I have a 2002 Yamaha Big Bear 400 2x4. The thing ran like a beast until last winter I was living somewhere different and made the mistake of storing the bike outside uncovered during the snowy wet Ohio winter. (I know this was STUPID)
when I went to start it up this spring it wouldn't start so I replaced the battery. Started up fine after replacing the batter but it only has about 1/2 the power it used to, the motor sounds like it's rev'd out but you'r only going about 30 mph in 5th gear and when you try to go up a hill it really runs out of power.

So I started with replacing/.cleaning the items I thought could be the cause. I cleaned up the front breaks, made sure the pistons weren't stuck and there was no drag, there also seems to be no drag on the rear wheels when I have it jacked up with the bike in neutral. I then replaced the air filter and spark plug thinking they could be the culprit, but no luck, bike runs about the same. So I moved on to the carb, took it all apart, cleaning all the jets, floats, bowl, slide and other sections I could find, put it back together and in the bike, but still the same issue. The only improvement is now the bike will idle without the choke on.

So I could use some guidance on what else could rob the power from the bike like it has, it's a very noticable difference.

Thanks,
Jason

 

Make sure there is not a bird nest in the exhaust pipe! Also check the oil and make sure it doesn't have water in it (milky looking), which won't rob power but could screw up something. If the engine sounds like it is over-revving but not going anywhere the clutches could be slipping, but more than likely if they were not slipping before you parked it they wouldn't be slipping now. One other thing to check is water in the fuel or old fuel. I've read gas is good for up to 6 months.

 

I don't think there's anything in the exhaust because the girlfriend has rode it for about 8 or 10 hours worth of time with it in this condition and the air flow out of the exhaust is strong, I'll check on that though.

As far as the gas goes this is the the 3rd tank since this all started, I even ran 93 to hope it might help, but no luck.

I need to change the oil in it, I looked at the dip stick and it seemed fine, not milky at all, but it could use an oil change.

Would there be anything i could check or diagnostically tell if the clutches are slipping?

Any other ideas?

Thanks again,
Jason

 

The best way to tell if the clutches are slipping is when you give it gas real hard and the engine sounds like it is revving higher than normal and you're not going anywhere. It kind of sounds like the engine is revved up but it never fully engages. This ought to be obvious, though. The only technical way to check clutches is to pull them out and measure the thickness of each clutch plate, but I wouldn't go thru all of that if they don't sound like they are slipping. I'm just thinking out loud here: shaft drive, right? Maybe something in there is dragging. But you said it turns over freely on jack stands. Or never seen this happen before, but what if the engine valves were not opening all the way?