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  1. #1

    Getting Back to my '95 SL750

    I am finally getting back to working on my SL750. During my last outing one of my friends rolled it. It seemed to be running fine but after letting it sit for about a half an hour it would not develop full power. The dreaded 4200 rpm! Compression measures 120 psi on PTO and center and 110 on mag, still within spec. I upgraded to the triple outlet fuel pump, cut back the spark plug wires and check the spark. The spark is blue and I can hear it snapping which I think is an indication of good firing but it was bright out and was hard to tell how fat the spark was. Took it out and still could only manage 4200 rpm. Compression remained unchanged and the mag spark plug was very wet. I did measure the spark plug boot resistance and believe that on this cylinder it is out of spec low.

    How important is it for the boot to be within spec? Could this be the reason that my mag cylinder is not working properly? If so, this could be a very easy fix!


  2. #2
    Is there anyone in the Baltimore-Washington-Annapolis area who would be willing to help me remove the engine from my ski, probably in a couple of weeks? I will provide plenty of food and beer!

  3. #3
    Moderator beerdart's Avatar
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    Sounds like a fuel problem. Water in the gas?? Carbs rebuilt?

  4. #4
    Water in the gas was my first thought so I drained the tank and refilled with fresh but I never saw any water in the separator. I have not serviced the carbs yet. I tried to take them off but two of the six nuts were so rusted that the wrench just spun right away. I went at one with PB Blaster and a small pair of vice grips but still no luck. I have been letting the PB do it's magic while I am waiting for the rain to go away, then I will try again. I fear that engine removal will be necessary to get the carbs off. I also want to check crank index. Can it be checked with the engine installed?

  5. #5
    Moderator beerdart's Avatar
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    Yes index can be checked in the hull. Bring any cylinder to TDC the other two should measure the same..

  6. #6
    Quote Originally Posted by beerdart View Post
    Yes index can be checked in the hull. Bring any cylinder to TDC the other two should measure the same..
    Sounds easy enough, thanks!

  7. #7
    Quote Originally Posted by beerdart View Post
    Yes index can be checked in the hull. Bring any cylinder to TDC the other two should measure the same..
    It is amazing what you can find when you READ THE MANUAL! I found the section called Dial Caliper Crankshaft Indexing which is as beerdart described but I think that I may have a more accurate way to check crank index.

    I found a piston travel vs crank rotation calculator and did some calculations of my own as well. The cylinders are supposed to be aligned to ±2° to the mag end cylinder. A 2° error in determining TDC is only .001" on a dial indicator but at 120° and 240° a 2° error is .033". If a take a lesson from the previous chapter in the manual and measure .100" (20.3°) ATDC my 2° or .001" error determining TDC will result in a less than 0.1° error. Even if I miss TDC by .005" (~5°) my error at .100" ATDC is only ~0.5°.

    At .100" ATDC one piston should be down 1.667" and the other 2.335". I should also be able to verify this at .100" BTDC. Whatever numbers I measure I can calculate how far off the index is and determine if further inspection is necessary.

    Sound reasonable?

  8. #8
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by pseudonym View Post
    ...At .100" ATDC one piston should be down 1.667" and the other 2.335". I should also be able to verify this at .100" BTDC.

    Whatever numbers I measure I can calculate how far off the index is and determine if further inspection is necessary.

    Sound reasonable?
    Yes, especially the cross checking of BTDC and ATDC

  9. #9
    I took some measurements today and think that my crankshaft is OK. My measurements were only good to about .005" so I rounded everything to the nearest .01" since I was looking for a piston location difference no greater than .08". Using the shop manual method I measured a .05" difference when the mag piston was at TDC. When repeated for the center and PTO at TDC they both measure .01".

    I then repeated the measurements when the pistons were at .10" ATDC and .10" BTDC and compared them to my calculations. For all six of these I measured .01" maximum, which is about 1°.

    I did two final measurements. First rotating the crank counterclockwise to achieve mag TDC then clockwise to TDC. I did this as an attempt to see if there was any appreciable slop in the bearings. When rotating counterclockwise the cen was up .02" and the PTO down .02". When rotating clockwise the cen was down .03" and the PTO up .02". These numbers initially concerned me but going thru the math a .03" error can be explained by a 2° rotational error which is about a .001" error at the TDC piston. I think that .001" piston slop is probably fine.

    In conclusion, I believe my crankshaft is still indexed properly and my bearings are still in place. I will think about it some more and will probably redo a couple of measurements tomorrow but for now I think I am looking at a carb rebuild and fuel line replacement to get me back on the water.

    Anyone know a trick for getting stripped nuts of the carbs without removing the engine?

  10. #10
    Click avatar for tech links/info, donation request K447's Avatar
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    You may be able to remove the intake manifold nuts, and lift the entire intake assembly out of the hull.

    Hard to see what you are doing way down there at the side of the crank case, but doable, I think.

    Then you can wrestle with it on the work bench.

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