650ccMatt,
I don't mean to insult your intelligence but I thought I would bring this up since it happened to me.
I did the stage one upgrade, you know; K&N, DynoJet with Karl's 2" exhaust tip w/o spark arrestor. That was a wheelie'ing machine. I opted to go with 25" MudBugs on mine with the stock wheels. I got em installed, pulled out into the yard and nothing. No more wheelies, as hard as I tried. I figured it had to do with the minimal unsprung weight I had introduced into the equation. No biggee, I don't do wheelies that much anyway. Then, a couple of nights later, my bud who just got a new 400 Artic Cat that he thought was fast, decided we should race. It just so happened that we were on a tar road. I goosed it and damn near flipped over backwards. Come to find out, the MudBugs just don't grip on grass and dirt as good as the stock radials did. The tires spin instead of gripping and bringing the front up.
You're probably a bit smarter than I am but for a couple of days there, I was bummed out for no reason!