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  1. #1

    1997 SLT780 Running Rough

    I'm new to the forum trying to see if you guys have any answers for me. I have a '97 SLT780 compression is 140-140-135 I've replaced the following:

    -reed valves
    -rebuilt carbs
    -rebuilt jet pump
    -new fuel lines
    -spark plugs

    popoff is set at 20psi on every carb.

    Problem is it runs really rough, like only off 2 cylinders or something. So I thought it may not be getting fuel like it should, so I checked the pulser and turned the motor over and it just started spitting out fuel. So I did it until all the fuel was gone and it was actually pulsing again.
    Anyways, I thought it may need the fuel pump rebuilt so I have a rebuild kit coming in tomorrow for the "pentagon"
    I really have no idea what else would make the lower engine fill up with fuel since the reeds are brand new, some of the old reeds had over 1/16th space and weren't seating properly, and yes the engine was drained of any fuel from the old reeds, so the new fuel in the lower engine has happened since all the new parts.

    Sorry for such a long first post but I've been on "other forums" and nobody seems to know what's going on and I need some answers or at least I'd like to know if I'm on the right path.

    Thanks


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Welcome to the Hulk

    Polaris Fuji (blue) engine

    Is the fuel tank building up excess air pressure?
    The tank venting system is supposed to limit tank pressure to under 2 PSI. If the check valve sticks closed, excess pressure can develop.

    Did you clean/service the fuel selector valve when you did the other stuff?
    Are you turning that valve OFF when not using the engine?

    Have you confirmed strong spark on all three cylinders?

    Correct spark plugs?

    Trimmed the plug wires 1/4 inch and re-attached plug caps?

    Ohm tested plug wires and ignition coils?

  3. #3
    Quote Originally Posted by K447 View Post
    Welcome to the Hulk

    Polaris Fuji (blue) engine

    Is the fuel tank building up excess air pressure?
    The tank venting system is supposed to limit tank pressure to under 2 PSI. If the check valve sticks closed, excess pressure can develop.

    Did you clean/service the fuel selector valve when you did the other stuff?
    Are you turning that valve OFF when not using the engine?

    Have you confirmed strong spark on all three cylinders?

    Correct spark plugs?

    Trimmed the plug wires 1/4 inch and re-attached plug caps?

    Ohm tested plug wires and ignition coils?
    spark is great, already trimmed back wires, & correct plugs

    I feel it is a fuel supply issue, FedEx just delivered parts so after I close I'm going home to rebuild the fuel pump, I'll also check the tank and anything fuel related.

    Also, what should the head bolt torque specs be?

    Thanks for the info very helpful so far.

    Maybe I'll have this thing running properly by the weekend!

  4. #4
    blegan's Avatar
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    cyl. heads should be torqued to 18 ft. lbs.

  5. #5
    Quote Originally Posted by blegan View Post
    cyl. heads should be torqued to 18 ft. lbs.
    Thanks for that blegan, I removed the head to see if there was any damage to the piston or cylinder wall. I may try to use the old gaskets temporary if it brings back the compression. If not I have new gaskets coming in tomorrow. It depends how much I accomplish with the fuel pump rebuild and installing everything else.

  6. #6
    So I got the fuel pump built and installed and ran all new hoses. Fired it up and it still has a hesitation in the idle, then midrange is very choppy, and higher rpms are smooth.

    The mag cylinder is over 200*f while the other two are 120* or less and this is only approx. 5 minutes on a water hose. I would take her into the water but I don't want to overheat that one cylinder. Unless putting it in the water will make it run cooler? not sure.....someone chime in and tell me where I'm going wrong.

    Low speed adjuster is set at 1 1/4
    High speed adjuster is a tad more than 1/8

    The only thing I think it may be is the head gasket placement. I put them back the way they came out but wasn't sure if they were right. The smaller holes were towards the intake while the larger holes were on the exhaust side. They have a small tab on them but no reference mark to align them with. New gaskets will be here tomorrow, maybe it'll have a diagram.

    BTW the old head gaskets torqued down to spec kept my compression exactly the same 140-140-135. I'm still installing the new ones though

    Could there be blockage farther down in the cylinder? It looks immaculate as far as I can see down it with the head off. Anyway to blow it out or run some high pressure water to see if it's blocked?

    Just trying to bounce some ideas around maybe someone can help

    Thanks again

  7. #7
    blegan's Avatar
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    Quote Originally Posted by RayRay420 View Post
    So I got the fuel pump built and installed and ran all new hoses. Fired it up and it still has a hesitation in the idle, then midrange is very choppy, and higher rpms are smooth.

    The mag cylinder is over 200*f while the other two are 120* or less and this is only approx. 5 minutes on a water hose. I would take her into the water but I don't want to overheat that one cylinder. Unless putting it in the water will make it run cooler? not sure.....someone chime in and tell me where I'm going wrong.

    Low speed adjuster is set at 1 1/4
    High speed adjuster is a tad more than 1/8

    The only thing I think it may be is the head gasket placement. I put them back the way they came out but wasn't sure if they were right. The smaller holes were towards the intake while the larger holes were on the exhaust side. They have a small tab on them but no reference mark to align them with. New gaskets will be here tomorrow, maybe it'll have a diagram.

    BTW the old head gaskets torqued down to spec kept my compression exactly the same 140-140-135. I'm still installing the new ones though

    Could there be blockage farther down in the cylinder? It looks immaculate as far as I can see down it with the head off. Anyway to blow it out or run some high pressure water to see if it's blocked?

    Just trying to bounce some ideas around maybe someone can help

    Thanks again
    The tab on the gasket should line up with water rail connections, there is an up and down side its usually clearly marked. The manual says your pop off shoulc be 20 - 24 lbs low speed 1 1/4 on all and high speed is 1/8

  8. #8
    Quote Originally Posted by blegan View Post
    The tab on the gasket should line up with water rail connections, there is an up and down side its usually clearly marked. The manual says your pop off shoulc be 20 - 24 lbs low speed 1 1/4 on all and high speed is 1/8
    that's where I have the gasket set at and it is correct side up. Pop off is 20 on all 3 carbs and the low speed and high speed is exactly that.

  9. #9
    Why does the mag cylinder get over 2oo degrees while the other two run fine? this is on a hose in less than a few minutes.

    I removed the head from that cylinder and ran water through the water channels and it drains perfect.

    Does the ski have more water flowing through the cooling system when it is in the water?

    or is there some other problem making it run hot?

  10. #10
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    If you do some research on here, you'll find the MAG cyl always gets hot on the garden hose because it's last in line to get any water. Which really sucks because it runs the hottest with the magneto directly behind it creating heat.

    You'll also find that's it's not good to run the engine on the hose for more than a minute or 2 tops. UNLESS you disconnect the driveshaft from the engine. You'll overheat your thru hull seals and pump bearings because there's no water to cool them.

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