PC, valve lash probably but I would also be checking the compression.

 

The hard to turn over issue is most likely a groove has worn on the exhaust rocker arm where the decompression ball makes contact. With that groove the decompressor is basically non functioning. It is intermittent depending on what stroke, exhaust or compression, the engine ends up when the ignition is turned off.

The stalling and backfire through the intake is most likely a valve lash issue.

 

PC, Jungle and BPS, This sounds like what our '03 Ranger 500 4x4(carb) is doing, too. Acquired used with little background history in early Jan, '22 and has had this problem since. I pull the spark plug and cycle the engine with starter and replace the plug and it usually turns over and starts or a least gives an opportunity to start. The next time the start is attempted it "seizes" and a fresh batt. will not turn the engine over. I'm thinking the decompression system is faulty. Should I plan to replace the camshaft/decompressor and rocker arms? Can this be done (reasonably easily?) without removing the engine, altogether? When it starts and runs it seems to run well, no power issues or backfiring. Also, I think maybe a side issue is that it runs rich and initially fouled plugs with soot but has gotten better by changing out the fuel and trying to eliminate any water in the carb/ fuel tank. Air filter seems good/new. I have not yet replaced the inline fuel filter. (Many years ago, I was a motorcycle mechanic and raced motocross. This"seizing" reminded me of those big ol' BSA 500cc single 'crossers that used a compression release to "assist" starting.)

 

Should I plan to replace the camshaft/decompressor and rocker arms? Can this be done (reasonably easily?) without removing the engine, altogether?

All you have to replace is the exhaust rocker arm if it has a significant groove worn in it. Yes, it is done with the engine in place. All that is required is removing the valve cover and rocker arm assembly. Make sure to check/adjust valve clearance upon reassembly.

 

Addendum to my previous post. Hour meter reads 0573.1.

 

BPS, Which gaskets/seals/o-rings should I have on hand in case they need replacement? Anything "tricky" or peculiar I should watch for or be aware of? Thanks!

 

I'd change the valve cover gasket, it'll be the only one affected. You'll want to get the engine rotated to top dead center (TDC) before removing/installing the rocker arm assembly and you'll need to have it set there for the valve clearance adjustment. The service manual outlines the procedure for this.

 

Got it! In anticipation of taking on the problem, I got one of these [Camshaft Cam Rocker Arms Gasket Kit for Polaris Sportsman 500 2x4 4x4 1996-2012 | eBay] which is a bit of overkill, I think - but (being chinese-made) it wasn't too, too expensive... so I have the rocker arms, camshaft and decompressor and some odd & ends on hand, too. I've also gotten one of the PDF service manuals. Again, thanks very much!

 

Another (maybe dumb) question: How does one rotate the crankshaft / engine without the electric start... ie: How / where do I get access to the crank to rotate it to find TDC thru the (minuscule!) timing inspection hole?🤔🧐

 

Pull your clutch cover and rotate the primary clutch.

 

Thanks! I was hoping it would be easier, like waving my magic wand but ...

 

BPS was exactly right. I waved my magic wand and POOF! Pics of the damaged /worn exhaust valve rocker camshaft pad and the camshaft with decompressor ball. I'm not sure this is "done" but I really appreciate the help! I'll try to update further when the job is complete.

 

What's the main cause of this problem?

 

Just wear or the exhaust rocker arm was not hardened enough to resist wear from the ball. It also looks like the rocker surface is plated, if so maybe the plating was not sufficient to combat wear from the ball.