KX85 to 100cc

[Kneumeier]

I bought a used 2004 KX85. It came with a new 100CC cylinder and piston kit. Can I just change out the cylinder and piston, use the same head, valve and carberator? I need some direction. My son races the bike and needs a little more power and speed. Will this make the bike quicker? Thanks!
I've heard I have to grind off the valve assenbly. How do I do that and how much do I grind off?

 

[gwcrim]

In addition to the jug and piston, you need a 100cc head and powervalve. The stock parts can be modified to fit or you can buy new. I got a new head and ground the PV to fit.

 

[griffbones]

The 100 has quite a bit more low end, but less top end, this worked out well for my son riding trails and harescrambles type stuff, no MX.

The 85 & 100 use the same carb, but they use a different needle, and stock jetting is identicle except for the main being on step leaner. On my son's bike I had to go up one on the pilot jet to get it to idle decent, I really don't know why as most people seem to lean them, but it runs a good clean plug and I have not been able to find any vacuum leaks so everything should be good. On the needle I have leaned it out one clip position and may have to go another when it gets hot outside, and the main for now is the stock 138, but again that may get leaned out when it gets hot outside. As I said, so far we have a nice looking plug.