Ok I'm a newbie here guy's so go easy on me. I bought a 03' KFX400 last Fall and am loving it. It's all stock with a few bolt ons. Question is it's running like it has water in the gas under hard riding (sputtering or cutting out). Any gear also. I talked to the guys up a JPMX and they have told me it was due to Vapor Locking and told me to run racing fuel in it. I'm running 93 in it now but they told me to try 100 or better. Has any of you heard of this problem or have it going on with yours? Thanks guys/gals!!

 

Sounds like a jetting problem to me. There is no way you need race gas for "bolt ons". What are your current jetting specs???

 

Yup, sounds like a jetting problem. Pull a spark plug out and check it, ,if it is white (and it may be if it still jetted stock) then it is running lean which means not enough fuel and too much air, if it is black then it is rich, meaning too much fuel and not enough air. If you are running lean and it sounds like you are then get it jetted ASAP, that is an easy way to trash your motor.

 

Does it do this at quater to half throttle, half throttle and up, or when your wide open only?

 

It happens at half and full throttle. The jetting is stock. When I said bolts I didn't mean exhaust or engine mods sorry. So this does not sound like Vapor Locking to anyone?

 

I know this may sound stupid but do you have to pull the fuel tank off just to get to the spark plug? If not what's the easiest way to it?

 

You have the tools in the tool kit under your seat to get the plug out w/o removing your tank. You use that long socket in there and the 17mm wrench. It takes a minute, but it'll come out. Also, when I first got my z, the air filter had so much oil in it, I could barely blow through it with 80psi. A couple dunks in the old gas tub does the trick for getting out some, but not all that thick gooey oil zuki puts on there. If the plug is black, it's probably the filter is too gummed up.

 

So the filter would cause this problem also? I clean the filter after each ride with mix gas from my wife's dirt bike.

 

The amount of oil in your filter can make a difference. Too much oil and it'll run rich, not enough it'll run lean. When I piped mine last week, it was lean till I put oil on my new filter, now it's just a touch on the rich side and I need to step the main jet down a size.

 

So when you guys say jetting is this something simple to do yourself?