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  1. #1
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    NEED HELP / OPINIONS on '95 SL750 High Revving

    Hey folks, I need some help in diagnosing a problem I am having with my winter project. I recently picked up a '95 SL750 from someone who started a top-end rebuild but either got confused or simply gave up. I purchased it from the guy and went ahead and proceeded to start from scratch by pulling the entire engine from the hull, removed the fuel tank, removed the oil tank and de-greased the hull. I separated the crankcase and checked the condition of the crank (it was ok), lubed up the crank and rods and put in new seals in the case, cleaned up the pistions and put on new rings, rebuilt the carbs, and now I have put it all back in the hull. I did install an oil block-off plate so I will be pre-mixing the fuel. As you may have already figured out, I removed the jet pump and shaft so that I could remove the engine. As part of this rebuild, I replaced all the fuel lines (new fuel filters and I ensured the restrictor was in my return line) and I installed a 3 output fuel pump. So here is the problem... when I first started up the ski (which did not take much effort), within 5 seconds the engine seemed to race up to about 5500 RPM. Now, I have not re-installed the shaft or jet pump yet, but I did not expect the engine to race like that. I have checked my idle screw and am sure it is not the culprit. I set the low-speed and high-speed needles to factory specs for my factory carbs. I am sure my throttle cable is not causing the problem. Does anyone have any other ideas? Is this kind of behavior to be expected with absolutely no load on the motor (e.g., out of the water, no jet pump installed)? Any help or insight is greatly appreciated. Thank you!!


  2. #2
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Welcome to the Hulk booze.

    Be careful revving the engine with no load on it. It can run away and cause issues for sure.

    It's recommended to do an engine leak down test prior to starting the engine. This will indicate an air leak before you ever start the engine.

    Make sure the throttle plates are closed all the way. (sounds like you pretty much did by checking idle screw and throttle cable).

    If it's not the carbs, then you have an air leak most likely.

    Air leans out the mixture and usually causes the engine to race like you described. It's also known as the "death rev".

    When you assembled the engine, did you install the exhaust manifold before torquing the cylinders down?

    Did you pay attention to the rear crank seals? There is a certain way they need to be installed.

    Double check your head bolts for proper torque. Did you torque them in a star pattern?

    Did you strip any intake manifold bolts? (easy to do)

    Carefully check the intake manifold and carb gaskets for any possible issues.

    Check your fuel lines and pulse hoses for tight clamps.

    If all this looks good, check the pulse fitting itself. I had one pull right out of the case this past fall when removing my engine for the winter.

    Good luck and let us know what you find.

  3. #3
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    xlint89... thanks for the info. In answering your questions...

    I did not install the exhaust manifold prior to installing the cylinders. I torqued them down per the factory specs and order.

    I am sure that I assembled the crankcase and seals correctly.

    No issues with the intake manifold, again torqued to spec.

    All gaskets were new from complete gasket kit.

    I will go thru and check the hose clamps of all the fuel lines, and when you said "pulse hoses", there is only one pulse hose coming from the crankcase, right?

    I am pretty sure you are right about the air leak though, I can't think of any other reason why the engine would rev as it does upon startup. A friend suggested also that I might spray WD40 around the seam of the crankcase and other points (cylinders, intake/exhaust manifold, etc.) and listen for a change in engine noise/speed. It would indicate an air leak. I'll doudle check the hose clamps as well and get back with you guys later tonight when I am home. Thanks again!!

  4. #4
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    The WD 40 trick would work OK if you let it race, but that's not a good idea. Not to mention, you won't be able to access every part with it inside the hull to spray it on.

    It's best to do a leak down test, rather than running the engine in it's present state. You'll see why you should run this test prior to install. What you need to do is:

    Remove the carbs and seal the intake manifold openings air tight. (some have used block off plates, while I've used racquette balls taped in place)

    Unfortunately, you need to remove the exh pipe from the exh manifold. Then seal the manifold air tight. (once again a racquette ball works for me)

    Then you need to apply 9" of vacuum (preferred) or 9 PSI of pressure through the pulse hose fitting.

    It should hold that for 10 mins without dropping but 1 or 2 " or PSI max. If it drops, you have an air leak. Now would be a good time to spray soapy water around the engine looking for the leak.

    The reason I asked about the exhaust manifold is because you can disturb the base gaskets if you torque the cyls down first. Imagine if you will, the cyls are bolted in place. Now you "pull" one or more cyls into alignment with the exh manifold being torqued.

    That's why it's importatnt to do the exh first, it will align the cyls before torquing them down. i.e. no gasket issue


    Yes, there's only 1 pulse hose, sorry about the confusion.

    Another member on here messed up installing a reed gasket somehow, that's why I asked if you made sure the intake gaskets are correct

  5. #5
    Lake Mead Bum & BTLS Mark starflight's Avatar
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    Quote Originally Posted by xlint89 View Post
    Another member on here messed up installing a reed gasket somehow, that's why I asked if you made sure the intake gaskets are correct
    I have a feeling X is referring to me.
    One thing is for sure, I can screw up anything.
    Here is a link to the evidence.

  6. #6
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    I will be picking up a leak-down tester this afternoon and pulling the engine from the hull so I can do this right this time. Assuming I am good with the crankcase seals and intake/exhaust manifolds, I'll focus on the carbs.

    And the raquet balls are a great suggestion!!

  7. #7
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Quote Originally Posted by starflight View Post
    I have a feeling X is referring to me.
    One thing is for sure, I can screw up anything.
    Here is a link to the evidence.
    I never mentioned any names...

  8. #8
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Quote Originally Posted by boozpolaris View Post
    I will be picking up a leak-down tester this afternoon and pulling the engine from the hull so I can do this right this time. Assuming I am good with the crankcase seals and intake/exhaust manifolds, I'll focus on the carbs.

    And the raquet balls are a great suggestion!!
    All you need is a vacuum pump (hand held). You can often rent them from an auto parts store.

    As for the racquette balls, try cutting them in half to fit inside the intake manifolds. A whole one works well for the exhaust. (Walmart for $4)

  9. #9
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    Well, I have good news and bad news. The good news is that the leak-down test passed. The bad news is the leak-down test passed.

    I pressurized the motor to 9 PSI and over the next ten minutes it dropped by only 1 PSI, so I am confident that all the seals are good, but now this is pointing to my carbs.

    As I said earlier, I have done a re-build on the 3 factory Mukuni carbs (38mm w/34mm venturi). In addition to installing the parts with the re-build kits, I also installed new needle valve assemblies. I am hoping the springs that came with the kits are of sufficient tension, they are able to close the needle valve and not allow the regulator diaphragm to hold the needle valve open thus allowing all the fuel from the fuel chamber to rush into the engine upon startup (which is happening with no throttle action).

    From data that I found on Mikuni's website and the fact that I ordered 2.0 needle valves, the specs from Mikuni for a 2.0 needle valve with 18 PSI pop-off pressure would require a 65 gram spring (which is supposed to be a shiny silver spring). Tonight, I will go ahead and tear down the carbs again and try to verify these parts.

    As all ways, any input from folks is appreciated.

  10. #10
    Lake Mead Bum & BTLS Mark starflight's Avatar
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    Before you open the carbs back up, check to see if they are synchronized. Even if your popoff valves were sticking open, the engine shouldn't rev if the throttle plates are near closed or closed. There wouldn't be enough air for a bunch of fuel to burn properly. Even having the wrong pop pressure wont cause a reving condition. I've gone from pop pressures as low as 13#s to 24-25#s with spring changes w/o problems idleing.
    I would back off on the idle adj. screw till the throttle plates are closed then use light to shine through the carbs and ensure that no light shines through. I found mine were out of sync. that way, and I adjusted them using the plate adjustment screws between the carbs till no light would shine through.
    The pics show how out of sync the carbs were. Sorry they are kinda blurry, but you can see the issue I had.
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