Actually, anytime you increase the air flow into the engine, two stroke or four stroke, you must increase the amount of fuel also (jetting) or the mixture will be lean. If you don't you aren't developing any more hp at all. The air has to mix with something. That's of course if the jetting was correct in the first place. Doesn't matter which engine. I've found the 4-strokes to be actually a little bit touchier than 2-strokes. When I put the K&N on a SP500 I could not even get it to run past 1/3 throttle untill I changed the jetting. If you want to check for lean, just apply the choke slightly while running and see if performance improves or gets worse. That should tell the story. The duct tape trick also works because you are decreasing air flow into the engine. I can't even think about running with the airbox lid off. It's way too lean then. Many don't think that they need to adjust mixture with a 4-stroke like they do a 2-stroke, but that is not correct. You must read the plug the same way to check your mixture setups at various throttle settings.

<A HREF="http://www.godigital-design.com/schultzmotorsports">Schultz Motorsports</A>

 

Tomcat, I was talking to my dealer yesterday, and he mentioned that the SP500 would not run quite right with a regular K&N filter. He said that the carb requires a slight vaccume. He also said that K&N makes a filter that will efectively work like a stock filter. I would think that an easy way to check this therory would be to install the stock filter, and check the action. Then, install the K&N and try it again. I have not tried it, but it is logical. Dave

 

Dave, someone from DynoJet told me the same thing, about the vacume. He was talking about my Scrambler 500 though. It has a different carb the Sportsman, but I felt an increase in power with the K&N.

After I installed the K&N I didn't re-jet and still runs better than stock. It just depends how it came jetted from the factory and if the dealer changed it any for your altitude or temps.