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  1. #1

    701 Raider Pop Off pressure

    Hey guys,

    To start off I have downloaded and read every document I could find related to tuning carbs. Here is where I need help.

    I just rebuilt my 2 Mikuni 38's with authentic rebuild kits.
    My current settings are as follows:

    NV 1.5
    Jet Low: 67.5
    Jet High: 120
    spring: dull silver

    Carb 2 (PTO):
    NV 1.5
    Jet Low 67.5
    Jet High 130
    spring: dull silver

    My issue is that the pop off pressure on both carbs with the dull silver spring is around 22psi. I tried the shiny silver spring and the pop off pressure jumps to 42psi.
    There is no leak and the pressure holds fine. I checked the NV to make sure the tip wasnt messed up.

    What is puzzling me is that the shiny silver spring is supposed to be 65 g and the dull silver 95 g. Also the shiny silver spring is longer than the dull silver.
    How come with a lower gram spring my pop off pressure is higher? shouldnt it be lower?

    This engine is totally stock except for FA's.



  2. #2
    Click image for larger version. 

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ID:	232008Its a weird one. I compared the ones I replaced on the carb and they are the dull ones also. So I put those back in and sure enough the pop-off is the same, right around 24 to be exact.
    The pressure holds so there isnt a leak around the needle and seat. I even tried swapping them around from one carb to the other and get the same readings.
    The shiny silver which is supposed to be the 65 gram spring reads 42 PSI, the dull spring reads 24 psi. This is on both carbs. I tested both thru the fuel inlet with the pump cover on and then I tried it with the pump cover off holding the hose nipple on the fuel filter port on the carb.
    My carbs are pretty spotless with no corrosion inside or out. The NV's arent pitted or show any sign of wear on the tips, even if they did it would not hold pressure.
    I thought about the pop-off pressure gauge being bad, but I have no way of checking it. I added wd-40 to the hose leading to the carb just to make sure.

    The other thing that puzzles me is the color of the springs and the difference in pressure and size. Does anyone have any extra springs laying around that they could compare?
    Attached is two pics of my current springs. The left spring is the dull silver and the right spring is the shiny silver.


  3. #3

    Join Date
    Aug 2008
    Mobile, AL
    The larger spring is the 95g spring. Taller spring + larger diameter = more seat pressure on the nv. I know the reference manual says the shiny spring is 65g, but that's not the best way to distinguish them anyhow.

    Harry @GroupK states that the 95g spring is wound the opposite direction from all the others, but according to your pics that's not the case.

    According to the table in the mikuni sbn manual, the popoff for the 95g spring w/1.5nv is 42psi. Your gauge is pretty accurate according to that.

    Your reading for the 65g spring is low, however. The sbn manual states 32psi for 65g w/1.5nv. Don't think that matters, but if both carbs have the same reading, I would say it's accurate.

    Try to find the stock specs for your carbs (maybe even test the old springs?). If your motor is stock, & running the stock airbox, you just want the popoff pressure to match factory specs.

    Higher popoff = leaner low-mid range fuel mixture (but since you read the sbn manual you probly know this).

  4. #4
    Hey jamn,

    Weird I only just saw your post. Im guessing my cache was still showing only my posts.
    Im pretty much stock except for Riva FA's and choke plates removed. The factory specs call for it being around 35-40, but couldnt get exact numbers.
    I tested with the old springs and i get the same reading. I havent had a chance to take it out on the water yet and test it that way but hope to soon.



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