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02-06-2021, 05:34 PM | #1 |
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Fuel Pump Bubbles
Hi guys. I just rebuilt and refitted the mechanical fuel pump on my '54 Merc. Car starts and idles fine, but stumbles on acceleration. I have since noticed some large air bubbles coming from the spot noted in the attached photo. Just a couple of bubbles at idle, more when I rev the motor. The glass bowl stays about half full, but I'm thinking its lowering the fuel pressure to the carb, especially on acceleration. The bubbles seem to be coming from the intake port, so is it safe to assume I have an air leak somewhere upstream from the pump? Or could it be a leak in the pump?
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02-06-2021, 08:37 PM | #2 | |
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Re: Fuel Pump Bubbles
Quote:
Remove fuel line inlet at tank and outlet to carb, seal the openings up and see if the line holds vacuum with a MITY-VAC. If good, then possibly fuel pump diaphragm. Is the diaphragm ethanol rated?
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02-07-2021, 08:42 AM | #3 |
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Re: Fuel Pump Bubbles
The diaphragm is new and ethanol rated. The car is also on an ethanol-free diet, as we have the luxury of buying ethanol-free gas at the local gas stations here in Florida.
A couple other notes - I do NOT smell gas from the oil fill tube or the dipstick. I see no gas leaking from the pump or fittings and all the fitting are dry to the touch. I see the bubbles as soon as I start the motor, so I know its not a gas boiling problem. Wonder if it could be sucking air from one of the fittings? Do you guys tape or putty your fitting threads or leave them bare?
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02-07-2021, 10:01 AM | #4 | ||
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Re: Fuel Pump Bubbles
Quote:
Quote:
What is the fuel pressure at the carb?
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02-07-2021, 10:28 AM | #5 |
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Re: Fuel Pump Bubbles
Looking again -
The air bubble appears to be right at the fuel inlet. Maybe change clamp styles as a worm clamp is not the best for fuel lines. See Below -
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02-09-2021, 05:23 PM | #6 |
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Re: Fuel Pump Bubbles
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02-09-2021, 08:12 PM | #7 |
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Re: Fuel Pump Bubbles
Well ...
I see you have a locking gas cap. The cap on your setup was vented from the factory. The cap has to be vented for the system to work correctly The vent you repaired is for expansion/contraction of the actual fuel in the tank (pre-EVAP). The cap is a one-way vent. It only opens to the atmosphere as the fuel level is depleted to allow even draw. If pressure lowers/builds in the tank (outside temp change), it is relieved by the vent system you repaired, not the cap. With no cap vent it is possible for the tank to collapse if the pump is strong enough. You were lucky to notice the bubble(s).
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02-10-2021, 11:32 AM | #8 |
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Re: Fuel Pump Bubbles
My chassis parts catalog shows the cap was non-vented. Only the station wagon (model 79) shows having a vented cap.
There is no check valve on the vent tube that I cleared, so it provides for pressure equalization as fuel is consumed. Only if it is blocked, as mine was, will it build negative pressure as the fuel is used. And in my case it eventually starved the pump of fuel and started sucking air instead.
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02-10-2021, 02:59 PM | #9 |
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Re: Fuel Pump Bubbles
Sorry I bothered you.
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02-10-2021, 09:52 PM | #10 |
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Re: Fuel Pump Bubbles
No bother at all. Thanks much for the advice. Always appreciated!
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