Needle / Seat and Spring Questions
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  1. #1
    PWCToday Newbie
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    Needle / Seat and Spring Questions

    Hey gang...I have a few questions regarding pop-off. I've read through so many threads, posts and blogs from Mr. O'Neal and he did a fantastic job of answering questions and explaining things. Because of his dedication to this board, I now have a much better understanding of these carburetors. Unfortunately I never really got a question in my mind answered....

    We'll take this example since it is pretty simple:

    A 2.5 N/S and 95 gram spring will give you 17 psi pop off. A 2.3 N/S and 80 gram spring will give you 17 psi pop off.

    Based on Bill's explanation, and correct me if I'm wrong, the 2.3 N/S and 80 gram spring will be richer down low off idle due to the weaker spring force even though the pop off is the same. In other words, even though the 2.3 N/S is smaller (and the fuel pressure is pushing less on the needle due to the reduced N/S area), the 2.3 N/S with 80 gram spring will open sooner / easier than the 2.5 N/S with 95 gram spring since the weaker spring doesn't have as much preload on the lever, requiring the diaphragm not needing as much force to push the N/S open.

    I ask this because of my current situation with my carbs...not sure if specifics are needed here since I would like to keep this more generic if possible:

    It was recommended by the tuner to use a 2.5 N/S with 115 gram spring. The boat would die off the line unless the LS screw was opened up past 2-1/4 turns. I kept the 2.5 N/S but reduced the spring to a 95 gram spring. The boat immediately took off much better but still needed 1.5 turns out on the LS screw. I'm wondering if going to a 2.3 N/S and 80 gram spring will help me close the LS screw some since the weaker spring will open the N/S sooner...and the 2.3 N/S would be less prone to leaking or dribbling.

  2. #2
    Tech Guru PWCToday.com Is My Home Away From Home Watercraft Magic's Avatar
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    Re: Needle / Seat and Spring Questions

    Sounds like you are overthinking things a bit. First of all, there is nothing wrong with running the lows at 1.5 out. And fresh needle and seats with healthy (rust-free) springs don't leak or dribble. So don't make any choices based on that. If it takes off cleanly and feels good throughout the powerband, gives you something like a cardboard brown color on your plugs after a high speed plug chop and leaves the piston crowns with some wetness over the entire surface (as opposed to a dry or sandblasted spot in the centers or over the exhaust edges) you are good to go. Go by feel first. Then verify with RPMs. And always confirm viability with plug color and appearance of pistons crowns before hammering on it too hard. If you want to try even more fuel, consider trying 80g springs with your 2.5s - but that definitely could be too far based on your details and scenario. Advice from a tuner or engine builder is always just a starting point. If your ski seems to want more fuel, always give it what it wants and don't be stingy! If you want to be a micro-tuner, purchase some extra 95g springs and cut 1/2 circles off one end and keep testing popoff until you get 16psi and try that. Keep in mind that lowering popoff is sorta like installing a larger main because the mains will be open sooner and more frequently. So it's also possible that your lower popoff may be what's needed, but end up giving you a bit too much fuel overall. Sometimes, it's necessary to lower the popoff and also drop the mains down 2.5 jet sizes as well. But do one thing at a time whenever possible. Meaning lower the popoff first and test. Then perhaps drop the mains a bit (if the plug color looks too dark or the RPMs dropped noticeably or it feels soggy in the middle) and test again.
    Last edited by Watercraft Magic; 07-20-2018 at 12:29 PM. Reason: typo
    email Chris at Watercraft Magic cdofixr1@live.com

  3. #3
    Resident Guru JC-SuperJet's Avatar
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    Re: Needle / Seat and Spring Questions

    First, follow Watercraft Magic's advice, as they have extensive real-world hands-on experience building and tuning PWC engines.

    Just for fun, regarding the Spring Force question:

    At first glance, both combinations yield the same pop-off pressure (ideal conditions= same pump pressure, perfect diameters, same friction, same vibrations, etc)

    When you compare a 2.5 Valve vs a 2.3 Valve, the resulting force ratio is (2.5x25)/(2.3x2.3) = 1.18 (since Needle Force = Needle Cross-Sectional Area x Fuel Pressure, and Fuel Pressure is same for both). So 2.5 valve exerts 1.18 times the force of a 2.3 valve.

    Then compare the Spring Forces: 95gr/80gr = 1.18, so at first calc, the 95gr spring appears to exert 1.18 times more force than a 80gr spring.

    So at a ratio of 1.18 on both sides, it appears that both force combinations(pop-off) are equal and balanced.

    But here's the catch: the 95gr Spring is about+1mm longer than the 80gr spring, but has to fit into the same pocket length as the 80gr spring. So the 95 gr spring has to compress(shorten/travel) even more than the 80gr spring, thus exerting even more pre-load force. Spring Law (Hooke's F=kX): Spring Force = Spring Rating Constant x Actual Spring Compressed Length.

    However, Mikuni recommends "always best to use the smallest needle valve to obtain the correct pop-off pressure".

    Mikuni Spring Comparison Photo and Force Chart.jpg

  4. #4
    PWCToday Newbie
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    Re: Needle / Seat and Spring Questions

    Thank you guys for the responses. I understand that I probably am thinking about this too much but I'm an engineer that must always satisfy my brain's need to understand why certain things do what they do and see if my understanding follows the real world experience.

    I've been putting a lot of this info into Excel to try and compare the different values to try and find the connection. One such thing that I feel is that the different gram springs aren't really describing the spring rate but maybe a preload force exerted by the spring on the lever arm? For example, a 115 gram converted to pounds is ~0.25 pounds...no way that little spring has that sort of spring rate (be it 115 grams per mm or what have you).

    For example, the pop off of 19 psi with a 2.5 N/S (and 115 gram spring) calculates to ~0.145 pounds of force acting on the needle from the pressure underneath. Since the lever arm distance between the N/S and the pivot is longer than the lever arm between the spring and the pivot, the spring providing ~0.25 pounds of preload force (if that what 115 gram means) sort of makes sense. I need to measure out those distances though to see if the math actually adds up.

  5. #5
    PWCToday Newbie
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    Re: Needle / Seat and Spring Questions

    So I measured up the distances on an old lever arm that has been worn so I can tell where the spring and the needle and seat make contact. Between the pivot and the N/S, it is 7.0 mm and between the pivot and the spring perch, it is 4.5 mm. Now time to check the math in Excel...

    Hey JC...are you going to be out racing this weekend?
    Last edited by hyoctane23; 07-27-2018 at 05:27 PM.

  6. #6
    Resident Guru JC-SuperJet's Avatar
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    Re: Needle / Seat and Spring Questions

    Will not attend Madisonville since I haven't been riding lately due to my local water spot being closed so I'm not in shape to race GP Ski in this 100 degree swamp heat.

    But it will be a good turn-out specially since there are cash prices in the GP classes.
    Last edited by JC-SuperJet; 07-27-2018 at 05:42 PM.

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