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  1. #1

    Question SLT 780 in a jetboat

    Hey Guys,

    I have been on here looking around and reading up for the last year. ive done my best to do what i can but now im not sure what to do.

    i bought a 96 slt780 and stripped it down to put in a 3m aluminum jetboat.
    so the engine was pooched and i did a full rebuild on it. bottom end right to the top. and the carbs. i should also mention when i bought the ski it was partly disassembled. tuning the carbs back up was a difficult task for me as ive no two stroke experience really. so i got a guy to help me with that and im confident they are pretty close. plugs look good etc. ive had it running twice now for about 6 hours total. did the break in. it has never reached high rpms. i got around 5k rpm out of it for the most part then it just seemed to clear right up and was doing 6000 rpm for ten mins. then it just died down again and im lucky to get 5k out of it. pto cylinder seems not to be burning. plug is always wet. ive been looking at the coil packs and particularly the way the wires push in there. seems really sketchy. is there better coil packs around? ive got to test the coil packs again and the stator. but when i put it all in the first time it tested fine .
    do you have any ideas where i should be starting? is it possible the timing is off too far? if it was timing i would expect the problem to be more uniform. but really its the only thing i didnt adjust or check. i just put it right back where it was before.

    those that are interested here is my project thread....
    http://www.nzjetboating.com/yabbse/i...?topic=22498.0


    and a youtube video....


    thanks in advance for any help
    Jamie


  2. #2
    urugol's Avatar
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    That vid is awesome. Good job on the boat but how shallow is that water? Looks like a couple of inches at best! I'm actually building a Polaris 700 twin for a friend here in Australia we are also doing a jetboat project.

    In terms of the rpms, I would be investigating the carbs as a starting point. Did you rebuild them using mikuni kits and did you follow the service manual for pop off pressure testing when you did it? Your mixture maybe on the rich side if you are getting wet plugs, but the only way to get a good plug reading is to do a wot run and switch off the engine at wot then check your plugs they should have a nice tan to them. Did you replace all of the fuel lines as well? Did you perform a leak down test on the motor also? A good 780 should be good for about 6500rpm the rev limiter kicks in at around 6800 rpm. There are a few threads here that are very helpful with carb tuning, I think one is called carb tuning for girls or dummies or something like that its quite informative.

    If your stator checked out beforehand I doubt you would have any issues there. The coil pack is what it is the fuji ignitions are pretty good I wouldn't be looking for alternatives.

  3. #3
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    Stators can sometimes act funny when they got hot though. It sounded to me like it may have been dropping a cylinder.

    That was a cool video. I was just waiting to hear that thing suck up a rock though.

  4. #4
    Click avatar for tech links/info, donation request K447's Avatar
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    650 Fuji engine?

    Is the return fuel flow restrictor in place in the return fuel line to the tank?

    Was the fuel pump rebuilt? Did you upgrade to the triple outlet fuel pump?

    Genuine Mikuni carb rebuild kits?

    Fuel systemj needs to be 100% perfect on these engines. Zero air leaks into the fuel supply from tank to fuel pump. Fuel supply hoses are under suction, not pressure. Make sure fuel shut-off valve, water separator, etc are all 100% air tight, as is every hose fitting. No zip ties.

  5. #5
    Quote Originally Posted by K447 View Post
    650 Fuji engine?

    Is the return fuel flow restrictor in place in the return fuel line to the tank?

    Was the fuel pump rebuilt? Did you upgrade to the triple outlet fuel pump?

    Genuine Mikuni carb rebuild kits?

    Fuel systemj needs to be 100% perfect on these engines. Zero air leaks into the fuel supply from tank to fuel pump. Fuel supply hoses are under suction, not pressure. Make sure fuel shut-off valve, water separator, etc are all 100% air tight, as is every hose fitting. No zip ties.
    Did you read my post? 96 slt780
    Fuel system is good I've read all there is on fuel and everything is good. Had a professional 2stroke mechanic look at it but thanks for trying

  6. #6
    Thanks guys yeah it's pretty shallow a good pair of boots would get you across with dry feet. have custom grate in the intake. It also separates every second bar so if rocks do get stuck in there you can push a bar and they fall out again withoutmotor running of course . Carbs should be good everything is new mikuni and new fuel lines.

  7. #7
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by JamieCraigGreenfield View Post
    Did you read my post? 96 slt780
    Fuel system is good I've read all there is on fuel and everything is good. Had a professional 2stroke mechanic look at it but thanks for trying
    Dunno where I got the 650 idea from

    Sorry about that!

  8. #8
    LaveyT's Avatar
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    I would like to see a picture of that (The Grate),Sounds cool. I wondered if anybody had ever modified a grate to keep rocks out.
    Awsome Video

  9. #9
    LaveyT's Avatar
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    Click image for larger version. 

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Name:	ez clean grate retracted.jpg 
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    After I looked a little further....A stomp bar it even has a name. That looks like fun!

  10. #10
    haha yeah cool you found my build thread. these boats are alot of fun for sure. like i said ive had it out twice and had a blast both times but i wish i could get the thing running 100 percent. cant imagine how much fun it will be then.
    i should be more specific

    i checked all the plugs and they were all good. then it cleared up and ran at 6000 rpm then when it came back down i checked the plugs again and the pto plug was wet but the front two were perfect. so i ran a little more back to the trailer ( i was 6km down the river) when i got back i checked the plugs again and it was still the same front two perfect and the pto was wet. i didnt do the pop off testing i had the mechanic do it. and also what is a leak down test? i shouldnt have to do that being a brand new motor? new rings new pistons new sleeves. it starts first time every time. cant imagine having a problem there.

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