even if they are both running in top form the KX100 should take the 80. I have a older RM100 that runs perfect and the kids up the street have 2 YZ 80's 02' and a 98' and both kids ride exstreamly well, and in a straight line drag my nephew can get about 3 1/2 lengths on either of them. but on the track they smoke my nephew but thats because he has no skills in MX

 

my son has one of each and races. he wins and prefers the yz 80. he tells me it is much faster on the track.

 

no cursing dave.

 

shut up dan