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  1. #1

    Sl750 mag cylinder hot?

    I have a 93 sl750 and the mag cylinder is much warmer than the other two. mag 170, center 130, pto 120. Temps taken with temp gun right after running full throttle. Fairly fresh overbored top end, fresh carb rebuilds, triple outlet fuel pump, 780 style airbox. I know one would assume it running lean but I have the high speed screw on the mag carb backed way out. The piston wash confirms I'm getting plenty of fuel to the mag cylinder.
    The pto cylinder looks the leanest why would it be the coolest temp? While I had the heads off I noticed the head gaskets were installed 180deg off. Could this cause a cooling issue?
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ID:	361337Mag 170 degrees
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ID:	361338Center 130 degrees
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ID:	361339Pto 120 degrees

    last fall after I rebuilt the carbs and replaced the fuel pump I had every cylinder running exactly the same temp. The only thing I have done since then it put the 780 style air filter on. In doing that I did have to add a couple inches to the pulse line. It about "14 long now. Thanks for any advise I'm stumped.

  2. #2
    BlueFishCrisis's Avatar
    Join Date
    Aug 2011
    Medina, Ohio, United States
    You could try replacing the head gaskets with a newer set of 750 gaskets, with larger openings. This will increase water flow and was used in the later 750s and in the 780s. What water rail are you using? 750? The 780 water rail has a restricted cooling passage on the PTO cylinder. The combination of larger head gasket openings and a restricted PTO outlet seems to have balanced things out...

    Doesn't answer why you went from all good to a balance issue though..... Did you adjust you carb settings when you changed the airbox out?

  3. #3
    I did open up the air screws with the new air box. 750 water rail. Today I put the head gaskets on correctly, went through all the cooling ports and ran it. Temps came down 150mag 130center 120pto. Fresh plugs didn't show much color and I backed the high speed screws out a long way. Motor runs good but kind of flat on the top end. I beleive I am running lean. The piston wash darkened up a bit too. I'm wondering if the rich piston wash on the mag piston was from before I changed the airbox. How long should it take to change the piston wash? How long should it take to get good spark plug color?

  4. #4
    Forgot to mention. I ran it one the hose before I went down to the river and all three cylinders were running the same temp at idle. About 80 degrees.

  5. #5
    BlueFishCrisis's Avatar
    Join Date
    Aug 2011
    Medina, Ohio, United States
    Can't really judge much on the hose. Wash will take some time. You can check the plugs right away by performing a plug chop. Hold the ski wide open throttle while on the water then pull the lanyard to kill the motor. While floating pull the plugs one at a time and take a look at them. Chocolate brown is good. Gray is not good. Really light gray is death. If the high screws are really opened up it may be time to go up a jet size. What size is in there now? The Airbox upgrade kit for the 750 skis came with 102.5 jets. Stock are 90.......

  6. #6
    I did several plug chops and the plugs didn't really change(maybe a dozen times). After the overbore it bumped the main jets up to 105. That was last fall, spark plugs looked awesome and ski ran well. The airbox kit I bought off eBay w/out jets. I ordered some larger jets yesterday. Could the flat spot at wot be caused by a lean condition? Can you think of a reason why the plugs wouldn't take color? Maybe water from reusing the head gaskets? There was I little steam coming out of the spark plug holes after I ran it, but it depended on the position of the piston. I assume it was making its way back into the combustion chamber from the exuast. It seems like I have multiple things fighting me here. Thanks for your input.

  7. #7
    The prong on the plug had kinda a yellowish color to it and the ceramic part stayed white.

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