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Discussion Starter #1
Hi folks, I need some real help here. This darn bike has got me baffled. I have a 1998 Polaris Sportsman 500. Here is what's going on.

Out riding with my son doin some mudding and everything was working fine. Bike went through everything I threw at it. After about 2 hours of riding we went through a ditch of mud and water at a level just to the centerline of the tires. When I went to get out of the ditch I lost power at 3/4 to full throttle. Put the bike in low gear, 4 wheel drive and inched my way out at 1/2 throttle.

When we got home I pulled out my factory service manual, pulled the carb for a rebuild and performed every trouble shooting test possible. I also noticed the muffler intake pipe broke loose from the muffler so I welded a new fitting onto the muffler.

I pulled the carburator, tore it down completely, cleaned it to where you could eat off of it. Replaced the inlet needle, inlet needle seat and O-ring, needle jet, jet needle, main jet. Checked float height adjustment to 13.0mm. Inspected vacuum slide diaphram, spring and vacuum slide for proper operation and excessive wear (all in good shape). Removed the choke assembly, replaced the spring (it was rusty) and cleaned. I did all this because it was due to be rebuilt anyway (it wasn't touched since new).

After the carb rebuild was complete and re-installed I had the same issue, no improvement. The bike idols fine and doesn't miss a beat up to 1/2 throttle. I did some more trouble shooting to make sure the following have been eliminated from the picture:
- Fuel filter and fuel line ok
- Fuel tank vent line clear
- Carb vent lines clear
- Fuel pump ok
- Impulse line clear and tight, no leaks
- Inspected all intake air seals and connections for proper seal
- Air filter clean
- Speed limiter tested no issues
- Removed spark plug (was clean and gapped correctly)
- Checked ETC (electronic throttle control) and adjusted to specs

I hope someone could point me in the right direction because I am completely baffled.

Thanks for looking and helping!!:Eyecrazy:
 

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I read recently that the main Needle jet is the only important part when working above 1/2 throttle. Occam's razor states the simplest solution is usually the correct one so id aim for cleaning the rest of the bike since u were in deep mud (Electrical connections for example). sometimes a bad spark plug may not be able keep up with the ignition system as well!
 

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My 500 did the same thing I did everything you did with no luck. I took it to the dealer and he found a crack spark plug and it ran find after that. Yes I had just put a new one in it before I took it to the dealer. It runs find now total cost was 65 dollars to change plug. I felt pretty stupid but it runs great now.
 

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Discussion Starter #4
Thanks for the quick response guys. I pressure washed the bike yesterday after I wrote my plea for help and made sure all my electrical connections were tight and clean and fired it up after it dried and it did not help. I will check the spark plug idea, I think that is the only thing I didn't replace because it looked like new when I pulled it to inspect. I think I went after the less obvious things instead of the really simple ones.

allenpace - I know how you felt with your repair, I've been doin that a bunch lately! Thanks again for your responses and I'll let you know if it works. If it does, I'll buy you a couple of beers (after we are done riding) if we are ever riding in the same area of the country!!
 

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Discussion Starter #5
Well, I bought a brand new spark plug, gapped it to .6mm and fired it up. Same problem. I got to thinking about air and fuel mixture and just for laughs I pulled the air filter, installed the air box cover and checked to see if there was any difference, still the same. Then I thought maybe not enough fuel so I pulled the air box cover, fired it up and watched to see if the slide moved freely at different throttle positions and it looked to be in working order. So I decided I would take a look at the fuel side and maybe I wasn't getting enough fuel flow at higher rpms. I removed the fuel pump and dismantled it and cleaned it. I found the air side of the pump packed pretty good with really fine dirt and I thought for sure that was my problem. Reassembled the pump, installed it and tried again. Still the same, bogging down over 1/2 throttle.

I am almost bald from pulling my hair out trying to figure this thing out.

Is there a chance it could be something internal in the motor? Last week I pulled the valve cover and adjusted the valve clearance to specs. the exhaust were about .2mm more than what they should ahve been but the intakes were perfect.

I'm just fishing for any clues now. All suggestions will be accepted!!
 

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While you had the carb apart did you happen to turn it over and check the setting on the air screw? Are you getting any sort of back fire or pop? Or is it just a straight bog? I know you put a new plug in, but have you checked to see if the spark is strong enough to jump the gap? Just a few easy things off the top of my head. Let me know how you turn out.
 

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Discussion Starter #7
Hey imhuntingcrazy (excellent name and picture by the way, you look like Ted Nugents brother!! LOL) thanks for the response. I thought the same thing about the air screw and when I had the carb apart and rebuilt it I adjusted it to specs but have not tried to adjust it since re-installing. I read through my service manual thinking maybe I adjusted it wrong. According to the manual I would have a rough idol if this was not adjusted correctly so I checked it off as a non-issue. At this point it's worth a try to adjust it. I check the new plug when I put it in and I am getting a good blue spark from it. Today I also checked all hoses and air intakes for blockages and they were all clean and clear.

I've got my mechanic neighbor baffled and his buddy baffled who has owned two of these beasts in the past. I'm almost afraid to bring this to a Polaris dealer because I think I have tried everything they would try.

Frustrating. I need a ride fix, having withdrawls. Ahhhhhgg (that was me yelling!)
 

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Discussion Starter #9
Hey NolanGerke, thanks for the response. I checked the float height for proper height specs once I installed the new inlet needle and inlet needle seat.

I found another thread on this forum that had the same issues and the guy reported back that his cam lobes were shot. I kind of find it hard to believe that this would be my problem but I'm not ruling anything out at this point. Would you happen to know if there is any way to test this theory without removing the cam?

Thanks again for your help!
 

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Before you remove the cam...verify you aren't getting any popping in the air box, backfire. You should be able to take the valve cover off and peak in with a mirror to look at the cam. How many miles do you have on the machine? Cam ever been replaced? There was a time about two years ago when we were putting in a cam every couple of days. Now it's down to about one a month.
Oh and thanks...I do love the Nug. Hunting isn't a hobby. It's a lifestyle
 

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Discussion Starter #11
Thanks imhuntingcrazy. I am not getting any popping or backfiring and that's what bugs the hell out of me. At 1/2 throttle or less this motor runs great. Over half throttle boggs with no power. I studied my OEM repair manual last night and for some reason I kept going back to the electronic throttle control info and thought maybe there is something to do with this. But.. I went through the trouble shooting before and disconnected the black wire from the ETC box and nothing changed. I'm not sure about the total miles because the speedometer was replaced by the previous owner so my best guess is over 4000. I am not sure if the cam was replaced in the past. Something else I read up on last night about the cam is that it has the automatic decompressor component. Can it be something with this?

Man, I really appreciate your input on this. Thanks!
 

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I've never run into a problem like that with the decompressor. With over 4000 miles, if the cam hasn't already been replaced....i'd say it's probably time. Take a look at it. Pop the valve cover off and see if the lobe is turned down(will have flat spot) Most of the machines we put cam's in have began to backfire, but by that time they wont even get a 1/4 into the throttle and they dont move very well if at all.
 

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Discussion Starter #14
I'm back and wanted to let everyone know what I found. It is the cam. I pulled it out last night (out of town for work this past week and could not look at it until now) and the center lobe is trashed. What baffles me is for as little as the exhaust valves were opening I'm amazed this motor could actual turn over and run to half throttle. I hope everything that I went through will help someone else out there.

I will have to replace the exhaust rocker arm as well. These must have been inferior cams or something with the decompressor screwed these up because that is the only thing different I see on the lobe that is not on the intake lobes. The intake lobes look like they are brand new.

Anyone out there have a inexpensive cam I can buy? I'm going to get this bike up and running and sell. It's time to get myself into a more modern unit that I can ride more than I repair.
 

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Just wanted to say thank you to everyone who offered suggestions with my bike. I can honestly say I would have never expected the camshaft to be the problem. I found a brand new cam that includes all the parts for the auto decompressor and the exhaust rocker arm for $250.00 and some change shipped to my house. If anyone is looking for the best price on new Polaris parts go to www.partspitstop.com

Can't wait to tear up the trails again!
 

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I am having a similar problem but my issue shows up only after running for about 20 minutes. I start out with snappy throttle response and top speed of 45+, after it watms up top speed is 35 and throttle response is slow and nothing from about 2/3 to full throttle. This seems to be heat related. 97 Xplorer 500
 

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need help, same problem with 1999 sportsman, but cam is fine. cant find anything wrong. had a bad backfire and hasnt run right since. carb cleaned and rebuilt, valve seating is fine, but do have gas blowing back thru carb. runs fine at idle, but bogs--leans out at 1/2 throttle or more. i have checked everything. main need diaphram fine as well. exhaust sems fine as well. any suggestions, need help? thanks in advance
 

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Why do you have gas blowing back through the carb? If so you jumped timing if your valves arent bent. A backfire could cause a tooth to jump on the timing or cause a bent valve. Id do a compression test.
 

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i suspected the same thing. checked the timing--ok, compression ok so valves cant be bent, although i do suspect it is an engine problem, not the carb. what else could it be? maybe one of the pulser or vac lines?
 

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I'm having a similiar problem with an xplorer 500 with a new curve ball added in.

If I take the air box off the machine and run it completely open the thing takes off and flat out screams. If I put the air box back on it without a filter the things bogs and coughs and anything over half throttle. Pull the air box back off again and nail it and away you go.

When I got the thing it was consistently boggy at more than half throttle but it would run around at a lazy pace. I pulled the carb apart and saw that the vacuum diaphram was pretty wimpy looking so I replaced it with new. Now it will run better than ever without the air box, but bogs out with the air box on. I looked through the big tube into the airbox too and it's clear.

This thing has me stumped!
 
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