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  1. #1

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    Jun 2011
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    1990 wr500 rev problems!!!!

    I had gotten a 1990 wr500 it ran good the first day but it started only going around 10mph for about 20 min held wot as i was trying to get back to the ramp it all of a sudden started working great and planed right out, now after sitting all night at home it will not rev over 1/3 almost sounds like the carb is trying to spit back at me It starts and idles great but just will not get over 1/3. this is what ive done so far- rebuilt the carb (sbn38 twice) checked the reed valves, they looked new and not broke, i have power out of the stator brown wire(not sure how much i should have but it has power) cdi has power passing threw the orange wire. new plugs, checked the compression (141psi both cyl) checked the fuel valve(ok) cleaned the fuel tank put fresh fuel in, the lines seem ok they hold vacuum. im at a loss to what to do next. it has a small exuast leak that appears to come from around the drive shaft area under the manifold and i did roll it over three times (with some water in it) the day i got it. there is a link to the video to hear it run.


  2. #2
    Old Ski Insanity w/rpm500's Avatar
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    Aug 2005
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    the 500 only has two fresh air intakes..they are small compaired to later skis... the exhaust leak must get fixed to enable the engine to pull fresh air...

    the thermo switch on the head is more than likely what shut your rpms down..
    if the engine overheats from restricted cooling ( check your pisser..) it goes into limp mode (2000 rpm +/-400 )

    there is a test you can perform on it to see if its good..
    The thermo switch on a 500 is an open and close switch.(tested with a continuity meter)
    you can test this in the kitchen with boiling water,thermometer and a meter..
    once the switch reaches 90 deg C it closes and will not reopen until the temp drops to 83 deg C .(sorry youll have to find the F equivelent #'s)

    also if the switch is shorted it will stay closed all the time and stay in limp..
    its possible it is cracked and when you rolled it the 3 times, you got water into the unit and its now shorted, in 'closed' limp mode

  3. #3

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    Thanks, I just pulled the engine and found the exhaust leak I think that could be it, the switch seem to work. Is there a way to bypass it to se if that's it for sure?

  4. #4
    Old Ski Insanity w/rpm500's Avatar
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    I would think that as its an open and close switch you should be able to remove (clip) a wire and test...
    I have looked at the limited wiring dia. YAMAHA prints and dont see this being a gounded unit to have to have it in the circut to run (two wires)..I'd still run a thermo switch as a precautionary device..I also read this as being a normaly open circut except when it gets too hot..

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