My brother owns a 97 sport.

Symptoms before blow up:
1: top of piston was melting
2: changed the jetting to be on the rich side and still ended up burning the piston.


What's been done:
1: New weisco piston installed
2: checked the compression and all is well
3: idles good, but problems start when you hit the gas, it won't rev up
4: When the belt is disconnected, the engine revs fine

My cousin believes the crank seal may be gone. what dose everyone think. I havn't heard it run so this is second hand information

 

oh, come on now, somebody's gotta have an idea of what's wrong

 

"4: When the belt is disconnected, the engine revs fine"

you mean the engine turn only when the belt is out?
there something wrong into the secondary,check for a cracked spring or
helix cam.

 

The piston melting is definitely a lean problem,it could be the crank seals or check the rubber boot between the carb and motor.Check under the boot they tend to crack in that area.Hope this helps.

 

we thought at first that it could be a clutching problem. After several adjustments and trying another setup up off of mycousins scrambler that we knew worked, there was still no difference. Our conclusion was that it wasn't in the clutch or tranny. Somethings not burning right in the motor. Before the melt down, We noticed that the top of the pison was flattening out. Kinda like it was heated and hammered. We replaced the piston and up the jetting, much richer then my sport. I think it's a 280. So I have a hard time thinking that the main jet is lean. I guess I should note that this is the third pison he's been through. The first was when the oil injection stopped working. We removed that and went to premix.

 

Check the carb boots and the reeds,head gasket,any where air could leak.

 

I would have to agree with ftwflh, sounds like an air leak. A coolant leak would do the same thing, but If it has been re-built that many times with new head gaskets, that is doubtful. I also think you could be right about the oil seals being bad for the crank. The oil seals in the bottom end are detrimental in the operation of a 2-stroke motor. I would also recommend the Polaris base gasket instead of aftermarket, I have heard it to be much better quality. If that is the 3rd piston, has the cylinder been honed each time to help seat the rings and is piston clearance (and ring ends) being measured each time??

 

it has been honed every time and is one size over stock bore. The compression is good when we test it. So I'm leaning toward a bottom end problem.

Well, since he also owns a Raptor, it's hard for him to put a lot of money into it. But it's also hard to leave it since we use it on the farm a lot

One more question, since it's a 97 and has been rebuilt so many times, would you suggest rebuiding from the bottom end up. I.E. bearings gaskets and seals

 

Yes, I believe that would be a good idea to re-build the bottom end. That is probably where the problem is anyway. Probably a worn oil seal letting air in. Just the fact that it is a 97 is enough reason to re-build the bottom end. I would go with all new bearings (3) new seals (2) new gaskets and a new rod end if it has play in it. If you are going to do it, do it right, and hopefully this would be the last time for a long time. I think I said earlier that the Polaris base gasket is better than aftermarket, so use it.