This is a really strange problem. My 2001 polaris scrambler 4x4 runs great idles perfect, its fast WOT just flies for about a mile or so. Then it sputters backfires and dies. It starts back up sometimes right away sometimes after a bit. Sometimes it idles really high like a lean condition and i just idle back home because if i hit the gas it does the same thing. I have the polaris and the clymer manual. What I have done. 8 years ago new shock towers 4x4 didn't work magnets were bad had dealer trouble shoot and fix one side. Its always ran rich 8 years now. Choke never seemed to work figured out it was on a little cable too tight, Fixed it as per manual. I don't know why the previous owner had set the choke a little tight but it always ran good before a year ago it was just rich black smoke out there EXHAUST. Then it started doing this. New polaris float, needle and seat. 4 carb rebuild kits over the years i found around here. Carb is spotless clean, set the float level, 9/16 seam parallel. went to 2.5 turns out. I connected a clear line to the carb drain while running to make sure the fuel level in the bowl is at the top of the bowl. So everyone says camshaft so i replaced it and the rockers. Set the valve clearance again. The cam wasn't bad and didn't fix the problem. New cdi same thing. Unplugged the reverse limiter, reset the throttle safety. Still the same problem. New regulator. Same thing. Checked the fuel pump put 5 lbs vacuum holds forever, fuel comes out when you pull line off, I got a new one coming anyways, bypasses fuel pump but holding tank on handlebars and connecting hose direct to carb, new fuel filter. Vent line not plugged. Blow on it to fill float bowl. Got it to pop and die in the shop throttle it up for approx 1/2 mile of time. I did some WOT runs and the plug was white. I raised the needle lowered clip, nothing put it back to stock. Pulled the carb again drilled out one of my 4 pilot jets with jet drill a little bigger. Got further down the road. Oh new spark plug, air filter. The chain and cam gears look great didn't remove chain and measure but the tensioner is doing its job. I got a new stator thats next and going to pull the muffler someone might have decided to make it louder somehow so the choke needed to be on a bit. Still don't explain why all these years it ran great just rich now last year started popping and acting up. Checked and re-checked timing. When i pull the stator i will check the lower cam sprocket. I also bought a new clutch and Polaris belt. I check and re-check things and nothing changes its very strange that's why im thinking stator or electical anyways. I make it about to the same spot every test ride. Took it to my dads made it 1/2 mile on gravel road. Thought i would get lucky. Going to tighten the drive chain, defection about 9/16 dont affect problem. I am super **** retentive. Everything works on this atv anything that stops working gets fixed. But i'm frustrated with this problem. I might buy a top end kit and get the valves ground but i don't think that will help because it idles great and goes fast. Anyone have any suggestions. I can make a YouTube video if that would help.

thanks John

 

I have a 98 sportsman 500. That was popping just off idle. I found the timing chain was stretched and I pulled the head to check the non moveable chain guide, so with the head on the bench I pulled the rockers off to check the camshaft and the EXHAUST lobe was just about flat. I thought about buying another cam and rocker arm set. But i measured the old one from the scrambler and it was in speck, looked like new, so I put that set into the head. Set the valves now I'm waiting for new chain and sprockets.

I'm working on both machines at the same time. Waiting for parts for one work on the other. Frustrated at one work on the other.

thanks john

 

Welcome to the forums.

On the Scrambler, check for a kinked or clogged fuel tank vent line.

 

I used the vent line to pressurize the tank to force fuel into the carburetor. But i did try loosening the cap. Still the same thing.

 

I had a similar problem with an single cyl. moto-ski snowmobile tried everything ! long story short turned out to be a small piece of the fuel line chipped off the end of fuel line at carb end and was being pushed against the seat in carb on so under pressure from fuel pump was closing off hole going into carb seat making no fuel go into fuel bowl . blowing out backwards with air must of cleared it[ CLEANED CARB MULTIPLE TIMES] but fuel pressure on it closed it like a flap .once I finally found that little POS ran fine might be something to check it had me stumped for a while Good luck

 

So i bought a new Aftermarket stator and a Aftermarket fuel pump and installed both of them. And one of them fixed the problem. Also a second new spark plug. Just for good measure.
Its my wifes atv and we put 2 tanks of gas through it over the weekend and it ran great. I couldn't keep up with her.
It still runs rich. It smokes about the same as always. In the past i have put a 135 pilot in it but that didn't seem to help. Its kind of a triple wammy against this atv. 3 gallon tank, runs rich, and is fast so your in the throttle more. I'm thinking about doing a top end rebuild, new 20 over piston and getting the head done. We ride once or twice a year she loves the thing and rides it hard so what can i do.
my sportsman got the camshaft from the scrambler but i still had to buy an OEM Chain and sprockets for it. I did a top end on the sportsman 8 years ago but didn't replace the cam or chain and sprockets. Putting it back together right now.

thanks everyone.

 

Glad you hopefully got it fixed!

 

So again this year running rich. Machine is 100% stock so stock jetting should be good but its rich so I went from the 155 main to a 150 and from a 40 pilot to the 35 and dropped the needle 1 clip. Turned the pilot screw to 2 turns. I bought genuine polaris needle and seat and float. Anyway it's way better no more black smoke. But it's still rich. Its pretty fast probably 65 mph. Compression check is kind of worthless with the decompression hole. Anyways I was wondering if it could just need a top end rebuild. Bore out the cylinder 20 over and grind the valves. Seems to me when everything was new it ran just fine with stock jetting. Why do I need to lean it out everything to get it to not be so rich. I've did a couple of sportsman and a 325 magnum top end rebuild and they all ran great with stock jetting. It's just kind of strange it's a 2001. 17 years old and never been rebuilt and been beat on. The Clutches were shot. I think its probably just a little tired. Any thoughts.