2000 Polaris Magnum compression issue

[Tyler Reed]

Came here for help, If it’s in the wrong place I’ll move. I’ve got a 2000 model Polaris Magnum 500 that runs strong as can be for about 25 mins until warm, then sputters and loses power. Took it to a shop and they say it’s low compression and they will have to swap the entire engine. Seems a bit much and costs more than the entire bike. Was wondering if anyone has seen this and what the fix was. When I hear compression I think of gaskets, not an entire engine. Seems like if it was that bad it wouldn’t have power cold. I’m at a loss. Any help is appreciated greatly. 

[Ajmboy]

You came to the right place! So low compression is usually piston rings or the cylinder head. Does it smoke at all? Is it burning oil? I’m assuming it’s still at the shop?

[Frank Angerano]

Welcome to Quadcrazy @Tyler Reed ! As @Ajmboy said you came to the right place! Any repair shop that recommends you need to swap out the engine based on your symptoms is a shop I would never return to my friend!  So what i would do is start With the basic testing first.  

Does this happen during all riding or in damp/wet conditions? 

Go get yourself a cheap compression test kit from Amazon or anyplace.  Take a compression reading while its ice cold and then a reading after it starts to act up.   Compare your readings and see whats what.   
Secondly I would locate the regulator/rectifier and see if its over heating after the atv acts up? It's cheap enough to replace if suspected.    
Im not sure if your machine is water cooled  but is the cooling fan coming on? Could be getting super hot and causing the problem. Don't trust temperature lights etc. Visual verification of the fan running is key. 

Next I would check your air box and make sure everything is ok inside, filter good, no overload of gas inside the box or water? 

Lastly is making sure your fuel is flowing properly to the engine weather it be some sh** at the bottom of the tank or maybe a condition where this happens when you get to a certain fuel level in the tank ? And we have been talking about vapor lock by maybe a bad gas cap? 
 

Start with this and see how you make out.  
 

 

[MarkinAR]

If it runs strong for 25 minutes, even if it's blowing smoke, I don't see engine replacement needed.  Even if rings are completely shot there's rarely any variation between perceived power cold and hot.  I'm leaning more towards what Frank mentioned above; fuel starvation or some component getting hot like coil, regulator, etc.  I'm willing to bet it's a pretty simple answer if you can do some basic troubleshooting.

[Mech]

I'm not familiar with polaris at all, and wouldn't know if it's two-stroke or four, but if it is a four-stroke...Check the valve clearances. They might be closing up when it's hot causing the valves not to close properly.

[green53]

Did you ever get the machine working ok?