I have a 2000 kx500 with stock bore, it has a fmf fatty gold series pipe and it looks like someone changed the flywheel to maybe a heavier one since it was use for trails, the bike has good smooth power but tops out at about 80 to 85, I'm wondering how I can make the bike get up around 100 without rejetting or bigger piston,

Only way to increase your top end speed is to change gearing. To push 100 your going to need a smaller rear sprocket, 43-46T will probably get you in that speed range

What sprockets do you have?  You also need a top end pipe, try a desert gnarly, you wont get the top end you want with a fatty.  15/47 you should see close to 100 if your engine is in good shape.  reeds, jetting, you need more fuel on top.  you can burn up an engine fast running it wide open if you are not jetted right.  if your main jet is not a 168 or bigger you are not going to be happy.  Good luck, 100 is fun!

A "stock" kx 500 goes 126 mph with proper gearing. I'll see if I can find the video on my phone and post it on here, if not I'll try to remember to do it next time I'm on a computer.

You might think about cutting down the front fender.

Ill check what sprockets I have and let y'all know, I have hot rod crank in it as we'll, the topend only has about 25 hours on it,I know I'm crazy but I really want something really fast,

Right now I'm running a 14/49 sprockets

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