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  1. #1
    PWCToday Regular
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    rebuilt carb question

    Ok, here we go. rebuilt sbn38 pto carb, did popoff test came out at 25-26psi, had finger over return fuel line nipple had air on inlet. noticed there wasn't any pressure on my finger on return fuel line. put air hose on outlet nipple cranked up to 50 psi, and there was no air leaking out anywhere. my question is, is there something stopped up somewhere? cant figure out where the fuel would come from that would be going back to the tank, I got to be missing something somewhere. i thought about hooking air up to inlet of pto carb, but realized it i did that it would lose air through jets. Is there a check valve in the fuel chamber that the fuel filter is in, that allows excess fuel from this chamber to exit trough the return line? WHEW.....need help scratching head.....thanks

  2. #2
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    Re: rebuilt carb question

    Here is page 6 of the SBN manual. Notice the fuel flow arrows. Did air flow OUT of the return line nipple during your pressure test, before you put your finger over it? Notice the tiny orifice in the return circuit.
    page 6.png
    2, 1998 TS 770 L, restored and running great. (Except for the one with engine that SBT put the WRONG crankshaft bearing into and is now sitting as I collect repair parts)

  3. #3
    PWCToday.com Is My Home Away From Home Myself's Avatar
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    Re: rebuilt carb question

    You have to scrape the green goo and crud out of the fuel fittings when doing a Seadoo carb.
    http://www.facebook.com/twinlakesjetskirepair

    '89 Kawi 650SX 42.4 GPS
    +3" stock pipe, ported ex. manifold, drilled waterbox, blueprinted pump, Ocean Pro ride plate

    '99 Yama GP1200 65U 61.8 GPS
    ported cylinders, matched cases, milled head, blueprinted pump, long ride plate

    '96 Seadoo HX 717 53.13 GPS
    light porting, massaged cases, lightened flywheel, port matched manifolds, Rossier pipe, stubby pump cone

  4. #4
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: rebuilt carb question

    Quote Originally Posted by KB2017 View Post
    noticed there wasn't any pressure on my finger on return fuel line.

    put air hose on outlet nipple cranked up to 50 psi, and there was no air leaking out anywhere.

    my question is, is there something stopped up somewhere?
    First off, inlet is the bottom fitting, return is the top fitting. If you were testing pop-off
    correctly, very little air comes out the return line because of a restrictor. You might check
    and see if it is clogged.

    The restrictor bleeds off pressure so the engine doesn't run rich at idle. It also bleeds
    air out of the fuel system so that air will not go through the needle and seat, and lean out
    the engine.

    If you pressurize the return line fitting, you are going against 2 one-way check valves
    (if that carb has the fuel pump), otherwise you are trying to flow air through a pin-hole.


    Bill M.

    Your pop-off sounds low. Have you ran the engine with that spring and seat combo before?
    Last edited by wmazz; 02-09-2018 at 07:45 PM.
    Horsepower == Speed, RPM != Speed



  5. #5
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    Re: rebuilt carb question

    ok, tore down pto carb again, and found that there was a tenny tiny pin hole that was stopped up. clean it out with a small wire and now I can get air out of return. Had to use a magnifying glass and bright flashlight to find it. Also got the other carb rebuilt today. I found during the rebuilds, that both carbs must have had the wrong springs in them, both carbs required about 50-55 psi to popoff. Changed springs, and now both popoff at 25 psi. Plan on reinstalling them hopefully tomorrow. And since we are on the subject, how will ski react with high pop off vs normal popoff, and normal popoff vs low popoff. Trying to educate myself. And thanks for the reply's to post. Also I removed oil pump, and installed block. Going to run premix. I've got low speed jets at 1 1/4 and highs closed. Its a 93gtx 587 engine. I haven't ran it yet. I found it last week, and it looks like its been in a garage for the last 20yrs. Doesn't have a scratch on it, looks brand new. I started it before I bought it, but ran very rough. For what I got it for, I thought what the heck, I'll give it a try. If anyone has anymore advice, I'm all ears. Thanks

  6. #6
    PWCToday.com Is My Home Away From Home Myself's Avatar
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    Re: rebuilt carb question

    If it has been sitting for several years it likely has a rotted wear ring too.
    http://www.facebook.com/twinlakesjetskirepair

    '89 Kawi 650SX 42.4 GPS
    +3" stock pipe, ported ex. manifold, drilled waterbox, blueprinted pump, Ocean Pro ride plate

    '99 Yama GP1200 65U 61.8 GPS
    ported cylinders, matched cases, milled head, blueprinted pump, long ride plate

    '96 Seadoo HX 717 53.13 GPS
    light porting, massaged cases, lightened flywheel, port matched manifolds, Rossier pipe, stubby pump cone

  7. #7
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: rebuilt carb question

    The low jet and pilot screw settings are closely related to pop-off.

    Jetting gets richer when you lower pop-off.

    Your old pop-off is an indication that something is wonky.

    It is unlikely someone would replace 2.0 seats with smaller 1.5 or 1.2 seats,
    increase the pop-off and still use the stock jetting.
    Unless they are not sbn carbs.

    1.2 to 1.5 seats are oem seats for SBN-i carbs,~94'-95'

    SBN carbs have 2.0 seats and lower pop-off.


    Bill M.
    Last edited by wmazz; 02-10-2018 at 12:40 AM.
    Horsepower == Speed, RPM != Speed



  8. #8
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    Re: rebuilt carb question

    Fired her up this afternoon, (woo hoo) ran a few minutes on hose, seemed to be doing pretty well. Will be a while before I can put it in the water (little chilly). Would there ever be a situation where you would open the high speed needles any? factory settings is high speed needle closed.

  9. #9
    PWCToday.com Is My Home Away From Home Myself's Avatar
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    Re: rebuilt carb question

    I always crack 'em open about 1/8 turn.
    http://www.facebook.com/twinlakesjetskirepair

    '89 Kawi 650SX 42.4 GPS
    +3" stock pipe, ported ex. manifold, drilled waterbox, blueprinted pump, Ocean Pro ride plate

    '99 Yama GP1200 65U 61.8 GPS
    ported cylinders, matched cases, milled head, blueprinted pump, long ride plate

    '96 Seadoo HX 717 53.13 GPS
    light porting, massaged cases, lightened flywheel, port matched manifolds, Rossier pipe, stubby pump cone

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