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  1. #1

    Unhappy help !! I give up.

    Okay friends, I give up. I have a semi-pristine 1995 SL750 that I thought I gave up on. Have had it bored .020 over, after the first rear piston holed itself about 10 years ago. Since then it has run now and then , between two more pistons on the rear cylinder. I blocked the oil line and started pre-mixing, thinking that would solve the prob. Same deal, another cute little hole. I have tried to sell , but have been bombarded by folks wanting a perfect unit for nada. The ski is in really nice shape, and I hate to just toss it out. All the forums I have read keep pointing towards the fuel system.. I think will make one more stab at keeping in family. Will one of you all, much wiser than I, kinda give me a list of what to purchase and where , of necessary replacement parts ( I am thinking along lines of fuel pump, block off oil plate, fuel lines , etc. to try to resurrect her one more time). Money is really not the issue, although I do not want to farm this out. A personal challenge to see if a tired senior citizen can still function (me). Thanks very much in advance. GAP

  2. #2
    Join Date
    Apr 2007
    dont give up yet............
    your on the right track,,,,,
    clean and replace anything involved with the fuel system.
    the experts will give you a list shorty
    once you fix the 3-4 truoble areas in the fuel system you will get lots of life from those pistons----I PROMISE

  3. #3
    Join Date
    Jul 2007
    Cedar Rapids,Iowa
    Get us some pictures of the offending piston and head. Could possible be a seal leak also.

  4. #4

    seal - head

    After last hole, just shut door to barn. First warm day will take off head and send pics. How difficult is seal to replace?

  5. #5
    Click avatar for tech links/info K447's Avatar
    Join Date
    Jul 2007
    near Toronto, Canada
    Welcome back to Green Hulk Polaris

    Burning a piston usually means there is insufficient fuel, or too much air, getting into that cylinder. This is called lean burn, and it causes excessive piston temperatures. The aluminum piston gets so hot it softens and melts the hole.

    Since you have had this happen more than once, there is the possibility that some of that material has contaminated the crankcase and/or the crankshaft bearings.

    If you really want to do it right, you might consider pulling the engine, and rebuilding it yourself. Not as hard as it seems, and there is plenty of advise and guidance available here to get you through it.

    There are also several people around here who can and will rebuild the whole engine, or just portions (crankshaft, carbs, etc).

    Do some Searching on here, there are many threads covering the updates that the old Fuji engine models need to be reliable.

    The main thing is to replace all the fuel lines, including those inside the fuel tank. If you have not done so already, install a triple outlet fuel pump upgrade. Be sure not to lose the tiny fuel line restrictor. It is often located inside the return fuel line, just and inch or two down from the last carb. The triple outlet pump was actually the stock fuel pump on the Fuji 780 engines.

    Some models have the restrictor in the line, others had it inside the last carb. Without the restrictor, your fuel pressure will be too low, and the carbs will run lean.

    Next step would be rebuilding the carburetors. They often accumulate junk and gum, which restricts fuel flow. Not that hard to do, and not expensive.

    Often the spark plug wires benefit from being trimmed about 1/4" shorter at the spark plug cap end (they unscrew from the wire). Having a fresh cut end on the wire ensures a solid connection between the wire core and the spark plug cap.

    You may need to check the engine for an air leak. Search for leakdown test. If any of the crankcase/crankshaft or intake seals are leaking air, it can cause lean combustion, which burns those holes in the pistons. The rear cylinder failing implies an air leak, possibly the rear crank shaft seal. Usually fuel starvation affects the front cylinder more than the rear.

    Replacing the seals on the crank shaft requires removing and disassembling the engine.

    There are some other things to check on those 750 Fuji engines, and I am sure others will contribute.

    If you want to, we can also suggest some performance boosting changes

  6. #6
    TempoGT's Avatar
    Join Date
    Apr 2008
    Montreal QC
    I'm no Polaris expert, but having 2 friends with 3 Polaris', I've heard them discussing quite a few issues, such as the fuel selector being bad, and could cause a lean running issue (and burn a cylinder). BRP makes an identical one, and should be easier to find.

    As well, check out this thread for another common issue:

    I know rob2337 has had his share of issues with Polaris (and updates, etc), and can probably point you in the right direction.



  7. #7
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
    Join Date
    Oct 2006
    Cleveland OH
    Geoff, don't sweat it.

    There are a few things that can cause a particular piston to fail. Simply replacing the piston and not correcting the problem will result in another failure.

    Possible causes:

    Obstruction in the fuel lines - Prob not in your case since it's 1 piston in particular

    Restrictor missing - if your fuel lines were ever replaced and the restrictor wasn't re-installed, then that can cause a fuel pressure drop. ESP in the PTO (rear) cylinder.

    Adjustment on carb is wrong - If someone has ever messed with the carb adjustments, thay may have leaned that carb out too much.

    Dirty carbs - may need rebuilt or thoroughly cleaned. There are filters inside the carbs that may be clogged.

    Wrong spark plug - I know the BPR7ES is recommended, but most feel the BPR8ES is better. It's a little cooler spark and should be gapped at .028"

    Timing off - if the timing is too advanced, it will cause a hole. Most likely not your issue as it will affect all 3 cyls.

    Out of phase crank - if you ever flipped the ski and got water inside it then hit the start button, you may have hydrolocked it. What that means is the water won't compress inside the combustion chamber, so the crank or rod bends or gives and forces that cylinder out of time or phase. Very good chance this might be something for you to look into.

    Air leak - You may have a faulty gasket seal somewhere allowing air to leak into the engine and lean out the fuel mixture. Common causes are crank seals, base gaskets, head gaskets, carb gaskets, intake manifold gaskets, or exhaust manifold gaskets. Another one for you to look into.

    Oil system has nothing to do with a hole in the piston center.

    If I were you:
    Remove the autocock if it's still installed on your ski, per the Polaris service bulletin.

    First check your carb adjustmets to the specs in the manual

    Rebuild the fuel system. Carb rebuild kits, new fuel hoses, new fuel filters, clean/replace the fuel selector valve, and install the triple outlet fuel pump.

    Perform a leak down test. Looking for an air leak. There's a procedure for this test.

    Index your crank. There's also a procedure for this as well.

    One of these will help locate the source of your problem, and fix it for good.

  8. #8

    food for thot

    Thanks to everyone for the encouragement. The next few weeks as time allows, am going down the list. Prob better ck the crank index first. The way I understand this, is to measure a piston at TDC, and then measure the other two, which should be same? Is that accurate???

  9. #9
    Race, wreck, repeat delagem's Avatar
    Join Date
    Aug 2006
    Homer, NY
    You just want to verify your crank hasn't twisted, and there's a procedure in the Factory Service Manual for that, without the degree wheel. Go to this site:

    92-98 Service Manual on page 4.111 shows how to do a crank index with a dial caliper. A Runout indicator helps to be sure you're at exactly top dead center, I bought mine for $12 at a cheap tool sale.

    According to the manual piston heights should be less than 2mm apart.

    Big thumbs up to Mattman and Beerdart for putting this info up!

  10. #10
    john zigler's Avatar
    Join Date
    Aug 2006
    well you found the correct place to be!! there is lots of good info, and the best polaris guys around, right here.

    get it out of the barn, and get it assesed. go through the fuel system with a fine toothed comb.

    i would remove, and clean tank, replace all lines. ( make sure to re-install return restrictor) replace fuel select, rebuild carbs, ( use ONLY genuine mikuni brand parts), upgrade to triple outlet fuel pump.

    we also remove the thermostat on any that come through the shop here.

    parts for the fujis are pretty easy to find.

    good luck!


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