The 2000 ZL's did not have power valves & are equipped with standard carb's. I think the 500 had 38mm. Check your carb boot, possible it has splits in it & it's sucking too much air. The Carb boots on those sleds were notorious for splitting, they made them a little too big for the distance between the carb & the air box which resulted in a wrinkle in them. Pull it off & check it, don't just look at the top. You might also check your oil pump adjustment.-Mezz