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Discussion Starter · #1 ·
I recently purchased a 2005 Yamaha bruin 350 4x4, I wasn't too crazy about it only because of the ultramatic transmission with no gears, mainly no low range\granny gear. I've heard though that these quads are a decent all around general purpose atv. Since the day I got it, it's had an intermittent vibration, but only when it's in forward gear and either being accelerated or when the engine is running against the gear, such as going down a steep hill from a higher speed and letting the whole way off the throttle. Sometimes it doesn't vibrate at all, other times it is so bad I have to be really easy on it because I'm worried something is going to come apart. I've jacked up the rear end and started it and put it in gear and revved it up and theres never a noticable vibration or wobble or anything. However, I recently was changing the rear brake shoes and noticed that the wheel bearings have quite a bit of play in them and I could move the axle up and down inside the rear end. I got online within 10 minutes of finding this play and ordered a set of new wheel bearings and seals. I have read numerous forums online that mention how notorious Yamaha quads are for having vibrations, specifically the bruin, moto4, big bear and similar 4x4 utility quads. I've read alot of comments about rear end bearings, this confused me because I thought rear ends only had wheel bearings in them. Where are these rear end bearings located and does anyone know what causes this vibration? The wheel bearing play is the only loose parts I've found. Thank you for any input!
 

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Location of a few bearings

Many things can cause a vibration. From aggressive tires on hard pack to a bad one way bearing to a wobbling front drive shaft to a bent rear axle. However replacing the axle bears is a good start
 

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Are you mis-reading vibrations with driveline slacking? It's common for an ATV to give clacking and somewhat cracking driveline noises during deceleration on the engine brake. This is especially apparent over bumpy or rough terrain. This is due to the tires losing then gaining traction. This causes unloading and loading of the drivetrain. This is going to cause noise. No way around it.

Replace your rear axle bearings. There is also bearings inside the rear differential final drive. You simply remove the differential from the swing arm to access them. Depending on model, you may have to split the differential to replace the bearings. Not a hard job.
Remember to replace the seals with the bearings.
 

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Discussion Starter · #4 ·
Update; rear wheel bearing and seal replacement went well, the old bearings literally fell into multiple pieces when I removed the axle. Unfortunately, the vibration is not only still there, but seems to have gotten worse, it is really bad if I ride a really bumpy dirt road and then go down a paved road. I have since found both sides of the front wheel bearings are also shot, and also learned about a ujoint on the front of the rear driveshaft. I have pulled the rubber boot forward on the swingarm and it doesn't seem too bad, but just to add something else to my list of eliminated possibilities, I have a new ujoint and yoke coming with the front wheel bearing kit. I have engaged the 4x4 while accelerating and there is a noticable improvement, but the quad seems to dislike 4x4 on dry pavement and walks back and forth. I'm very confused about this. I guess one positive is I'll have pretty much a whole new drivetrain when I finally solve this problem. Thank you for any input!
 

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A frozen/seized u- joint can vibrate like the tires have flat spots.
 
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