Is this engine rotary or reed valved? . I assume it is rotary but have not had it apart to check because it runs so well. What basic mods (bolt on)have been done to better this engine/drivetrain.Mine is bone stock and now we cannot do pipes in VT now . Clutch would be a good start, yes?

the 583 engine is indeed a rotary valved engine. CLutching never hurts if done properly. I believe you can also put in the 501 rotary valve, and short intake boots. I know you can do that to the 670, but not sure about the 583. http://www.snowmobileworld-s2.com/forums/iB_html/non-cgi/emoticons/confused.gif Anyone?

The 583 is a good motor stock. Run it. I don't think it's cost effective to mess with it. If you want more power get a 670. I don't want to sound harsh but I've been there.

RVR, not harsh at all. I also have a 98 mxz670. Jetting alone has made the 670 engine rock for me!