Hey, this is my first post here and it comes due to an issue that's been bugging my atv for a while now. BTW, I own a 2003 Yamaha Bear tracker 250 (YFM250X).

So the issue I'm having is when the engine is reving at higher rpms. It will start and idle fine, and even seems to put around ok at really low rpms. But when I rev it up in neutral it will either cut out and almost die or sputter and kind of misfire and pop.

I initially just thought it was the carb so I rebuilt it with a complete rebuild kit and went ahead and ordered a new K&N air filter. So in the process of this, I noticed that the air box had a good bit of oil in it. I'd say 1/8 inch over the entire bottom of the air box.

After cleaning it up and installing the new filter, I tried it out. It started fine and idled pretty smooth. I was feeling pretty good about it until I reved it up a little and noticed the same symptoms. I then drove around for a couple of min and again experienced the same symptoms. I finally decided I should checked the oil and it was in fact a little full. I've yet to change the oil because I'm waiting on a new oil filter to come in the mail, but it made me wonder if that was the problem all along.

Assuming it was overfilled with oil by a lot and that it was ridden a lot as well what kind of damage would/could it have done and do my problems sound like they go with that? What should my next step be? I plan to change the oil and refill to the proper level and install a new oil filter. I'm also going to add a good dose of SeaFoam to the gas to help keep the carb a little cleaner from any oil that may have tried to get in there during my 5 min running and test drive. Is it possible if I do that it may return to normal or did I ruin my rings or seals and need to take it into the shop? TIA to anyone who replies!

EDIT: If I posted in the wrong forum please help advise me where to re-post.

 

I wonder if it might be related to the rev limiter. Your quad maythink it is in reverse and won't let the revs climb. Just a thought! you got it working yet??

 

Here's my status. After rebuilding the carb and changing the air filter, it started and ran at low rpms ok. However when I would rev it up if would backfire, sputter and bog down. Since oil was in the airbox, I figured that just doing the carb wouldn't fix it. Then I changed the oil and oil filter. The old oil filter was terrible. The outer screen was totally disintegrated on about half of it. Also, the area where it was still there had a few tiny metal shavings on it which can't be good.

After getting it all done, I warmed it up and adjusted the idle. Then I took it for a test drive. At first it was just like before...it had no power and when I tried to accelerate and increase the rpm's it would sputter and bog down. However, it was able to go up hills in 2nd gear as long as kept it at walking pace. I also noticed if i was going down hill, I could put it in 3rd or 4th and gun it and it would actually accelerate and get up to higher rpms. After a while it got to where it wasn't cutting out or bogging down and almost seemed to be back to normal. The only thing is that it definitely isn't as quick or responsive as it was.

I'm going to keep riding it when I can to see if it gradually improves. Also, planning on checking the compression. Maybe all it needs is new rings which I'm sure I can change myself. For anyone who knows, what is the compression supposed to be on my atv?

 

I just came across your post from NOV 2011 and was wondering if you ever got it resolved? I recently had the same thing happen to TWO of my machines: a 1996 Polaris Sportsman & a late 90's Honda Recon. I was hoping it was just related to the air filters, but that did not seem to fix your problem.