I have a Cub tractor in the shop with a Kohler Command 18 motor (hydraulic lifters) on it and it will run at full throttle for about 5 minutes and then start surging and finally stall. It always restarts but on subsequent runs, it starts surging a little earlier each time and stalls. The gas is fresh.

It's been doing this for a few seasons, but in the beginning, it would run for 1/2 hour, long enough to do the yard in sections.

The carb was cleaned and rebuilt with no effect. Air filter removed without effect.

It has a fuel bowl solenoid, fuel pump and filter. Oil is full.

It has a Briggs style carb, so I temporarily replaced the solenoid with a solid fuel bowl nut. No change. The fuel filter always maintains gas. The float is moving freely and the bowl doesn't run out of fuel, at least not when I remove the nut. That leaves the fuel pump itself.

I'm going to try hooking gas directly to the fuel pump to further eliminate the gas tank and filter.

It obviously involves a heated motor and surging, but would a fuel pump issue cause surging when it warms up?

Any suggestions?

Thanks.