just bought an 04 500 auto w/ 405 miles. used on farm, ride out to check cattle and put it up. (so was stold) was told that the dealer had rejetted for more low end power. this brought up thoughts of not sounding right, but think it was good price(3500.00). when get home and go for longer ride than test, notice a backfire in breather box/ out exhaust/ no powerhalf throttle and up when over 20 mph. check plug and it was black and a little gooey. clean air filter, still has orig. new plug and another ride good for 1/8 mile and check plug- black around metal edge/threads real white on center. remove carb-disassemble and clean thouroly.... reassemble another ride...same on plug-black outer white inner. remove main jet (140) and use numbered bits and go up to 145. still have sputter at 20mph+ same plug. jet up to 150-still sputter and plug turnedblack as in too much fuel . have heard of some atv'shaving a limp mode in cdi if belt gets too worn. (or something to that effect). any other ideas of possible problem?????(coil/stator????)

 

First off why would you have to rejet for more low end power? That's controlled by the cam grind and the ignition box (CDI). Someone is blowing smoke. The main jet is not gonna change anything at part throttle. The needle controls part throttle fuel flow, the jet controls fuel flow at WOT (Wide Open Throttle).

 

Yes bear 4570 you are correct.... (i'm a long time bowtie hotrodder)cam profile is 90%-95% engine performance when proper air/fuel mix is pressent and adequate exhaust flow, any how........found problem tonight!!! used a well running '99 wolverine as the donner and started frankinstiening parts onto the cat until i found what it needed to purrrrr. tried the carb first--same result then had a brainstorm, what about a week spark????? tried the ignition coil next and of all things they are identical(except the yamaha has longer plug wire) and what a differance it made!!!!!! so back with the orig. carb. and wow this thing jumps when you nail the gas literaly...front end up every time. when i put the air box cover back on it sputtered again(differently) i take the cover off and cat is a tiger again! if i ran it with the cover on and no seat it was 90% but put the seat on and it falls on its face. so i did what any go fast guy would do.........I cut the air horn off half way down 3/4 inch at a time untill it run good again. So off to the dealer to get a new coil and reassemble both machines,also i am going to measur the jet i drilled out and get one the proper size to match. can't wait to go ridding!.!.!

 

Good job![img]i/expressions/face-icon-small-smile.gif[/img]