throttle surges

[52DAVE]

My 52 cub was rebuilt in 2022. It is bored .20 over with high dome pistons and the crank is .10 under. The tractor runs great until about half throttle then it starts to surge. When under load I can very slowly increase throttle. If I get into a heaver spot where the governor needs to increase fuel it starts surging again and won't stop until I move the throttle back to half. I have checked the float and it is set right. Is there anything that it could be other than the gov spring or worn parts in the gov? I'm a little hesitant to pull the gov off sense I've never seen it done before and I don't want to mess it up. We have been considering going to the Barnyard Bash this year if I don't figure this out before then.

[Jim Becker]

The surging is likely something that can be corrected with adjustments. First adjust the linkage from the governor to the carburetor. Engine off, pull the pin from that link. Push the speed control lever fully forward. Hold the carburetor throttle fully open. Adjust the length of the rod so the pin just slides back in without moving the other parts.

If it still surges, look on the front side of the governor to locate the governor bumper spring holder. It will be held with a jam nut. Loosen the jam nut and turn the holder in some to eliminate the surging. If turning it several turns has no effect, unscrew it completely and clean/inspect the spring. Try again. If the spring is rusty/broken you need a new one.

[inairam]

The first thing is what Jim said. I would also look for air leaks at the carb/ manifold gasket.

What carb are you using? The 52 would have had an IH carb. With the work done did you also convert to a Zeinth carb? Some members have posted about fit issues on the older manifolds and the Zeinth and suggest using 2 gaskets to get a good seal.

Some members suggest spraying WD-40 or starting fluid at the gasket area with the engine running and listening if the RPMs change. If so there is a leak somewhere.

[Gary Dotson]

Years ago, when I had my 1st Cub, I rebuilt the engine, much the same as yours. After the rebuild, it always ran a little lean. With the increase in compression, it wanted a touch more fuel. Along with the basic adjustments Jim mentioned, you might consider switching to the next size main jet. A lean fuel / air mixture will absolutely cause surging.

[SONNY]

could be gov. BUT carb sounds lean.

[52DAVE]

I'm sure the tractor was all original when I bought it. From the IH carb to the 6 volt to the seat that is in far better than average condition for its age. (It still has 90% of the cloth and padding.)
I had already tried the bumper spring. It didn't help at all. I have also adjusted the linkage between the gov. and carb. by the book. I think I will order a new larger main jet if i don't find an air leak.
It seems a little strange that higher compression would need a bigger jet but I guess high dome pistons means less air fuel mixture going into the cylinder.

[inairam]

An air leak at the carb /manifold gasket will also create a lean mixture.

If it was my cub I would try replacing that gasket first.

[Glen]

Hi,
I sent you a PM, you get to it at the top of the page.

[Barnyard]

We have been considering going to the Barnyard Bash this year if I don't figure this out before then.

Dave, whether or not you figure it out, come on down to the Bash. You will be glad you did.

[52DAVE]

I want to come to the Bash if We can. The weekend before the Bash is the Holland Vintage Car Club show and cruse on Friday and the West Mi. Mustang Club meet on Saturday. I will be at both of those if the weather is nice. We will have to see how a few other things go this spring.

[Jim Becker]

. . . I had already tried the bumper spring. It didn't help at all. . . .

If all the other suggestions fail to fix it, turn the bumper spring in a little more.

[52DAVE]

Well it's definitely not running lean. I was dragging for about an hour with it Monday when it started to miss. I went back to my shop and checked the spark plugs after it cooled down. They were coal black as is the inside of the muffler. I cleaned them and checked the gap turned the idle screw in 1/2 turn. Tuesday it surged less when I dragged for about an hour and then it started missing again. I'm guessing the problem is a piece of carbon in one of the plugs. Sense then the weather has been too wet and cold to try again. I will have to play with the idle adjustment to get it set right when the weather improves.

[Eugene]

Double check, then triple check the ignition system.

I'd also compression test the engine and valve tappet adjustment.

[inairam]

Tuesday it surged less when I dragged for about an hour and then it started missing again.

I still think if it happens after an hour that it could be an air gap developing between the carb and the manifold.

[Gary Dotson]

On the idle screw, in is richer, out is leaner, in case you were not aware.