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Thread: Popoff howto

  1. #1

    Join Date
    Jun 2006
    Peoria, AZ

    Popoff howto

    Disclaimer: I've never done this before so I need help.

    I rebuilt my carbs and now I'm trying to test the popoff so I can put them all back together. When I pump the tester, I am able to build pressure but am not getting a clean needle pop off. The needle starts to leak once I get near the range that I it probably should be popping off in. I am using lube liberally on the needle prior to doing tests. Twice I did get clean pop offs with the needle but the rest of the time I just get needle leakage.

    Am I just overthinking this or should I be getting a clean needle popoff?

    Thanks a bunch for the help, I really appreciate it.

  2. #2
    Moderator RX951's Avatar
    Join Date
    Apr 2005
    League City, Texas

    How to Check Popoff Pop Off Carb Pop Off Popoff

    byrtch, You can fill the fuel inlet hole IN THE MAIN BODY OF THE CARB with WD-40 then insert the end of the pop off gage into or over the hole and pump the gage slowly till the needle pops off it should pop.

    A good needle and seat will "Pop-Off" at the desired setting, drop 1/2 to 1 pound and hold. A very slow leakage from this hold pressure won't affect the performance of your carburetor but a complete drop from the desired setting to ZERO of the gauge will cause very poor carburetor performance and must be cured.

    Never check your "Pop-Off" Pressure with the carburetor dry.

    You can also fill the line of your pressure tester with WD 40 then when you pump this in to test the "Pop-Off" Pressure, the needle will be wet and closer to the condition it will be when running."

    Pop-off pressure testing:
    1) Before testing we must make sure that the carburetor is "wet" inside. If it was overhauled, before testing, it needs to have fuel or WD-40 inside. Otherwise the pressure value will not be accurate.
    2) Connect fuel hose to the fuel inlet of the carburetor.
    3) Start pumping until desired Psi of pressure ise achieved.
    4) Stop pumping. If the pressure holds, the needle valve closes tight. It the pressure drops the needle valve and seat needs to be replaced.
    5) Increase pressure until a sudden drop is noticed. This means that the needle valve opened. The value at which it opened is the pop-off pressure.

    1) Too high pop-off pressure leads to fuel starvation, hot engine, overheating and even seizing, if the engine is run at full power for long periods of time.A good indicator of this will be of course a high temperature reading on the CHT, EGT or a very "clean" white-gray spark plug insulator. This is NOT an indicator that the oil used in the mixture is good and burns clean! It shows that there is TOO much AIR in the mixture and not enough FUEL. The combustion is done in excess of oxygen. Such a combustion generates excessive heat.

    2) A too low pop-off pressure will cause a too rich mixture which is not really dangerous for the engine on the short run but will decrease its performance and cause too much carbon buildup. On the long run, the excessive carbon deposits may fragment, get caught in between the piston and the cylinder wall and cause scoring. If not detected in time, this deposits will sooner or later "load the spark plug" cause a short and an engine out.

    • engine flooding
    • difficult warm engine starts
    • rough mid range RPM
    • lowered maximum RPM of the engine.

    Also, When checking popoff also keep this in mind

    Measuring Fulcrum Arm Height


  3. #3

    Join Date
    Jun 2006
    Peoria, AZ
    Awesome info...thanks a bunch. Makes a lot more sense now, I was definitely testing with everything dry but the needle/seat.

    Tomorrow I think I'll have a lot better results.

  4. #4

    Join Date
    Jun 2006
    Peoria, AZ
    Well, I was finally able to get some time tonight to work on the carbs agian. Basically it boiled down to the pump side screws not being ridiculously tightened down. They were not tightened down very hard at all when I took the carbs apart. It was leaking a bit on the pump side where the gasket is but once I torqued em good and snug the leaking stopped and I was able to get good popoff tests.

    One was right at 43 with no adjustment, and the other two were 55 and 62 so I had to cut some coils on those but I'm in business now!

    Thanks for the help!

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