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  1. #1

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    1995 SL750 got too hot

    About three weeks ago I completely rebuilt the whole top end in my 1995 Polaris SL750 due to some prior damage to two of the pistons and jugs shortly after I bought it. Replaced all the jugs, rings, and pistons and all gaskets of course. I had purchased a new shop manual along with all the parts to do this rebuild, and reset all the low speed screws to 1/2 turns out, and the high speeds to M 1 turn, C 1/2 turn, P 3/4 turn just like the manual said. When I first tried to run it on a garden hose on the trailer it idled way too high and would runaway as you may have read in my first post. I opened up the low speed screws about 7/8 of a turn and it idled well and had snappy response on the trailer, and overall sounded like it was running very well. Took it to the lake two days ago and it ran perfectly, beginning to end on about an hour and a half run. Took it out again yesterday and it ran just like it did the day before for the first 30-45 minutes. Then it seemed like it had a rev limiter at around 3500 rpm but I know these machines do not have a rev limiter so I knew something was wrong. Got it home and took the plugs out and the rear one was shiny. Discovered a hole in the rear piston. I guess what this whole message is about is me wondering how it ran fine and didn't have any issues the first day then the second day it goes downhill. It obviously overheated and it was not out of oil. When I pulled it out of the water all three heads were so hot I could only touch them for a split second. I did check the middle and front cylinders when I got home and they both still have good compression. I am also wondering what the issue could be so I can fix it for good when I rebuild the rear cylinder. Sorry for the essay here just wanted to provide the proper background information. Thanks


  2. #2
    RLACEMAN's Avatar
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    possible thermostat stuck, sand or other debris in cooling lines. These days you want the high speed mixture screws turned out more, todays fuels are not what they were 25 years ago, also ethanol fuel is a no-no unless you upjet the carburetors, the mixtures screw even turned out as far as they go is not enough with E-10. When you assembled the engine did you note the position of the head gaskets? The need to be a certain way for proper cooling circulation. Did you also go over the entire fuel system, there are many places in it that need to be checked and/or upgraded, fuel pump, carbs, in tank lines.

  3. #3

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    Quote Originally Posted by RLACEMAN View Post
    possible thermostat stuck, sand or other debris in cooling lines. These days you want the high speed mixture screws turned out more, todays fuels are not what they were 25 years ago, also ethanol fuel is a no-no unless you upjet the carburetors, the mixtures screw even turned out as far as they go is not enough with E-10. When you assembled the engine did you note the position of the head gaskets? The need to be a certain way for proper cooling circulation. Did you also go over the entire fuel system, there are many places in it that need to be checked and/or upgraded, fuel pump, carbs, in tank lines.
    Thanks for the advice. I will definitely look into bigger jets as non-ethanol gas isn't available near me. I did verify the head gasket position when re-assembling. Before the engine rebuild I rebuilt all the carbs and it already had the Mikuni triple outlet fuel pump. Is there a recommended way to effectively flush out the cooling system?

  4. #4

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    Also should I up-jet both the jets or just the main jet?

  5. #5
    BlueFishCrisis's Avatar
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    A hole in a piston isn't necessarily a result of overheating as much as running lean. When you get back on the water be sure to perform plug chops and check piston wash. Plug chops consist of running at wide open throttle and killing the engine by pulling the lanyard from the kill switch. A good running engine will result in chocolate brown spark plug ends while a lean engine results in light gray ends... While you can up the jets to ensure more fuel is provided, ususally this isn't necessary until you cannot get good looking plugs with the high speed screw out more than 1.5-2 turns. At that point you are not really adding more fuel to the mix and the needle taper has maxed out. Confirm that you still have 90 mains in for the jets. Were you running with the stock airbox/flame arrestor in place? An aftermarket one will warrant a change in jet size.....

    You can verify cooling flow by connecting a flush kit to the ski, but engine must be running while you do this to be safe. The most likely clog is in the exhaust pipe injector, at the top of the U shaped exhaust pipe, but this would result in hot exhaust and a melted connector hose, not hot cylinder heads.

    Check the pickup in your jet pump also. There should be a small cross hatch style grille that the water must pass through. If this is clogged or covered, flow can be restricted. Pull your thermostat and boil it in some water to see if it still opens. Check the overflow valve for excessive wear - small plastic piece next to the thermostat in the housing - it can become stuck as well.

  6. #6

    Join Date
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    Quote Originally Posted by BlueFishCrisis View Post
    A hole in a piston isn't necessarily a result of overheating as much as running lean. When you get back on the water be sure to perform plug chops and check piston wash. Plug chops consist of running at wide open throttle and killing the engine by pulling the lanyard from the kill switch. A good running engine will result in chocolate brown spark plug ends while a lean engine results in light gray ends... While you can up the jets to ensure more fuel is provided, ususally this isn't necessary until you cannot get good looking plugs with the high speed screw out more than 1.5-2 turns. At that point you are not really adding more fuel to the mix and the needle taper has maxed out. Confirm that you still have 90 mains in for the jets. Were you running with the stock airbox/flame arrestor in place? An aftermarket one will warrant a change in jet size.....

    You can verify cooling flow by connecting a flush kit to the ski, but engine must be running while you do this to be safe. The most likely clog is in the exhaust pipe injector, at the top of the U shaped exhaust pipe, but this would result in hot exhaust and a melted connector hose, not hot cylinder heads.

    Check the pickup in your jet pump also. There should be a small cross hatch style grille that the water must pass through. If this is clogged or covered, flow can be restricted. Pull your thermostat and boil it in some water to see if it still opens. Check the overflow valve for excessive wear - small plastic piece next to the thermostat in the housing - it can become stuck as well.
    Yes the airbox and flame arrestor are stock. I have read in a few of these threads that the thermostat can be removed if it's causing problems is that true?

    I also am just puzzled as to how a hole was burned in the piston if I had the high speed needle set correctly on that cylinder? Are the factory settings just too lean?

  7. #7

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    These older fujis have fuel pump issues.

  8. #8

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    Jul 2019
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    Fuel pump is fine I have tested it.

  9. #9
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Leak down test. May have an air leak at the crank seals.

    Todays fuels are not the same as it was 24 years ago when they printed the carb settings. You need to be checking your piston wash and adjusting jetting until you are very comfortable with your settings.

    First day out you might have been a bit more careful riding the ski. Once you felt you were "safe" on day 2, you might have done a LONG WOT run which could have holed the piston. Maybe you took a passenger along for a ride this time. Could have been a cooler temperature day. There are many reasons why it could have failed.

    Here's a few things that can cause your problem. Could be from wrong carb settings, air leak, poor fuel supply, too much ignition timing, or even an out of phase crankshaft.

    Ultimately it comes down to this. You cannot aggressively ride the ski until you are comfortable with your jetting.

  10. #10

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    I rode the same the second day as I did the first after it proved to be reliable which I would later find out was not the case. Could the thermostat have stuck the second day and that was just enough extra heat to melt it?

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