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Discussion Starter · #1 ·
Hey guys,

I am restoring a 54 Bel Air with the Blue Flame 235 engine. I enjoy working on it, but I ran across an issue which I am not sure what is going on.

Issue: My 54 Bel Air will not start with the mechanical fuel pump only.

Testing:
I added a temporary electrical fuel pump to prime the mechanical pump, and once primed, every thing worked fine and the engine continued to run without the electrical fuel pump. Sounded really good! Apparently the mechanical pump will not prime itself!

I then added a clear gas line to the carburetor to see what was happening, and the fuel is siphoning back from the carburetor (into the fuel tank). The fuel line ends up completely empty!

Question: Why would the fuel siphon back into the fuel tank? Is this a carburetor issue? Shouldn't the mechanical pump prime itself?
 

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Probably leaky valves in the pump. I’d also check to be sure the tank is venting adaquatly.

The pump has a spring loaded diaphragm and at least two valves one inlet the other outlet. Both valves are lightly spring loaded on a seat. When the camshaft moves the pump’s lever to push the diaphragm so it expands this increases the volume in the chamber. At this point the inlet valve opens exposing the fuel line to a pressure reduction from atmospheric. This draws fuel up the line (actually its atmospheric pressure pushing it). This makes the pump self priming. As the cam moves off the pump lobe the return spring in the pump pushes the diaphragm back to its rest position reducing the pump chamber size. This allows the spring loaded inlet valve to close while the increasing fuel pressure opens the outlet valve against its little return spring.

One can see that if there is dirt or wear on these valve seats or the valves that at the slightest opening of the carburetors fuel inlet valve which will happen as fuel evaporates from the bowl or the pump never actually fills the bowl enough to close the inlet valve and the pump valves leak then a syphon will form that will return the fuel to the tank. Next start up you have to crank quite a while to refill the fuel line. But if the pump’s valves are leaky this could take a long, long time if ever. This is why your electric prime works, and that’s fine except eventually the mechanical valves will go out totally.

Diaphram leakage will show fuel running from its atmospheric side vent hole assuming it’s not pumping fuel into the crankcase.

Bogie
 

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Discussion Starter · #3 ·
Probably leaky valves in the pump. I’d also check to be sure the tank is venting adaquatly.

The pump has a spring loaded diaphragm and at least two valves one inlet the other outlet. Both valves are lightly spring loaded on a seat. When the camshaft moves the pump’s lever to push the diaphragm so it expands this increases the volume in the chamber. At this point the inlet valve opens exposing the fuel line to a pressure reduction from atmospheric. This draws fuel up the line (actually its atmospheric pressure pushing it). This makes the pump self priming. As the cam moves off the pump lobe the return spring in the pump pushes the diaphragm back to its rest position reducing the pump chamber size. This allows the spring loaded inlet valve to close while the increasing fuel pressure opens the outlet valve against its little return spring.

One can see that if there is dirt or wear on these valve seats or the valves that at the slightest opening of the carburetors fuel inlet valve which will happen as fuel evaporates from the bowl or the pump never actually fills the bowl enough to close the inlet valve and the pump valves leak then a syphon will form that will return the fuel to the tank. Next start up you have to crank quite a while to refill the fuel line. But if the pump’s valves are leaky this could take a long, long time if ever. This is why your electric prime works, and that’s fine except eventually the mechanical valves will go out totally.

Diaphram leakage will show fuel running from its atmospheric side vent hole assuming it’s not pumping fuel into the crankcase.

Bogie
Thank you for your response. You are correct! It is the fuel pump and I am in the process of rebuilding it. Waiting on parts. I put a clear gas line on the output side of the fuel pump and filled it full of gas. The gas slowly leaked thru the fuel pump. Thank you!
 
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