Thump, thump, thump mystery noise

[vbmike]

I've been hearing this thump, thump, thump noise when the sled is moving. Doesn't do it when just ideling. It only makes this noise occasionaly and usually when going slow. It sounds like it's coming from under the hood as opposed to the track but hard to say which side. I would think it's either a bearing or the chain. I've check all the bearings recently and they were in good shape. The chain has 9,500 miles on it. If it stretched, could it be popping when comming off the back side of the sproket? I went through the chain case 500 miles ago and all looked well. Any ideas?

 

[RobX-1]

I'd be inspecting your belt. Good chance it's beginning to delaminate and causing the noise.

 

[vbmike]

I thought about that but just put in on last year. However, it was probably the original replacement that came with the sled. It might have dry rot. I had it off the other day and it appeared okay but I'll take a closer look.

 

[sideshowBob]

I once had your described symptom on a long off trail ride on my 144" 2002 SRX. I was convinced the issue was caused by a serious chaincase problem so I terminated the outing and gently nursed it back home only to find out there was a 2 inch chunk of the underside of the belt missing. Doh!

 

[RAMSOMAIR]

Or it has a flat spot. Does it surge at very slow speed?

 

[74Nitro]

Burnt spot on belt. Just try another belt to confirm.

 

[Kenneth Rogers]

I would also put a few quid on it being the belt.

 

[vbmike]

I checked the belt this AM hoping for a simple fix. I don't see any issue with it. It does not appear to be delaminating and I don't see any burn or thin spots. I could try another belt but it does this very infrequent so it would be hard to know if this fixed it. Only 500 miles on this belt.

 

[kinger]

Jack up the rear and spin the track with the engine to see if you can replicate it. I had a square chunk of rubber flapping around where a stud ripped out one time.

 

[74Nitro]

I checked the belt this AM hoping for a simple fix. I don't see any issue with it. It does not appear to be delaminating and I don't see any burn or thin spots. I could try another belt but it does this very infrequent so it would be hard to know if this fixed it. Only 500 miles on this belt.

Bad spots can be hard to spot. It doesn't take much effort or time to install another belt and try.

 

[vbmike]

I know, I should try the other belt but if it was the belt wouldn't it do it continuously?

The track only has 500 miles and no studs. I did inspect the track and tunnel incase a stick got lodged in there. Everything looked good. I did discover that the drive shaft (track shaft) bearing was missing 1 set screw and the other was loose. Could the bearing be spinning on the shaft causing this noise? I removed the bearing to inspect it. I popped off the seal and cleaned out the grease. It had a little slop but otherwise seemed good. I think I'm going to replace it anyway. Is the Yamaha bearing recommended or is there something better?

 

[74Nitro]

I know, I should try the other belt but if it was the belt wouldn't it do it continuously?

The track only has 500 miles and no studs. I did inspect the track and tunnel incase a stick got lodged in there. Everything looked good. I did discover that the drive shaft (track shaft) bearing was missing 1 set screw and the other was loose. Could the bearing be spinning on the shaft causing this noise? I removed the bearing to inspect it. I popped off the seal and cleaned out the grease. It had a little slop but otherwise seemed good. I think I'm going to replace it anyway. Is the Yamaha bearing recommended or is there something better?

Because the faster you go, the larger the primary pulley becomes. Therefore, a burnt spot on the belt has its effects negated by the substantially larger sheave contact area.
I'm not saying for sure this is for sure the problem, but it sure sounds like it and I've seen it plenty of times over the years.

 

[vbmike]

I'd love for it to be the belt but I don't think it is. That being said, I'm going to give it a try because I have a new belt and I've heard that you are supposed to break them in before use. Good reason to throw it on a guess. The other reason I don't think it's the belt, the last time it happened it didn't want to stop so I went faster to make it stop but that didn't help.

What about by bearing theory in the previous post?

 

[STAIN]

I'd love for it to be the belt but I don't think it is. That being said, I'm going to give it a try because I have a new belt and I've heard that you are supposed to break them in before use. Good reason to throw it on a guess. The other reason I don't think it's the belt, the last time it happened it didn't want to stop so I went faster to make it stop but that didn't help.

What about by bearing theory in the previous post?

I have seen that left side bearing make the noise you describe if not properly locked down.
That said, try the belt first, it is easy and quick.

 

[radianguy]

I know you're saying it's under hood, but what rear suspension is under your sled? The reason I ask is I have an 08 Vector LTX GT with a mono suspension and going slow over stutters the transfer rod clunks. Just tossing that out there, sounds travel especially when you have a helmet on.