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1999 Polaris Sportsman 500 wont start


Paul Maple
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I have a 1999 Sportsman  500, it was running fine, than started to lose power in the winter, I adjusted the idle screw and away I went, than a few weeks ago it died, I have found that i am flooding the engine, compression was at 70PSI and due to the compression release mech. i assume i am good as it compared to the same PSI on a 2000 sportsman 500 i have. i cannot start the atv unless i go full throttle and it starts an runs for about 30 sec than dies, again it is flooded. i ordered a new carb off ebay as i could get one for around $40. put that in same issue, i have checked the timing, (good), valves out of adjustment i adjusted to .006 still same issue, i have found the exhaust lobe and rocker arm to have some wear, i have ordered new ones, but am looking for any other input in this area as well. would the exhaust cam and rocker arm cause the engine to flood? i have used a bore scope and the piston liner has good cross hatching still? any input would be great.

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  • Admin changed the title to 1999 Polaris Sportsman 500 wont start

What makes you think it’s flooded? Wet plug? Excessive fuel left in the head ? Just curious.

Valves should be adjusted to spec. A little wear is expected on the valves and rockers but would still be able to run. 

Have you tried to rebuild the original carburetor? I know aftermarket carbs are inexpensive but they don't work really well.

I would rebuild the carb and set the air fuel mixture screw as per the manual and go from there.  Then check ALL  air intakes for any restrictions like bees or rodent nests.

Check that the boot that connects the carburetor to the engine does not have a crack in it or a broken connection.

Check the fuel delivery system and make sure the vent at the top of the tank is clear and not building up pressure inside the tank forcing unnecessary fuel pressure through the fuel line.  I don’t think that bike has a fuel pump ? Make sure your fuel is clean.  

This should eliminate the fuel/carburetor problems. 

If the condition persists then you may have a break down in the ignition system and not a fuel problem.  Maybe get an ignition spark in line tester that will show you the spark while the bike is running. If the spark breaks down as the bike gets hot it could be a bad pick up coil or rectifier, cdi etc.  All can be tested with the exception of the cdi. 

 

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Thanks for the feedback, yes wet plug as well as when cracking fuel will drip out of the exhaust, I have a Manuel and completed test for the electrical system and just to be on the safe side, I replaced the CDI, voltage regulator, stator, coil, plug and wire. this is my daily driver, I replaced the carb, I have adjusted the fuel air mixture screw 2 5/8 out as well as tried to lean it, I raised the eclip on the jet to the top position to try to lean out the fuel even more, I have tested the spark with the plug out, its good. ( or seems to be). if I go full throttle, it will run for a few seconds than shuts down, (too much fuel) pull the plug and its wet. I have left the cap off the fuel tank,  fuel is clean. I agree on the valves and rockers, that is why I am posting it, there is excessive wear on the exhaust rocker arm and lobe for the cam, I can run my finger nail on it and there is a lip.

I agree on the aftermarket carb as well, but this issue started with the original carb and due to the price I chose to purchase one instead of rebuilding, I have done this on some of my other ATV/Dirt Bikes and they have worked well.

the one thing I haven't checked which im not sure on, I have confirmed the flywheel and cam timing marks line up, but am under the impression there is another key on the crankshaft and gear with the chain? I have not pulled the stator plate to verify this key is still good, but at TDC with the timing marks aligned, if I go slightly past the flywheel will automatically spin forward which indicates to me the piston is truly at TDC.

any more help would be greatly appreciated as I am not sure that I believe the exhaust wear on the rocker and cam will correct my issue, though I do know they need replaced...

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Really the only thing I can think of is the timing.  But even if the timing was off you would still get an ignition in the cylinder head burning off the fuel unless the exhaust stroke is pushing it out of the head and into the exhaust too fast before a full burn off? That’s a funny looking question! 

You’ve checked the spark is not breaking down after a few minutes of running ?

I've  seen some pick up coils out of spec with regard  to the gap at the flywheel so maybe it’s a shot in the dark to check? Second is the pick up coil failing after it heats up and that would show up on the in line spark tester after a few minutes of running. 

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ok, I purchase a inline spark tester and will see where that leads, I have talked to a Polaris Tech and he is thinking possibly the compression release mechanism on the Cam is failing??? I am not sure but have already purchased a new cam and rocker arms. this should be done this weekend and we will see. any more thoughts are greatly appreciated as I am kinda stumped on this one...………. no 100% sold on the cam issue as I do think it needs replaced but I think it should still be running.

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That wear and tear looks normal to me.  Make sure your not having ignition breakdown with the in line tester  and double check that gap on the pick up coil.  

Maybe the cdi is bad but thats not testable. If you have someone with another bike that’s similar in years you can swap em out to see. 

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well, I replaced the cam shaft, rocker arms, adjusted valves. same thing. excessive fuel. I took apart the old carb and inspected it, it looks really clean and all jets were clean, float doesn't stick. I put the inline spark tester on, looks great, great spark, no breakdown. fuel pump arrived yesterday and I will hopefully install tonight. I holding out hope, but not sure. I really don't understand why out of no where I would have an issue with too much fuel on two different carbs????? hoping for the fuel pump, but not sure how a simple mechanical fuel pump would cause over fueling, would not the float stop this from happening? I have adjusted the eclip up (reduce fuel) still same issue..... ill post the results of the fuel pump upon replacement.... any other ideas....

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well, I replaced the cam shaft, rocker arms, adjusted valves. same thing. excessive fuel. I took apart the old carb and inspected it, it looks really clean and all jets were clean, float doesn't stick. I put the inline spark tester on, looks great, great spark, no breakdown. fuel pump arrived yesterday and I will hopefully install tonight. I holding out hope, but not sure. I really don't understand why out of no where I would have an issue with too much fuel on two different carbs????? hoping for the fuel pump, but not sure how a simple mechanical fuel pump would cause over fueling, would not the float stop this from happening? I have adjusted the eclip up (reduce fuel) still same issue..... ill post the results of the fuel pump upon replacement.... any other ideas....


I would double check that the choke is adjusted correctly and the plunger isn't stuck. Check the pilot screw adjustment, should be 2 turns out from lightly seated. Test the needle and seat, should be able to hold 5 psi.
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