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  1. #1

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    Bad day at the lake!

    Today I went out to break in my new to me ski and it didn't go very well. Been working on this project off and on all summer. Last week I had a fuel delivery problem. Got a new triple outlet pump, installed, primed and everything looked good for break in. Ski ran fine through the intial steps of break in, warm up cool down, vary the throttle, you guys know the drill. Its when I got to the higher throttle settings that the problem surfaced. On one pass it just died, but then restarted. I was thinking maybe I had an air bubble so I resumed break in. Just before I headed in, it died again and was hard to restart and didn't have much power. I got it back to the ramp thinking maybe I fouled a plug. I am running something like 65:1 premix along with what the oil pump is dumping in there. Wanted a safe margin for ring seating. Checked the plugs, mag is oily, center is a dry brown, and PTO was black. It wasn't till I got home did I find the carnage.



    The piston looks just like the one I removed when I went through the motor. Its the PTO cylinder by the way and the old piston didn't have the hole though it like this one, but it was close. Guess it ran too lean again and thats what I don't get. I set up all the carb adjustments to factory specs. Checked all the diaphragms and replaced the pop valves when I had them apart. One thing I didn't do is check the jets to make sure they were the correct number, maybe thats the problem. I just didn't think it would burn down so fast. Like my friend said, its about $100 to rent a ski for an hour these days and thats the amount of enjoyment I got out of this one so I shouldn't be complaining right?


  2. #2
    rob2337's Avatar
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    What model of ski? 780?

    Did you replace all your fuel lines? Also, the fuel selector has been known to causes restrictions after it gets old.

    Finding out what jets you have in there is a good idea, but I am guessing the whole fuel system needs to be cleaned / replaced.

  3. #3

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    Sorry, its a 96 780 in a 95 SL750 hull. Completly brand new fuel system except for the tank pickup. New lines, selector, water strainer and now pump. I think when I rebuild this cylinder I am going to switch to clear lines so I can see whats going on in the fuel system.

  4. #4
    AWA Member 32DegH2O's Avatar
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    Did you do a leak down test on the motor? Might have a bad crank seal on the back. Also, it could be a bad carb or reed gasket.

  5. #5
    Lake Mead Bum & BTLS Mark starflight's Avatar
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    Using the 780 carbs or the 750 carbs? If using the 750 carbs, you will need to change the jets. You do need to see what they are.

  6. #6
    xplayer2885's Avatar
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    Did you notice when you started the ski on the trailor that the rpms would climb high an not come back down? Its normal for the rpms to climb when your running the ski on the hose. But if rpms werent comming down then you have a leaky crank seal...Deffinatly do a leak down test. I just find it odd as to how fast the cylinder burnt down. Like it was mentioned check your jet settings since you have a 780 motor with 750 carbs....I strongly feel that you accendtly did not change jet an the lean condition caused your burn down.

  7. #7

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    All parts are 780 I just used the old hull. No I didn't do a leak down test, kinda wish I had when the motor was on the stand. When I was going through the carbs, I removed the intake and checked the reeds and they were tight. Could still be a gasket leak, the pressure test would have told me that. Newbie mistakes, thought I had covered everything.

    Also, when I started it on the trailer out of the water and on the hose everything was normal. It would idle and rev just fine. When I put it in the water it was hard to start, but once warmed up it idled at about 1350 rpm

  8. #8
    xplayer2885's Avatar
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    Deffinatly perform a leak down test...Im pretty sure it will pass, if your rev's were fine.

  9. #9

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    Just reading some other posts and got to thinking. This engine has OP F/A's on it, it came like that when I got the wrecked SL780 this powerplant came from. Does anyone know if those F/A's were stock on a 96 Sl780? It could be possible that they are contributing to a lean condition. The only other performance part on the ski is a different intake grate. Ahh well, motors gotta come out again to do a proper leak test and change the piston. I did get one ride in this summer, right?

  10. #10
    Bing-A-Ding-Ding-Ding, Brrrrrap! Brrrrrrrrrap!!! Polaris_Nut#1's Avatar
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    Quote Originally Posted by bigboatman View Post
    Just reading some other posts and got to thinking. This engine has OP F/A's on it, it came like that when I got the wrecked SL780 this powerplant came from. Does anyone know if those F/A's were stock on a 96 Sl780? It could be possible that they are contributing to a lean condition. The only other performance part on the ski is a different intake grate. Ahh well, motors gotta come out again to do a proper leak test and change the piston. I did get one ride in this summer, right?
    they came on the 96 SLX not on the 96 SL780

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