99 GP760 Running with plug wires off???
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  1. #1
    PWCToday Newbie
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    99 GP760 Running with plug wires off???

    Hello,

    I posted last night with regard to rebuilding the carbs on this ski. Got it done and the carbs installed. Primed it a couple of times and it burned those off fairly quickly but I could see the fuel filter was indeed filling (wouldn't do that before). About the third prime it kept running and seemed to run good. About when I was going to shut it down (maybe 20 seconds) it started reving even though I was not pulling the throttle. I pulled the lanyard, kept running at the same rev. Pulled the choke, kept running at the same rev (like 5k rpm or 1/4 to 1/2 throttle). I covered the intakes but didn't phase it (didn't stay with that tactic very long). So, I pulled the plug wires. No change, kept running at roughly the same rpm. I was completely confused and ran to get a plug wrench to pull the plugs but then it died. I am not sure why it died. I don't think it seized in that while the cylinder was hot, not very hot or to hot to touch (1st thing I checked).

    So, first question is - WTF??????????????? Anyone ever seen this? Makes no sense to me. I would have said it was impossible but it happened. I've seen things diesel but not like that (half thorttle'ish and fairly steady). I did just rebuild the carbs, was careful and don't have any idea if I could have done something wrong to cause that problem.

    Any thoughts, or ideas would be appreciated. I'm a at a loss and don't want to start it again until I know I can stop it.

    Strange days!

  2. #2
    Top Dog Bionic racing's Avatar
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    Re: 99 GP760 Running with plug wires off???

    Is this ski new to you ?

  3. #3
    PWCToday.com Is My Home Away From Home kcr357's Avatar
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    Re: 99 GP760 Running with plug wires off???

    lean conditions will cause pre ignition without a need for spark. You have an air leak/fuel blockage/carb issue.
    How many gerbils could a gerbil jammer jam if a gerbil jammer could jam gerbils?


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  4. #4
    PWCToday.com Is My Home Away From Home Myself's Avatar
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    Re: 99 GP760 Running with plug wires off???

    That also happens sometimes right after a carb overhaul before the carbs get full of fuel. It was "dieseling" the ends of your spark plugs were hot enough to keep the fuel ignited with no power. Scary isn't it!!??
    If it happens again you have 2 options. Grab a rag and shove tightly over the exhaust exit....it will choke out. Or, pull the choke AND give full throttle, the extra fuel with choke pulled will cool the plug enough to shut her down.
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  5. #5
    PWCToday.com Is My Home Away From Home Grumpy Old Guy's Avatar
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    Re: 99 GP760 Running with plug wires off???

    Pull lanyard & give full throttle...shuts right down.

    Also make sure your idle screw is closed & throttle cable has freeplay at throttle lever.

  6. #6
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    Re: 99 GP760 Running with plug wires off???

    Thanks for all the replies ... obviously not as rare as I had thought. I have worked on a lot of older standup Kawi skis but my first venture into a yamaha sit down. Yes BR, it's new to me, the person that had it before me, while nice enough, had no mechanical skills and I think tried some off the wall techniques (based on sprays and fuel additives on shelf next to it when I looked at the ski) to try to get it running. I think it got hotter than I realized and carbon on the head or the plug tips were igniting the fuel (as mentioned by most all of you).

    Long story short, I ended up hooking up a water line to it (duh, not sure why I didn't do that at first, guess I was just planning to see if it fired and pumped fuel at all) and primed it. Would burn prime off, then run very rough and die. I thought it might be a venting issue and took off the gas cap. Same thing. So I filled a small bottle with 14oz of gas and put the fuel line in it and tried to start. It fired with no prime. Choked it, started right up, (had water running through it at this point - while it's slow, I do learn), rev'd nicely, idled perfectly, ran 12oz of fuel through it with no issues at all. I am not a big fan of blaming "bad gas" (too easy and usually wrong) but it seemed like the next thing to eliminate. I pumped the tank dry (jeepers, these things hold A LOT of fuel) and put new gas in. Fired right up and ran fine for varying periods of time many times (you know that feeling you get like you are a kick-a wrench when something you worked on runs so you just keep starting and running it? - yep, I had that).

    The only thing I had a little concern with was how much it smoked. Not a crazy amount but certainly a light cloud while and after running it and an amount that, if I saw a guy tooling across the lake with that much smoke trailing him I would have thought - "you're putting too much oil in your gas friend." (Now it it was a good looking girl riding it, I wouldn't notice the smoke at all ... yet I digress) But, giving it some thought, I came to the conclusion that these things, when the injectors or being used (not switched to pre-mix) probably smoke a lot when started on land or not under a load because the volume of oil pumped is the same as if it's under a heavy load yet the volume of gas is much less and thus, real rich on oil and lots of smoke. Would anyone that reads this concur with that or am I uh, well, ... just blowing smoke?

    I may have an air leak but I don't think so. Once on the lake I will have a much better idea. Yes "Myself," it was scary. After pulling the plug wires I literally thought, "is this a dream?" Has anyone had this happen on the water with someone riding it. I assume not under a load but want to know because I only got this thing for the people that go to the lake with us that can't ride the standups or wakeboad. Translation - kids or old peeps with little to no mechanical know how and I'd hate to see one of them tooling across the lake at the speed of sound (in their opinion) unable to "shut er down."

    Again, thanks for reading, considering and replying.

    Great forum!

    "Grumpy ol Guy" sorry for referring to you as "Old Goat" in my prior thread. Not sure where I got that but probably what I should have chosen for my user name.

  7. #7
    PWCToday.com Is My Home Away From Home Grumpy Old Guy's Avatar
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    Re: 99 GP760 Running with plug wires off???

    Thats OK I answer to just about anything. LOL

  8. #8
    PWCToday.com Is My Home Away From Home kcr357's Avatar
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    Re: 99 GP760 Running with plug wires off???

    Yamaha oil pumps run stupid rich. Something like 20-1. Lots of smoke is normal.
    If you search for a post I made a cpl days ago, you'll also find yamaha runs a ridiculously high popoff pressure on these carbs. They are hard to start if left sitting. PLus the choke bufferfly has a hole in it, so pulling the choke to help get pull into the lines doesn't do much. Stupid design. As per grumpy's advice, I changed the spring and it draws fuel MUCH better now.
    How many gerbils could a gerbil jammer jam if a gerbil jammer could jam gerbils?


    "You build cheater motor's & still get smoked !!!!"
    "Its not your fault. You just don't understand what you're talking about. "

  9. #9
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    Re: 99 GP760 Running with plug wires off???

    Yes, it does seem to pump plenty of oil! I think I will switch to premix soon. lI did not change the pop off spring but may in the future. After the carb rebuild and changing out the gas it seems to be running great (not on the water yet, just hose). Starts right up without having to choke, even cold in the morning, and revs and idles quite nicely. I'm really curious to see how it runs on the lake (unfortunately that's a 2 1/2 hour drive for us). Soon though.

    Thanks for the input, it's appreciated!

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