i have had 3 xt660 now (still have 2 of them) Two with the kev mods one without never had any problems with any of them at low speeds never had any problems with them surging at all they run fine if you know how to ride them mine have anyway they are no sports bike but lots of fun. i have 4 other big bikes but still love the xt660. most important is the cush drive rubbers check for free play and the chain slack keep that adjusted right and you will have no real problems