Farmall Cub

I did a search about engine hunting and read that if the choke is pulled slightly and the engine smooths out that the carb needs to be adjusted. So, I tried that and when the choke is out a tad the engine smooths out. So I adjust the carb and there is no difference... I screwed the adjusting screw all the way in and set it at 1.5 turns and there was no difference. Then I tried 2 turns and still the same. Then I just let the engine run and ran the screw all the way in and the engine never died. With my Cub Cadets this would have stopped the engine. No matter where the set screw is placed, I still get the same hunting unless I slightly pull the choke out... I am stumped.

The main jet has gunk in it. It is the brass hex "nut" on the bottom side of the carburetor bowl.

I just went through the same thing here.

Lurker Carl wrote:The main jet has gunk in it. It is the brass hex "nut" on the bottom side of the carburetor bowl.

Not the bottom, it is on the side. The only 3/8 hex on the carburetor. A 3/8 box end wrench is the best as it fits only the jet on the carburetor.

AFAIK, the adjustment screw only controls the idle air circuit. Most tractors will still get enough air through the regular intake to run at idle, and the needle has no effect at anything but an idle.

Cubs with OEM IH carbs do not have any "high speed" adjustment for the carburetor mixture. High speed adjustment is achieved by resetting the float height slightly higher or lower as determined by the way the tractor is running. Normally it is not necessary.

Is your governor rod timed right to the carburetor?

Engine hunting is usually fuel flow related.

Does the engine start hunting immediately upon start up? If so the problem is probably carburetor related. If the problem shows up after a minute or so the problem is fuel flow.

Remove the plug from the bottom of the carburetor and check the fuel flow for 30 seconds or a bit longer.

Have you removed the carburetor? There is suppose to be a screen at the inlet to block debris.

Read your previous posts. Engine runs. Have you conducted a 100% tune up including compression tests?

The problem is resolved. I took the "plug" out the side of the carb and it was full of gunk! I mean, the oriface that runs through the center of the shaft was clogged and the center tube portion was clogged too. I dropped it in some carb cleaner, cleaned it out and ran air through it. I took some carburator cleaner and sprayed it in the idle adjusting screw and cleaned that too. Then I put it back together and fired her up. She ran better, with no hunting, but the engine sounded weak and sputtered. I ran her around the field and up and down the road with the deck on to create more load for the engine and after about 10 minutes or so she stopped sputtering and ran smooth. So thank you for the pointers. Now I also know how to adjust the high speed portion by adjusting the float. I didn't know that either. I appreciate all your help and suggestions.

And that was the brass hex "nut" that Carl was referring to. Suggest you read his Lurker Carl's Cub Carb Fixes, it is a great set of articles.

Bus Driver wrote:

Lurker Carl wrote:The main jet has gunk in it. It is the brass hex "nut" on the bottom side of the carburetor bowl.

Not the bottom, it is on the side. The only 3/8 hex on the carburetor. A 3/8 box end wrench is the best as it fits only the jet on the carburetor.

Carl said the bottom side of the Carb.

It is fortunate that the carburetor cleaner did the job in this instance. Fuel gum must have been the problem. And some of it apparently caused problems for a few additional minutes. Additional gum could have been in the bowl of the carburetor. In normal operation, the only way fuel leaves the carburetor bowl is by passing through the jet.
Frequently rust flakes will clog the jet and only mechanical cleaning (with a smooth fine wire) will dislodge those. Carburetor cleaner does not dissolve rust flakes. I know of no substance that will dissolve rust while leaving brass (the jet) undamaged.
Often I carry a 3/8 wrench and smooth tip cleaners when I am using the Cub. Occasionally they are needed.