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  1. #1

    Question 98 GP 800 - Runs at idle to mid and than full bog

    I just replaced all the fuel lines and rebuilt the carbs (Genuine Mikuni). My settings are listed below. I took my time and replaced all the old parts with new parts. One thing I didn't do was the pop off setting. The fuel lines are the same inside diameter although larger outside diameter. I will confirm if a restricted fuel line is the issue. The ski is at the lake and I only have 1 day/week to look at it.

    The ski starts up and runs perfect at idle. I put it in the water for the first time yesterday after the carb work and let it warm up at idle for about 5 mins. I than took it nice and easy out of the water and all seemed good. Once I knew it was at temp I gave it throttle and it completely bogged and it almost sounded like it was running on one cylinder. I worked the throttle with little variation. There was no difference from half too full throttle. I will change the plugs this weekend to start with the simplest solution although I'm pretty confident it is a fuel issue. Could it be my high screw settings? or more so the pop off setting? I was diligent to put all the new fuel lines on correctly. I removed the old lines one at a time and immediately put on a new line one at a time. Same with the carb rebuild. Oh I also cleaned the power valves and they seem to be working fine.

    I have a 94 Seadoo GTX and did the same to it. That one is running fine so it gives me some confidence that I didn't bugger up the reassembly of the carbs.

    Any suggestions would be great. I wouldn't have got through all this work without this forum. Great help and people.


    800 - Low Speed – 2-1/4 +/- ¼ (M),1-7/8 +/- ¼ (P)
    High Speed – 1/2 +/- ¼

    1.2 Needle/seat
    157.5 Mag main jet
    155 PTO Main jet
    85 Mag Pilot jet
    90 PTO Pilot jet


  2. #2
    DAGO RACING CREW 97GPSLEEPER's Avatar
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    You might have a needle sticking in the seat and its dumping too much fuel.

  3. #3
    Yeah possibly. What would the engine do if the pop off psi was not set correctly? It is the pop off that determines the open/close of the needle and seat correct? If the pop off pressure is too high I may not be getting enough fuel when the engine requires it and vice versa. If my pop off pressure is too low it may be dumping too much fuel into the engine. Does anybody fully understand how the engine will react with the incorrect pop off pressure? I dont have the pop off pressure tool but may need to buy one if this is my issue. What is the specification for pop off (psi) on a 1998 GP800?

    Thanks for the reply.

  4. #4
    DAGO RACING CREW 97GPSLEEPER's Avatar
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    You have a good understanding of how it works. Also the control arm (the piece that the needle attaches to, and the spring rest underneath), needs to be be straight, (not bent upward or downward) so the needle is not able to lift too high out of the seat and get stuck, or not be able to lift high enough.

  5. #5
    Thanks. I may have to bring the unit home from the lake to work on things. Looks like I have to take the carbs off again and do a once through.

    Thanks for the input.

  6. #6
    DAGO RACING CREW 97GPSLEEPER's Avatar
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    How did it run before you rebuilt the carbs? Did you replace the needles and seats?

  7. #7
    Yamaha artisan Cutlass's Avatar
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    Pop off should only affect just off idle fueling and basically doesn't nothing above 1/4 throttle. At least thats how it is on my GP1200 carbs.
    Sounds like something is wrong with the high speed jet portion of the carb, like maybe it clogged on one of the carbs.

  8. #8
    The machine ran good prior to carb rebuild although was a bugger to start when cold. Thought the fuel pumps may have been getting weak. Would have to play with the choke a little until warm. I replaced the fuel lines and rebuilt the carbs with mikuni rebuild kits including new needle/seat for preventative maintenance. The jets are original but I cleaned them. The engine runs nice at idle and up to 1/4 throttle. Must be either needle stuck in the seat or high speed jet? Any suggestions on how to narrow it down without removing the carbs again? If the needle was stuck wouldn't it flood out at idle and up to 1/4 throttle?

    To start I bought new plugs and will put my fuel selector on reserve to rule out the simple things. I do remember on the water when I was between 1/2 and full throttle I pulled the choke and the engine pretty much died out. I guess this just proves the chokes are working lol.

    I may tap the bowl of the carb to see if the seat might re-align. I will do a water test. If not better I will load back on trailer and back to the drawing board. I guess I have to pull the carbs and examine. Would improper high speed screw adjustment cause this? I don’t want to lean out and damage the motor. The more I turn out (counter clock wise) the leaner it will run correct?

    Thanks for the needed advice guys. I called my dealer today and they didn't even know what "pop off pressure" was. A little discouraging. I'm going to figure out this problem on my own with help on this forum.

  9. #9
    Yamaha artisan Cutlass's Avatar
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    Turning out richens the mixture. The high adjuster has the greatest impact on 3/4 to WOT...so that shouldn't be the problem. My carbs have a little filter in them. I suspect yours do to and maybe its mostly plugged up on on one of the carbs. If not, then do you have access to compressed air and a blow gun so you can blow through the high speed jet and passages to make sure they are clean??

  10. #10
    When I rebuilt the carbs the kit came with new internal filters. I blew everything out with an air compressor. I did notice that the new o-rings on the high adjuster were a lot smaller than the ones replaced. If for some reason the seals are the wrong size (hard to believe seeing as the kit was for my exact model) and loads of air is entering this may cause the mid to high end bog?

    What can I do to try and start an elimination process? Problem is my stinger needs to be removed for any card adjustment or work so it is hard to fix and test one thing at a time. If the seat is not seated properly in the carb would the engine react like this?

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