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  1. #1

    Carb Failing Pressure Test

    96 Mikuni SBN 40-I. Trying to track down a fuel delivery problem. The pump diaphram test passed at 4 psi. The pump valves are not stuck, but fail to hold pressure and release vacuum as specified in manual. Needle/Seat did not hold 10 psi and was leaking at the o-ring around the seat body. I replaced the o-ring and it no longer leaks at the o-ring nor the N/S, but it will not hold pressure and I can not seat bubbles ANYWHERE. Where can this leak be ocurring on the N/S test? I only know of the two that I already mentioned and are now clear. Could it be the valves, but they are on the pump side not the regulator side?

  2. #2
    I spray the entire carb out side with wd when ever I am testing them, and also make sure you tester is not leaking and what ever you may have blocking off the return

  3. #3
    I have done many pop off and N/S tests and had never seen this before. I did check the tester for leaks and no problems. I simply hold my finger over the outlet nipple as always. I may submerge the carb and see where the leak is coming from. Can the pump valves not sealing off cause this? They are on the pump section and the N/S is on the regulator side, but they do connect. The valve on the intake side did not open well under pressure and did not completely hold on vacuum. The outlet side did not hold completely on pressure, but that nipple is blocked on this test. I have never seen this before.

  4. #4
    I submerged the carb in gas and pressurized it. The leak is at the main fuel supply nipple. I blocked off the outlet nipple and put my line on the inlet nipple. It leaks more at 10 psi than 20 psi. I think this goes back to the faulty pump valves that did not seal and release as specified in the manual. Am I supposed to block off the main fuel supply nipple as well? I never have before and the valves should not let the pressure in the regulator section to the pump section. Am I doing something wrong or does this show a problem?

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