1996 Polaris Sportsman 500 severe misfire - Polaris ATV Forum
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post #1 of 3 (permalink) Old 08-13-2019, 04:38 PM Thread Starter
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Join Date: Feb 2015
Location: Holden, MA
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Unhappy 1996 Polaris Sportsman 500 severe misfire

Hello!
I have a 1996 Polaris Sportsman 500 that I rebuilt and now after a couple test drives, it ran excellent and was just about ready to put the plastic back on, but it suddenly developed a terrible mis-firing issue. It will start, but will only fire erratically about every half-second. Immediately dies if you bring it down to idle. It can be throttled up, but it will never run smooth. I took it in to let the experts figure it out. That should be easy, right? They put in a new plug and noticed that the air filter was flooded with fuel. They checked the cam timing and said it was good. When I got it back from them it actually ran OK long enough to drive it from the trailer into the garage. The next day it was back to its old ways. I noted that it had leaked a great deal more gasoline into the engine oil. When I went to put a new carb in it, the oil cooler was full to the top with fuel. I drained that, put in oil and put the new carb in it, thinking that the float wasn't closing. I'm not seeing fuel leaking into the engine oil any longer, but it still fires only sporadically. The spark coil and CDI box were not very expensive, so I figured they would be a good gamble. I lost that bet. Still runs like crap.

What now? I'm done with repair-by-crapshoot.
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post #2 of 3 (permalink) Old 08-14-2019, 06:31 AM
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Location: Illinois
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Check the fuel pump - if the vacuum side is leaking it will fill the engine with fuel and cause extremely rich running at all speeds.

Also, if you shut the fuel off when you park the vehicle, it will not overflow the carb or fill the crankcase with fuel. What you referred to as the oil cooler is the oil tank or reservoir.

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post #3 of 3 (permalink) Old Today, 04:20 PM Thread Starter
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Latebird, Thanks so much for the pointer about the fuel pump. Makes so much sense! I put a new fuel pump in it, being certain that it would run great. Sadly, it runs the same. Crapola!!!

I did learn one thing though - It is definitely running super rich. As a test, while running I shut off the fuel valve, and as it begins to run out of fuel, it perks up and runs great for a few moments before it dies off. This has a new ebay carb - I put one of these on my Magnum 425 and it worked miracles for that machine.

I'm back to scratching my head.
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