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  1. #1
    750sl's Avatar
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    Carb and Compression issues with 95 750 sl

    Problem #1 I was out today prepping my skis to take out this weekend. On the 95 I squeezed the newly installed primer bulb and heard gas gushing in the carbs. I took the flame arrestor off and looked down the carbs as I squeezed the primer bulb. In the center carb gas was actually GUSHING down the carb into the intake. What would cause this? Is this an easy fix? I do have a good spare carb set I bought from John Zigler, should I just go ahead and swap them out?

    Problem 2# I tested the the compression as well. I noticed that the compression on the MAG has gone down and now stands at 125PSI. Before it stood at 145PSI. On the center and pto they stand at 150 and 145PSI which is consistent from previous tests. High and lows were upped a bit from settings when I went to premix. The wash looks good. Spark is good.
    One possible reason for this is I had to perform the "rope trick" to tighten the crankshaft nut. The rope got caught up in there where I had to remove the head top to get it out. I remember reading on a past thread that this can cause damage. I looked for the thread but failed to locate it. Could this explain the compression discrepency? Also when I reinstalled the gasket I didnt use that copper spray that has been mentioned around this forum......

    Any thoughts? Many thanks.


  2. #2
    Polarisitis loonatik's Avatar
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    #1 - Seems like needle is stuck open on that carb.
    #2 - Check your crank index see if it's any good. Best way is to pop the head open and check for any visible damage on piston, ring, cylinder etc,. I think top of that piston may have melted a bit.

  3. #3
    thester71's Avatar
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    My guess is.... you damaged the ring with the rope trick. You are supposed to make sure the piston is above the ports in the cylinder before you install the rope.

  4. #4
    Canadian Beaver Inspector jkindt's Avatar
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    Quote Originally Posted by 750sl View Post
    ... I squeezed the newly installed primer bulb...

    ...I noticed that the compression on the MAG has gone down and now stands at 125PSI....One possible reason for this is I had to perform the "rope trick" to tighten the crankshaft nut. ...

    Any thoughts? Many thanks.
    First of all, a primer bulb like used on an outboard motor will not work for priming these carbs. You need to use a plunger style primer pump otherwise you will end up with fuel issues. I am sure someone else can explain why, I don't remember exactly at the moment.

    Second of all, if you were tightening the PTO coupler, you need to use the rope in the PTO end of the engine otherwise you could twist the crank and throw out the index. If it was the flywheel nut, then you use the MAG cylinder for the same reason. I use a light and inspect the top of the piston for damage, then check index on the crank, then pull the head and inspect for damage that you may not have seen with a light.

  5. #5
    750sl's Avatar
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    Seems like needle is stuck open on that carb.
    Check your crank index see if it's any good

    **Im going to take apart that carb this weekend and clean/replace internal parts. Crank index looks ok using the "redneck" method.

    My guess is.... you damaged the ring with the rope trick.
    I think top of that piston may have melted a bit.

    **Im thinking my Cheap-O compression tester is screwy. I tested the compression again today and on the MAG it up to 138psi compared to yesterdays 125psi. But then again the center and pto remained at 145 and 150psi....wtf? Im borrowing a friends digital compression tester and retest. I removed cylinder head and there was no hint of the piston melting anywhere. I might replace the ring anyway.

    First of all, a primer bulb like used on an outboard motor will not work for priming these carbs.

    **In that case I'll remove it.


    THANKS!

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