I just picked up a 03 kfx400 and I'm having the same issue. It starts up and runs fine but I'd say 3/4 to WOT it cuts out, sounds like its not getting enough fuel maybe. The fuel Petcock only works in the PRI position, if I turn it to RES or ON it does not allow any fuel through. Is there a quick fix for that or should I just buy a new one?
The spark plug is in good condition and looks like it has a good blue spark.
I pulled the carb out only to find that most of the screws are stripped and I didn't want to take it apart and not be able to put it back together. I sprayed carb cleaner through the fuel inlet and the bowl drain holes but can't really clean it till I get new screws. Any info or tip will be greatly appreciated.
Also the guy that had it before me had the top end rebuilt, I've heard these bikes have issues with the timing chain, if the timing was off a little would it cause it to run crappy while WOT? Thanks again in advance.
#1 - Option 1 New petcock, option 2 mod the stock vacuum one you have to be manual/free flow. Option 3 Yamaha raptor petcock.
#2 the float bowl screws are a PITA. if they are stripped, try a good pair of vice grips. The head is soft enough metal to usually get a bite. If that doesn't work try slotting them with a dremel so you can use a flat screwdriver on them. At some point you're going to have to get that bowl off to clean and check the needle & float.
#3 If timing were off it would run crappy through the entire rpm range, not just cut out at higher throttle. The chains are known to wear quickly, you should check the tensioner and see how many clicks are left. Some people have taken a 1/4" to 3/8" off the spring to ease some of the tension, or you can switch for a Hayabusa manual cam chain tensioner.
For all of these, be sure you understand what you are taking apart and modifying and how to get it back together and adjusted correctly
