Quote:
Originally Posted by ford1
the fuel cannot be sucked out of the carb by the fuel line draining back past the fuel pump,
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Even though the fuel can't be sucked out of the float bowl, it can drain clear back to the gas tank leaving the pump high and dry and, unless the pump will pump air, make it hard to refill the carburetor bowl after the fuel has evaporated out of it from sitting too long.
I think the check valve either just before or just after the pump may help keep the pump "primed" so that, even if the fuel is gone from the bowl, it can be quickly refilled and start.
Of course, if the pump is working right, it has it's own check valve that should prevent back flow. It would seem that the pump may be defective if it's allowing fuel to drain back.
One more thought. A very small leak in the fuel line from the tank to the pump would admit air while sitting to allow the fuel to flow back as well. No check valve, either in the pump or otherwise would prevent that. I've had that before. Such a small leak would not be detectable under normal operation because the fuel in the line from the tank to the pump is under vacuum such that it wouldn't be pushed out of such a leak. But, if such leak is low enough in the line, it might be found while just sitting. But, if it's high, like just before the pump, you'd have to actually pressurize the line while full of fuel to find it.
My 2 cents.