Post by Scoutpilot on Sept 5, 2015 6:12:59 GMT -5
The engine is warm and idling but you see air bubbles in the fuel pump. There is only one cause, but several possible sources of this problem. Straight clamping of rubber tubing to straight steel tubing is not a good idea, except in a temporary field repair. When the hose clamp tightens on the rubber it does not do it evenly around the circumference of the metal tube. The rubber will bunch up and allow small passages to form. These quickly become air leaks. The use of barbed brass fittings or at least some flaring of the end of the steel tube is recommended. Properly designed and manufactured fuel lines are strongly recommended here. And I mean from the tank to the pump and the pump to the carb. The more obvious source of the problem is a bad bowl gasket or a less-than tight bail or even loose screws in the body of the pump.
Another possible source is any old rubber tubing. With age, it dries out and begins cracking. Eventually, these cracks reach the interior of the tube. Since the fuel is being pulled by the pump you won't notice fuel leakage, just the air bubbles in the bowl.
And one more thing about glass bowl pumps and filters. Do not be concerned if the bowl(s) do not fill completely with fuel. As long as there is enough fuel in the pump to cover the mesh filter on top of the pylon you're golden. A small amount of air in the bowl causes needed back pressure on the outlet valve in the pump for maintaining pressure in the line. While there are no valves in the filter, the same air pressure is needed here as well.
Another possible source is any old rubber tubing. With age, it dries out and begins cracking. Eventually, these cracks reach the interior of the tube. Since the fuel is being pulled by the pump you won't notice fuel leakage, just the air bubbles in the bowl.
And one more thing about glass bowl pumps and filters. Do not be concerned if the bowl(s) do not fill completely with fuel. As long as there is enough fuel in the pump to cover the mesh filter on top of the pylon you're golden. A small amount of air in the bowl causes needed back pressure on the outlet valve in the pump for maintaining pressure in the line. While there are no valves in the filter, the same air pressure is needed here as well.