I have a mystery problem, and I would love it if someone could help me solve it. I have a 2003 KFX400, it's been parked for about 2-3 years now. Before we parked it, it was running perfect. After 6 months or so, we tried to fire it up and it wasn't running right. It will start very well, and within 30 seconds to a minute it idles up and stays there. We've been trying to figure this out on our own. To date we've replaced the carb, the choke cable, the plug, and the fuel valve. Several hundred dollars later and we still have the problem. I called the dealership and they said to check the air filter, intake, and compression. They are going to charge me $80 an hour just to diagnose. I thought I would try one more time to solve the problem at home. Any thoughts?

 

I know this is a long shot but check the header to cylinder gasket for cracks, tightness and such may try just replacing it. After the exhaust heats up it may be letting air in leaning it out and idling it up. Also check for cracks on the intake boot by spraying a little starting fluid between the carb. and cylinder if it idles up there's a crack or worn gasket. That's all I can think of right now.

 

The only way a 4 stroke engine can gain RPM is with an air leak.
Finding the air leak is difficult sometimes.

The air leak could be in the carb slide. (by this I mean the cable might get pulled on and cause the slide to lift a bit.

The air leak could be in the carb boot. That little crack can let air in.

I had a Breeze carb that had an air leak in the enrichment circuit. When the choke was pulled open the plunger opened an air passage so the engine had a fast idle.

One of the best ways to find an air leak is with a can of carb cleaner. Spray it around the carb and see what you get.