I have a ZR600 that has had a history of sticking valves. Last year I ran the Cat Oil and from what I understand, that is part of the problem. I switched to Spectro this year, had the cable update done, oil pump is set so I''m using a quart every 160-65 miles. I have noticed that when the sled is cold (has not been run in 12 hours) the valves move but they don''t "clink" back like everyone says they should. However, when the sled is hot (even if I just start it and let it run for 2-3 minutes) they "clink" back as easy as pie. Sled is running great (last year it wasn''t because a valve always seemed to be sticking). Is this difference between hot and cold "normal" or should I pull and clean them :wacko: . I have close to 400 miles on them. Any help is appreciated, thanks.