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  1. #1

    96 SL 780 won't rev above 2500 rpm

    Hi All. New to the forum and PWC. I just inherited 2 96 SL 780s from some friends who didn't want to deal with them anymore. Didn't cost me anything except for garage space so pretty good deal for me... I hope.

    A little background. These skis haven't been on the water for at least two years. One of them will start up and run and the other won't turn over fast enough to fire (even with a freshly charged battery, starter problem?). So I decided to focus my attention on the one that fires up to try to have at least one running ski. Once I get that working well I will turn my attention to the other.

    So I put the running ski on the water this week and found I was not able to rev it above 2500 rpm. When I fired it up right before I put it in the water it easily reved to 4k and the throttle response was good. In the water I had to be extremely light on the throttle or it would die. If I let off the throttle completely it would die. In order to keep the rpms up at about 2500 I kept feathering the throttle. If I held the throttle in one position it the rpms would slowly decrease until the engine died.

    Not sure what to think of this. I have new spark plugs installed (previous owners had a mixture of heat ranges in there). Battery was fully charged. She fires up almost immediately (with choke and 1/4 to 1/2 throttle when cold) but won't accept any throttle in the water. One thing that the previous owners did was cut the pink wire which from the wiring diagram looks like it goes to the fuel sending unit. From what I understand, they did that because they thought the ski was going into limp mode.

    Could this just be old gas (not sure since it does rev up quickly when out of the water)? Should I try adjusting the carburetors (have motorcycle experience so I won't be going in blind)? If so what corrections from stock setting should I make for being at about 5500'?

    Any input would be appreciated.

    Thanks


  2. #2
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Welcome to the Hulk Twin.

    Spark plugs should be a NGK BPR8ES gapped at 28 thousandths.

    If she sat for years. I'd look into going through the carbs and giving them a thorough cleaning. Replace the fuel hoses, and clean/inspect the petcock. Remove the spark plug boots, cut the wire back about 1/4" and rescrew the boots back on.

    Set the carb needles to the factory specs for the model year. Then add some fresh fuel.

    See how she reacts this. And let us know what you find.

    It's not uncommon to run good on the trailer and very poorly in the water. There's no load on the engine on the trailer.

  3. #3
    Click avatar for tech links/info, donation request K447's Avatar
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  4. #4
    johnsonmtz's Avatar
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    Quote Originally Posted by xlint89 View Post
    Welcome to the Hulk Twin.

    Spark plugs should be a NGK BPR8ES gapped at 28 thousandths.

    If she sat for years. I'd look into going through the carbs and giving them a thorough cleaning. Replace the fuel hoses, and clean/inspect the petcock. Remove the spark plug boots, cut the wire back about 1/4" and rescrew the boots back on.

    Set the carb needles to the factory specs for the model year. Then add some fresh fuel.

    See how she reacts this. And let us know what you find.

    It's not uncommon to run good on the trailer and very poorly in the water. There's no load on the engine on the trailer.
    Badda Bing Baby, that's good advice right there.

    TwinSL780, you said you have to go light on the throttle or it will die. That is the classic symptom of hesitation due to not enough fuel.

    There are 2 different ways an engine will stumble when you gun it off idle. If it just dies and/or RPM cut out that is a hesitation due to a lean engine. If the engine blubbers and gurgles that means it's too rich.

    As Xlint said, the best thing you could do is disassemble and completely clean out the carbs. There is possibly some fuel that dried into varnish and is plugging one or more fuel passages. The reason Kev suggests replacing fuel lines is they can degrade and swell internally causing a restriction. Also, the petcock's are known to corrode and cause a fuel restriction as well.

    You might also make sure the oil level is correct. I don't think these ski's have a limp mode due to low oil, but I can't remember for sure. 2500 RPM rings a bell as limp mode for something.

    These are all very common problems and are the best place to start. Once these are all addressed, you with either have:

    1) A great running ski
    2) A ski that still doesn't run right, but has all the usual culprits eliminated.

    If you end up with #2, you'll at minimum have a good baseline to start troubleshooting the problem.

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