kohler surging

[2 man crew]

I'm having a problem with a Kohler 20hp. When it's running at full throttle it surges and sputters. If I back the throttle down to half it runs fine. I replaced the fuel filter, air filter, spark plugs and cleaned the carb. It still does the same thing at full throttle. I can feather the choke to about half and get it to run ok at full throttle.
Has anyone had this problem? Or know what it might be?

[General Grounds]

:blob3: 2 man, you can move the govenoer spring, if it is a vertical shaft engine its almost right behind the muffler, use a pair of needle nose pliers and try moving up one hole and see, if not try backing down a hole or 2, tony

[khouse]

Clean and blow out the carb with compressed air. The main jet or passage is partially plugged. You don't have a fuel feed or governor problem since you can apply some choke to fix the problem.

[2 man crew]

Thanks for the help.
I went to clean the carb again and I noticed the gasket between the carb and engine block was worn. so I used a little gasket seal. Put the carb back on and it ran great. Should work fine untill I get a new gasket.