions have to be made.

1 - Stock M38
2 - Carb is a YS637S.
3 - By new you mean a rebuilt YS637S.
4 - You actually removed the vent tube by disconnecting both ends of the tube.

Since everything improved when the vent tube was removed the issue is obviously a plugged vent system. Have you inspected the fitting and jet in the crossover pipe to insure it is not plugged?


_________________
Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

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[KHH37435]

Thanks Wes. In response to your assumptions, all are correct with one exception. I should have included that info in my post.

I didn't actually remove the vent tube, but rather I disconnected it on one end and looked into it towards the carburetor. I could see that it is open to the carburetor. Also, I did inspect the crossover pipe and verified that the fitting and jet in the crossover pipe is not plugged. That was my first thought where I thought I might find a problem.

I'm assuming that the obstruction must be somewhere from the air cleaner to the entrance to the crossover tube, and I intend to look at that tomorrow. I'll verify what I find.

Kenny