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19 Posts
Discussion Starter · #1 ·
Hey guys, and gals.

I am working on a 98 sportsman 500 4x4 and I am stumped so I figured I'd come ask yall for a little help with it.

This bike was brought into my shop NOT running. I diagnosed it as a fouled carb to start with.

I cleaned the carb & replaced the jets.

Bike cranked right up.

Problem I have now:

With the air filter hooked up, the motors bogs and stalls out at about 1/4 throttle when held there or anywhere else up to full throttle. But idles fine and runs fine up to 1/4 throttle.

Without the filter/box hooked up it throttles up just fine and runs good but lightly backfires coming back down, almost indiscernible backfire, kinda sounds like a two stroke coming down.

I have adjusted the air/idle everywhere between all the way in to all the way out without really effecting much change. I know factory is 1 1/2 turns out.

I was starting to think that I had either a bad cam or valve clearance probs,

but after thinking about it - I have some questions

  • The 98 models had a cam problem - the exhaust lobe would wear off - but if that were the case here -it wouldn't hardly crank much less run good or decentunder any circumstances - am I right about that or not?
  • Valve clearance issues? Maybe, but I am not cracking the lid just yet until I have assurance of that's where to go next.
  • I am guessing that I still have a carb issue and don't want to get side tracked - right or wrong?

When originally working on the carb, it was badly fouled. I Couldn't find a carb kit for it so I ended up buying a kit for a '00 sportsman, and only replaced the needle valve and the pilot jet - needle valve was the same and pilot jet was the same - everything else was different so I just cleaned up the rest really, really good.

Any ideas or further tests I need to perform to get a direction on which way to go.

Any help would be highly appreciated.

Other info:

Pull cranks just fine, first try cold with choke (good sign)

Pull cranks just fine, first try warm without choke (good sign)

Does not leak gas - Gets all the gas it wants - pump works fine and I changed the filter in line and cleaned the ones in the tank.

Oil and filter changed as well.

(elec start is dead - thats why I have to yank it)

12,692 Posts
Did you remove the mixture screw? If your not getting any change when you adjust it, the tiny o ring and/or washer in there may be bad or missing or they may be installed in the wrong order. Don't ask me how I know about this problem...its easily done no matter how many you've rebuilt.

If that's not it, you may have a leaky or pinched diaphragm in the top. A little motor oil rubbed on it and around the edges will soften it some. Here's the exploded view.

When you're sure you have the carb all right, the next step IMO would be to check the valves and cam.
Good luck and let us know what you find.

526 Posts
DDO we are fighting an issue on a Magnum of that era.

Look in the Polaris shop manual if you have one or can get one online to view.

Our 2002 manual (not with me) has a page that states what jets come into play at different RPM's. If it will set and idle from our limited experience the fuel/air is setting is OK.

Try spraying carb cleaner so as to make it spray in each direction through each jet. It sounds starved for air to look at where that may be occurring.

Starting with a dead machine calls for a brave or otherwise tech. :)
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