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Old 05-05-2013, 10:27 AM
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Electric fuel pump to prime???

I read the old posts on adding a low capacity electric pump to prime the carb after sitting. I understand to mount it at the tank and use either a relay or fuse.

Here's a question about this we debated at length at yesterdays show. If you leave the mechanical pump in place---isn't there a risk of diaphragm rupture and filling the engine with gas? A manual switch or timed relay would cut this off.

Does the mechanical pump seal the diaphragm chamber from the engine? We thought so but one was sure.

Another interesting question that came up is the evaporation rate of gas to ethenol?? A local mechanic said 3 times as fast and maybe faster if there's a lot of heat transfer from the engine?

Help settle the argument.

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Old 05-05-2013, 11:22 AM
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You use one or the other, You shouldn't need both,, Your car should start after sitting for a while with either one..If you have to prime it to get it started something else is wrong..
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Old 05-05-2013, 11:42 AM
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they sometimes use a elect pump on the old flat head engines for they have vapor lock issues.. but it's on for a few seconds and not running down the road. but if u want to use one i highly recommend u do a bypass on the fuel line and put in a ck valve on the manual side. i feel there is too much restriction in the pump to allow it to be in line and the mach pump pulling through it.
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Old 05-05-2013, 11:53 AM
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The mechanical pump has a valve on the discharge side that is controlled by the pressure changes. This valve closes when the diaphragm is in vacuum (pulling gas from tank). Then this same valve opens when the diaphragm is in pressure (pushing gas to the carb). This happens anytime the engine is cranking or running.
The valving in the pumps will likely hold any conflicts between the (2) pumps at 7 psi but I wouldn't depend on it. Separate check valves are needed so the pumps do not see pressures on the discharge they are not designed for.
One more thing. IMO When an engine is idle long enough to evaporate the gas in the carb, I like to crank it for few seconds to get the oil pressure up before it runs. I just crank the starter until I see the oil pressure gauge move, pump the pedal few times and wham.
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