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  1. #1

    Onside Bill's Carb Drill Mod vs another idea

    So I was contemplating somthing; Thinking it might be a flaw possibly with OnsideBill's carb drillout modification. The issue is; at low rpm and cranking there is very very low fuel pressure inside the carbs.

    The carb gets pressure via the the pumps pushing against a 95 needle on the return line - a direct corolation to the pressure in the carbs. At crank and low rpm the 95 jet bleed much more of the pressure than at higher rpm. This is why we see ~1.5psi at idle and ~6psi at full throttle.

    So - whats to say a 6PSI pressure relief valve wouldnt make this affect better than a 95 jet? Wouldnt it be better for cold-starts and low rpm atomization of fuel if the carb pressure stays at 6PSI all the time - even while cranking? Of course jet changes would probably be in order. A 6psi ( Or Adjustable) check-valve can be made out of a spring, ball bearing, and brass fittings fairly cheap from HomeDepot. Another neet benefit of a check-valve of this type is that it would be one-way as well - preventing pressure in the form of fuel tank vapor (air) from pushing back on the carbs through the return line.

    Thoughts?

    Thank you


  2. #2
    OLD # 7 IEGPR's Avatar
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    If you think you can make a valve that will work then by all mean try it. That is the fun of this forum. We all try things and post our results. Its sounds like a valid plan. And by the way it's Osidebill not Onside. Good luck and cant wait to read about your results

  3. #3

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    The problem will be the increased fuel pressure at low speeds and idle will change the whole calibration curve of the carbs. Besides, if you want to validate if it would work, try pinching the return line clear closed, and see what fuel pressure you have at idle and low speed. You will find it is not 6 psi. Remember the fuel pump's output is based on rpm. Also, fuel pressure does not push gas thru the jets, fuel pressure just makes sure there is gas available for the engines vacuum at the carb to pull the fuel thru the jet. Its not like fuel injection.
    PS: O'Side Bill's mods have been done by hundreds on here with nothing but improved performance.

  4. #4
    Hydrotherapy's Avatar
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    I miss him and glad to of spoke in person to a true gent !

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    mudslanger's Avatar
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    I think this would be worth trying as a starting aid only. the return line is designed to do just that and Bills mod did aid in equalizing the 3 carbs and aided in tuning them. what i like about this idea is that some fuel pressure stays in the carbs and doesn't bleed out the return line. so if you have the check valve set to 6 psi you have 6 psi against the needle seat or more. it would be worth trying on a ski that is hard to start after sitting a few days. the down side would be if the valve stuck closed and the return line could not return fuel to the tank and cause the carbs to pump gas through the high and low jets and flood the engine.

    You should try this and report back. there are a lot of ski out there that could benefit from this.

  6. #6

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    A bigger down side with a check valve in the return line is if, and there always is, some air in the fuel system, you would have to build up 6 psi or so to pass it out thru the return. That is one of the reasons you have a return line at all, so air can purge from the system.

  7. #7
    Yamaha artisan Cutlass's Avatar
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    I'm with LT1 on this one. The fuel pumps probably won't create 6psi at idle.

  8. #8
    mudslanger's Avatar
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    Quote Originally Posted by LT1GMC View Post
    A bigger down side with a check valve in the return line is if, and there always is, some air in the fuel system, you would have to build up 6 psi or so to pass it out thru the return. That is one of the reasons you have a return line at all, so air can purge from the system.
    AIR that makes NO sense where does the air come from on a closed loop system. Maybe if you ran it out of fuel but even then it would be removed when you put fuel in the tank.

    And cutlass the needle seat unless its bad should be holding over 30 psi and unless there is something wrong with the check valves in the pump of the carbs the fuel should not bleed back when the engine is off. The 6 psi check valve in the return line will act as a regulator maintaining 6 psi weather the engine is running wide open or not running at all. The only time the pressure should change is if it exceeds 6 psi. Think about it some more.

  9. #9
    Every adventure comes to an end........ Not Today! wreckingball's Avatar
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    Quote Originally Posted by Hydrotherapy View Post
    I miss him and glad to of spoke in person to a true gent !
    I only knew him via GH and a few phone calls. He was a very intelligent and well spoken man.

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    Yamaha artisan Cutlass's Avatar
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    I just don't think the fuel pumps will pulse hard enough and fast enough at idle to build 6 psi. I could be wrong. I could also easily check it out on my ski. I'd just have to hook up my fuel pressure gauge again and pinch off the return line.

    Another thought I was having was, isn't this solution to a problem that doesn't exist? Or at least a complicated solution to a problem that may or may not be a problem. ZeroClient is concerned about low fuel pressure while cranking and at idle. If I'm not mistaken, his concern comes from his ski being hard to start cold. That easily solved with a primer. A lot of people use them and it makes the ski easy to start cold.
    The other problem I see, is the possibility of needing to rejet and change pop-off. The original built in restrictor has already been engineered in as a fuel pressure regulator and the drilled return mod mimics that exactly...only its an improved updated way. And in a way that doesn't require rejeting and pop-off changes.

    Those are just my thoughts, not saying its a bad idea or it won't work or you shouldn't try it or experiment with it. I do know I'm happy with my primer and 1 to 1.5psi fuel pressure at idle and 6psi at WOT.

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