air in fuel line

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    I am having a problem that I think is fuel related. The engine is running rough, I have checked and it looks like there is spark to all of the plugs. While sitting if I step on the accelerator peddle you can see bubbles in the fuel bowl on the fuel pump. I have checked the connections and it does not look like there is in leakage in the fittings. Thoughts would be appreciated.


    Hi Syd,

    Modern fuels can attack and deteriorate the diaphragms in original fuel pumps. I also found that a lot of ignition problems disguised themselves as fuel issues. If the fuel pump is OK I’d then check the distributor point gap, then swap out the condenser, next the coil, then the rotor and then the cap. At some point it all ought to start working again! Good luck!


    It is possible that you have air leaks between the tank and fuel pump, however it is more likely that you are seeing bubbles of vaporized fuel in the bowl. When you step on the gas the fuel pump tries to suddenly accelerate all the fuel in the line between the pump and the tank, this momentarily suddenly drops the pressure in the sediment bowl to a level less than the vapor pressure of the lighter constituents of the fuel and fuel vapor bubbles are formed. The bubbles will likely re-liquify before getting thrashed around in the fuel pump creating more vapor. The fuel pump was designed to deal with a lot of fuel vapor. This happens with higher temperature, rpm and throttle. It doesn’t mean that the fuel pump isn’t delivering adequate liquid fuel to the carburetor.

    Yesterday I chased a similar problem on a 1960’s brand X – turned out to be ignition timing.

    I would first check ignition timing. If you have dual points check timing on both sets. Then I would pull each plug and make sure none are fouled and they have the correct gap. Then I would check the distributor points to make sure they aren’t burned and have the correct gap. Double check the rotor to make sure there isn’t any evidence it has been hitting the metal tabs inside the cap. That would be a sign of worn distributor bushings.

    If the roughness occurs only after it is warmed up on a hot day it could be vapor lock related, you might read my boring tomes in recent PASB’s. I see vapor bubbles pulsing in my sediment bowl without any vapor lock, and when it is in the process of vapor locking it isn’t so much a matter of running rough as it is just slowing down and dying as it leans out, or dying suddenly after hitting the throttle.


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