My 2003 z400 is breaking up at high RPM's. It sounds to me like a lean condition. Any suggestions are appreciated. How should I test and what should I look for? Thanks.

 

Define "breaking up". Popping/hesitation under acceleration is usually a rich condition. Have you done any recent mods to the bike, rejetted etc.? Popping under deceleration is usually a lean condition. Top end "break up" can be associated with detonation/knocking usually from running too low of an octane rating. Mine will break up/detonate above 8k rpms on anything less than 94 (12.1:1 compression w/ hotcams).

What are your mods and what is your jetting currently at? Without knowing more about the bike and the situation I would do the following: Check/change plug. Clean the air filter(a dirty filter can cause a rich condition all by itself). Clean the carb (concentrating on cleaning/probing the main jet). If none of these works and your plug reads good or rich then you can bump the octane up a bit to stave off detonation.

 

I'm sure this has also bean posted on before but I'll throw some of my own knowledge on here about plug reading.

For a plug check on modern 4 strokes the best method is a "plug chop". Used plugs give better results than brand new ones here. Warm the engine up to operating temps then run it full out for a long stretch. I run mine wide open in 4th for about 1/2 mile in the field behind my house. Without decelerating, grab the clutch and hit the kill switch then roll/coast it back in. Doing this gives you your top end/main jet reading. What you're looking for is the color of the base ring, not the porcelin. The base ring is the "top" of the threads. You want a nice tan/brown color. Lighter is lean, darker or black/sooty is rich. This is going to be the closest thing to the color of your piston. The porcelin insulator will show you if the spark plug is the right heat range and if there is any detonation or pinging occuring (this is in the form of tiny black or metalic specks on the porcelin. Use a magnifying glass)

 

WOW...That's a lot of info. I just purchased the bike used. The previous owner said that it has been sitting for a while. It is completely stock. When I get to the high rpm's it feels like there is a rev limiter. At low rpms' it is fine. I have the bike scheduled for a carb service with a local shop in 2 weeks (they are a bit behind). Hopefully that will do the trick.