I'm working on a Kohler M20 (sorry, numbers wore off the tag). I've replaced the fuel pump and cleaned the carb. At this point the engine runs fine without the air cleaner tube (looks like a radiator hose)connected. If i connect it (and I've tried multiples), with the engine running at governor speed and I engage the deck, the engine starts to spit and sputter. I had another small engine friend over and we tried drilling out the jet as a last resort, but it made it run worse. This makes us believe it was running too rich to begin with and the little bit less air from putting the tube on messed everything up. I'm going to try another carb, but wanted to know if anyone had any other ideas at this point. Thanks for the help.

Check the oil level in the engine, may have fuel contamination. This would cause some fuel vapor to be drawn from the breather tube causing a rich running condition.

It does not take much to mess up the jets. One # bit to much can make it worse.
Dean

Thanks for the help. I ended up changing carb to a different used one and it seems to be running fine. I'm not sure what was going on with the other carb, but anyway.