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Old 12-05-2009, 09:39 PM
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How long does your pump hold pressure?

My mechanical fuel pump (Edelbrock 1725) holds pressure about 1 minute after engine shut down. I believe fuel drains past fuel pump check valves overnight. I have seen posts where pump holds pressure several hours post engine shut down. Should I pull pump and clean internal check valves?

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Old 12-05-2009, 11:06 PM
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Nope. This is perfectly normal. If there was a problem you`d know it as it would give you a much bigger clue such as flooding after shut down.
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Old 12-06-2009, 04:03 PM
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Quote:
Originally Posted by DoubleVision
Nope. This is perfectly normal. If there was a problem you`d know it as it would give you a much bigger clue such as flooding after shut down.
You are right.

Curiosity got the best of me so I dissected the fuel pump.

The check valves are the same as a scuba mask; no springs.

My large downstream fuel filter would sometimes drain back into the tank after a prolonged shutdown; took a few cranks to fill it up.

I am relocating the fuel filter upstream of the pump to keep the fuel pump check valves clean as well as keep the fuel filter full post extended shut down.
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Old 12-22-2009, 02:55 PM
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Edelbrock quick start

I moved medium volume fuel filter upstream of the mechanical fuel pump.
Now fuel filter elevation is below fuel tank elevation; therefore, fuel filter remains full after extended shut down.
The fuel pump check valves don't prevent fuel back flow indefinitely.
Fuel filter is in a cooler location which minimizes vaporization.
Upstream fuel filter also helps keep fuel pump check valves clean.
I no longer have to crank engine long enough to fill fuel filter and carb bowls.
Sometimes edelbrock fuel bowls will evaporate dry after extended shut downs.
My engine starts much quicker now post extended shut down.
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Old 12-23-2009, 09:15 AM
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Mine holds for days. It would not be unusual for me to not use the car for 2 or 3 days and the gauge still reads 4-5 lbs
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Old 12-23-2009, 09:44 PM
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Quote:
Originally Posted by T-bucket23
Mine holds for days. It would not be unusual for me to not use the car for 2 or 3 days and the gauge still reads 4-5 lbs
Wow!

What pump?
What carb?
Any add on check valve?
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Old 12-24-2009, 06:24 AM
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What you're probably running into is fuel evaporation, after a warm engine shut-down. The fuel percolates or evaporates out of the carburetor bowl vents, and as it does it's replaced through the carburetor fuel inlet until the residual fuel pressure is gone. In extreme cases, the fuel will boil out into the intake, flood the engine and prevent a fast hot start. On an engine that's not enclosed with fender wells, hood, etc., as in a Hot Rod, the carb cools a bit faster and will hold pressure longer. Modern gasoline is formulated for a closed (fuel injection) system, and evaporates MUCH faster than the old type leaded gasoline...
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Old 12-24-2009, 09:08 AM
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Quote:
Originally Posted by 001mustang
Wow!

What pump?
What carb?
Any add on check valve?
Holley mechanical pump and a Holley 750 blower carb. As long as everything is working, I dont see why the pressure would bleed away. The pump has a valve to hold pressure and the needle and seat in the carb is on the other end to seal it.
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Old 12-25-2009, 07:05 AM
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As the fuel level in the fuel bowl drops (evaporation/percolation), so does the float...allowing the needle to drop down from the seat...allowing fuel to flow into the bowl...
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Old 12-30-2009, 11:32 PM
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Quote:
Originally Posted by 345 desoto
Modern gasoline is formulated for a closed (fuel injection) system, and evaporates MUCH faster than the old type leaded gasoline...
Amen!

Haven't found supporting data, but modern gasoline sure does seem to evaporate at lower temperature than the old gasoline.
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