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  1. #1
    qbzonk's Avatar
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    Need carb tuning help on 98 gp800

    I purchased this ski back in 2005. It was previously raced and was pretty modded up when I got it. This year I decided to replace the wear ring and rebuild the carbs because I noticed an 8-9 mph loss of speed on my dream-o-meter.

    When I first got her, she would show 62 on the dreamo-o-meter. Now she struggles to muster 53 mph at a max of 7000 rpms. Should she be revving higher?

    I took it out today and checked it with my gps. She was running 50.5 mph and the speedo said the same.

    During my carb rebuild I didn't pay close enough attention to the NS spring color and when I realized there were different colors and went back to the trash to see what was previously in the ski, I realized my wife dumped the trash and everything was gone.

    Current set-up is this:

    it has FAs, 44mm Mikunis, 120 lows and 130 highs, 2.3 NS with the 115g springs (too much cause plug chops revealed lean misture),a different intake manifold, primer instead of choke, oil injection bypassed, an aftermarket waterbox, 15/18 solas prop, worx intake grate & sponsons, Riva drop nozzle kit, UMI bars and finger throttle & a riva quick turn nozzle.

    Did a compression check today and it was 130psi front and 140psi rear.

    Not sure what NS springs to go down to 95g or 80g. I think my LSA screw is 1.5 turns out and the HSA is 1.75 turns out.

    If you have more questions, don't hesitate to PM me or ask here as I will be checking back frequently.

    Thanks for your help guys.

    Here are some pics of the engine bay and carbs:
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    Last edited by qbzonk; 07-28-2007 at 01:18 PM.


  2. #2
    qbzonk's Avatar
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    Nobody? can anyone point me in a different direction as to where to seek help?

  3. #3
    skraggy's Avatar
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    If you are turning 7000 rpm's, I would look towards the pump. Where did you get the suggestion for the impellor at ? Maybe try the stock or previous prop back in it and see what it does.

    I have only worked on an '00 as far as the 800's go. And I never noted what jets or N/S were in it. I do remember that the carbs were pretty dirty and locked up,and I remember bringing the popoff down to the mid 40's from upper 60's-70,and going back to stock F/A-mixture settings for a stock ski.

    Maybe not much help, but I would start at the pump. Possible cavitation.

    Did you clean up the leading edges on that vien ? And make sure you were within tolerance in the wear ring/impellor area ?

  4. #4
    cheatin' piston popper addicted's Avatar
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    popoff really only affects starting and acceleration. Overall mixture is controlled by the jets, and fine tuning by the needles. Ideally, from what I have read, you want the needles as close to 1 turn out as possible. especially on the low screw. otherwise you will get funky idle and holeshot problems.

    Read up, http://www.greenhulk.net/forums/showthread.php?t=15473

    I can't help you on the jetting, but if the engine was put back to stock, and is not ported, stock jetting should be a good starting point. search for a thread called yamaha servive manuals, or look for the link in Billy, RX951's signature.

  5. #5
    master yoda
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    check your power valves .the higher compression on the rear cyclinder mite be a dropped valve this would account for the slightly lower rpm, and speed .

  6. #6
    qbzonk's Avatar
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    Prop was on the ski when purchased in '05 and when I was seeing the higher speeds and has not been changed since. It also came with a Solas YB-SD-XO, but I have no idea what the pitch is. See pic below.

    Could I had lost a reed petal? Here's a pics of the intake, I have no idea what type it is or where to get a replacement gasket should I need to remove it for inspection.

    I doubt the PV dropped as the ski has waveeater clips installed and they seem to be working fine.

    Going back to stock jetting isn't an option as this ski has 44mm carbs and not the stock 38mm carbs.

    Thanks for the help guys, I'm completely baffled and would hate to start tearing off the heads and such, just to see if she's been ported or not, but if I have to, I will.

    The waterox is a Tony Doukas from what I can tell, not that it makes a difference.
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  7. #7
    skraggy's Avatar
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    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

    Of course it doesnt tell you what pop off should be.

    That is your stock 800 settings...seems a little steep on the low side,but thats what the manual says,thats what it says for the jets and N/S also.

    If it were reeds, you would be noticing some back flow through one of the carbs,instead of just suction. In the one pic,you have the carb off and can see the reeds,if the motor is still in the boat,you may need a mirror to see the top reeds,or I always find it better to pull the reed cages out so that you can check the gap,you cant really check the gap with them installed.

    I believe the stock carbs were 44's on all 800's. Thats what my manual says anyways. Yes,I have a manual,so if you need any pages out of it,send me an email and I will email them to you.

    I would double check your fuel line routing also.

  8. #8
    cfgp927's Avatar
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    Skraggy,
    The spec's you supplied are correct for the 44mm Mikuni I-body carbs (stock on all 66E motors). What he has on the boat now are some aftermarket Mikuni SBN 44mm carbs. The jetting will be very different due to the way the carbs meter fuel.
    Qbzonk,
    What kind of atomizer do the carbs on the boat have in them? Is it a bombsight?, "d" shaped atomizer (novi), stock, annular discharge (blackjack, others)? My 44mm Blackjacks were jetted as follows: 1.5n/s, 95g spring, pilots 135, mains 165. If you know the mfgr of the carbs, share that info please. The manifold looks very similar to one that ADA put out, but not exactly the same. Looks like someone with access to a machine shop might have copied it from Billet Aluminum......There should be a name stamped or engraved on it somewhere. The specs I just gave you were for a twin piped boat, but other than both the pilots and mains being bumped up a few points for the pipes, they should be pretty close as a starting point. I had some Riva Billet carbs that were jetted completely different (reverse jetting) 2.5n/s, 115g spring, 120pilot, 100main.......
    Mike

  9. #9
    qbzonk's Avatar
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    Not sure what type of atomizer? I don't see anything in the venturis, it's just a straight shot down to the reeds. As for the manifold, I can't see anything stamped anyhwere on it, that's why I am so hesitant to take it off to see what's up with the reeds. I am afraid I'll never be able to find the right gasket for it.

    I took it out today after switching back to the 95g springs. She hesitated down low when going from closed throttle to WOT. I kept trying to open the low speed adjusters 1/8 turn at a time, but gave up after my buddy got tired. Currently I am at 1 1/4 turns out on the low speed circuit and 1.5 turns out on the highs. Max rpms were 7100-ish (hard to tell with the boat bouncing).

    Speedo read a max of 52 today. Weather was 92-ish, water temp was mid 80's, humidity was 85%+ (I think).

    Thanks so much for the help guys.

    Here are pics using a mirror to see down the venturis.
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  10. #10
    qbzonk's Avatar
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    bump

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