Trying to determine baseline pop-off pressure for '95 SL750 with upgraded intake
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  1. #1
    PWCToday Newbie
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    Trying to determine baseline pop-off pressure for '95 SL750 with upgraded intake

    I'm rebuilding the top end of my '95 SL750 after I burnt a hole in a piston last fall, and also rebuilding carbs and replacing fuel and oil lines.

    Many years ago my stock air intake cracked so I replaced it with what might be a '97 SLX PRO 785 air intake, which I'm guessing is less restrictive than the stock intake with the snorkel...

    Screen Shot 2021-05-01 at 5.12.27 PM.png

    (This may be one of the factors that contributed to my lean condition.)

    I'm still running the stock jets and needles, so do I lower the pop-off pressure to run this intake, or do I need to change my jets or needle, and if not necessary what pop-off should I be aiming for?

    I also have a 97 SL780 intake like this one. Should be going back to a more restrictive air intake to match my current stock jets and needles...

    Screen Shot 2021-05-01 at 5.11.27 PM.png

    I'm really just looking for long term reliability at this point and not maximum performance.

  2. #2
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    Re: Trying to determine baseline pop-off pressure for '95 SL750 with upgraded intake

    Your pop off will change because the new flame arrestor is less restrictive so there's less back pressure. You need a gauge and to check it and to put the popoff PSI back where the manual recommends, because after you changed that flame arrestor you changed how much air enters the engine and how much pressure it takes for the needle and seat to open up.
    90 Superjet - Coffman pipe, other mods
    98 Polaris Pro 785 - stock graphics, just ported and polished
    98 Polaris Pro 785 - black top, cylinders milled .020, ported, polished, watcon cooling
    95 Kawasaki 750 sxi - R&D pipe, ported, polished, bored 1.5mm over
    01 Polaris 600 sc xp sled - Stock

  3. #3
    PWCToday Newbie
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    Re: Trying to determine baseline pop-off pressure for '95 SL750 with upgraded intake

    I do have pump and gauge to check my pop-off pressure. I'm trying to determine *what* pop-off pressure I should be aiming for.

    The Polaris Service Manual has my stock '95 SL750 pop-off listed at 10-18 PSI but the '96 SLX780 stock pop-off at 20-24 PSI, which is also using the Mikuni SBN38 carbs and 2.0 N/S and I assumed has a less restrictive air intake than my original stock intake, so this seems a bit backwards to me.

    Anyway, I can't get much lower than 10 PSI without risking leaky needles.

    Should I aim for just around 15 PSI as I'm also using the Mikuni Triple Output fuel pump, which I'm guessing also raises the pressure on the other side of the needle, and maybe balances the less restrictive intake.

    Still can't find SLX Pro785 pop-off pressures listed anywhere.

  4. #4
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    Re: Trying to determine baseline pop-off pressure for '95 SL750 with upgraded intake

    I think the pop off is different on the 780 because the pipe has a different tune. I believe you should still want to use the pop off for the 750, but the spring should need to be changed because the back pressure from the arrestor has changed.

    Basically, and I would get a second opinion, you want the original 750 pop off pressure but you will probably have to change springs to get there.
    90 Superjet - Coffman pipe, other mods
    98 Polaris Pro 785 - stock graphics, just ported and polished
    98 Polaris Pro 785 - black top, cylinders milled .020, ported, polished, watcon cooling
    95 Kawasaki 750 sxi - R&D pipe, ported, polished, bored 1.5mm over
    01 Polaris 600 sc xp sled - Stock

  5. #5
    Top Dog JC-SuperJet's Avatar
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    Re: Trying to determine baseline pop-off pressure for '95 SL750 with upgraded intake

    Look, the Pop-Off Pressure matters the most on the Low End of the Fuel Circuit, where the Needle Valve is forced open by the Pulse Vacuum Signal from the low revving Engine.

    At higher Engine RPMs, the Air flows faster thru the Carbs and thus, the Venturi Effect takes over, easily popping the Needle Valve open as needed.

    Since the original Pop-Off is already plenty low, 10-18 psi, then either richen/open the Low Speed Adjuster or increase the size of the Low Speed Jet (Pilot Jet).

    Just back the Ski strapped to the Trailer into the Water and fine tune the Low Speed Circuit. Adjust the Low Speed Screws until the highest Idle RPM is found. Then, OPEN THE LOW SPEED SCREWS ANOTHER +1/4 TURN. This is necessary because the highest RPM on the Trailer is perfect for a Static Non-Moving Ski. But, when actually riding, the (Dynamic) Load on the Engine is higher, so it needs more Fuel, hence the extra 1/4 Turn.

    If the Low Speed Screw is over 2 Turns, go to bigger Low Speed Jets +5.

    For the High Speed Screws, adjust until the highest RPM reading is found while riding at Wide Open Throttle.

    And Exhaust Pipes do affect the required Pop-Off Pressures because the Pipes actually produce a Vacuum (Suction) that helps draw the Fuel Mixture thru the Carbs. The Pipe Vacuum helps Pop the Needle Valve. So the Pipe Vacuum adds to the Crankcase Vacuum Pulse Signal to force open the Needle Valve.

    The Crankcase Pulse Signal actuates the Pump to "Push" fuel against the Needle Valve, and the Pipe Vacuum Signal "Pulls" on the Needle Valve from the Carb Throat Orifices by acting on the Regulator Diaphragm.

    Learn to Tune...

    Tuning by Ear.jpg
    Last edited by JC-SuperJet; 05-03-2021 at 07:54 AM.

  6. #6
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    Re: Trying to determine baseline pop-off pressure for '95 SL750 with upgraded intake

    Great info, thanks!

    I realize the need to tune the low and high adjustment screws from stock settings once it get it in the water, but was more curious about the recommended pop-off pressure and where I should start. I appreciate your input on this. This PWC is old school and I've been working on it for over 20 years, but my skills still fall into the noob category. So I'll stick with the stock pop-off and tune from there.

  7. #7
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    Re: Trying to determine baseline pop-off pressure for '95 SL750 with upgraded intake

    All this info is in the mikuni manual Bud and it's very easy find
    90 Superjet - Coffman pipe, other mods
    98 Polaris Pro 785 - stock graphics, just ported and polished
    98 Polaris Pro 785 - black top, cylinders milled .020, ported, polished, watcon cooling
    95 Kawasaki 750 sxi - R&D pipe, ported, polished, bored 1.5mm over
    01 Polaris 600 sc xp sled - Stock

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