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  1. #1

    94 Polaris 750sl Lean Piston - Need Help

    Before I get started - I am new to both PWC and Message forums so bear with me. I have worked on many different types of internal combustion engines over the years but this is my first experience with one of these dang things!

    I recently acquired (for free) a '94 750sl that had been stored for 11 years after a hole was burned in the center piston. I completely disassembled the engine (did not split the crank-case) and inspected/cleaned every part from top to bottom...nothing else was wrong mechanically. New center piston installed, new rings and honing of all cylinders performed (125-130psi compression on all cylinders after assembly). I cleaned the fuel tank spotless, Internal tank lines replaced with rigid poly tubing and screens verified clean, fuel selector valve cleaned/inspected/cleared...all lines/hoses outside of tank replaced with 1/4 ID fairly thick-walled black fuel hose. I purchased the Mikuni triple outlet fuel pump recommended somewhere on this forum and routed hoses to feed each carb individually as suggested. I chose the shortest routing possible to each carb with the fewest bends...the lines run between the carbs and the jugs parallel with the fuel rail. The fuel restrictor is installed on the return line and the auto-cock has been removed.

    I disassembled/cleaned/inspected all 3 carbs and set all idle screws to factory settings per the Clymers manual for this model. I did not purchase/install carb kits because the diaphragms, needles, filters, etc. looked brand new. The bike only had 50 original hours of run-time on it.

    I removed the oil injection system, blanked everything off and mixed my own fuel....the same way I have done for years on the outboard engines I have owned.

    The engine started, idled and ran beautifully at the house (on the water hose) and performed flawlessly on the open water for 3 different outings of about 1 hour each. On the 4th outing....after about 30 minutes of fun on the water, the engine cut-out and died while at wide open throttle. Fortunately I had headed back near the boat dock so I only had to kick like 'flipper" for about 200 yards to trailer it. I pulled the plugs from all 3 cylinders and it was obvious from looking at the center plug that this cylinder ran lean (white aluminum residue covered the center plug while the Mag & PTO plugs had typical black carbon residue). I removed the center jug to find the sides of the piston fairly melted and the top of the piston nearly burned thru.....again....11 years later.

    This "free' bike has racked up a cost of about $800 in complete new gasket set, piston, rings, new battery, new trim motor and cable, hoses, fuel pump, etc., etc, etc. I'm looking at another $160 bucks or so to replace this piston and rings again.

    I love the's my first PWC (I've always had boats)....but I obviously have not solved the original problem that started 11 years ago on this engine...or maybe more like 18 years ago in the FUGI engineering offices!!!

    I can't understand why the center piston is 'leaning" out and the other two are O.K. Is this typically the piston that fails on this engine series? Has anyone seen this type of recurring failure on the same piston?

    Suggestions are welcome....please bear with me if I don't respond quickly...I'm extremely busy with my "paying" job right now.

  2. #2
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
    Join Date
    Oct 2006
    Cleveland OH

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