Mechanical Fuel Pump Question
Lead Driver
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Lead Driver
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If Galileo drops a gold bar and a small stone off the tower of Pisa they both get to the ground at the same time, no matter the cost.
Lead Driver
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Back to our bargain brand mechanical fuel pump. The big diaphragm spring is what controls the output pressure. The actuating arm pushes on the spring, which in turn moves the diaphragm. When the output pressure reaches a predetermined value, the spring collapses and limits diaphragm travel and thus pressure. This big spring directly controls output pressure. It would not surprise me one bit if the wrong spring was installed at the Fling Dung factory. Or maybe the spring dimensions are slightly wider and it didn't seat fully, increasing the effective length and bumping up the pressure. QC seems to be non-existent with some Chinese stuff. That could be why this new problem suddenly started after installing this new pump.
Lead Driver
Joined: Apr 2002
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I'd be careful not to base what's possible on whether you've seen it or not. For example, I'm pretty sure black holes exist, but I've never seen one...
Back to our bargain brand mechanical fuel pump. The big diaphragm spring is what controls the output pressure. The actuating arm pushes on the spring, which in turn moves the diaphragm. When the output pressure reaches a predetermined value, the spring collapses and limits diaphragm travel and thus pressure. This big spring directly controls output pressure. It would not surprise me one bit if the wrong spring was installed at the Fling Dung factory. Or maybe the spring dimensions are slightly wider and it didn't seat fully, increasing the effective length and bumping up the pressure. QC seems to be non-existent with some Chinese stuff. That could be why this new problem suddenly started after installing this new pump.
Back to our bargain brand mechanical fuel pump. The big diaphragm spring is what controls the output pressure. The actuating arm pushes on the spring, which in turn moves the diaphragm. When the output pressure reaches a predetermined value, the spring collapses and limits diaphragm travel and thus pressure. This big spring directly controls output pressure. It would not surprise me one bit if the wrong spring was installed at the Fling Dung factory. Or maybe the spring dimensions are slightly wider and it didn't seat fully, increasing the effective length and bumping up the pressure. QC seems to be non-existent with some Chinese stuff. That could be why this new problem suddenly started after installing this new pump.
It's possible that the float needle valve was leaking before but a small leak in the diaphragm compensated for that. Or when the diaphragm failed a piece of it made its way to the valve seat and is preventing shut off. Yah he said there is a filter but stuff can bypass a filter if not installed correctly.
I highly doubt that Fling Dung accidentally installed super springs.
More likely the valve seat is not sealing and that can be due to age and alcohol in the fuel. How many miles or years on the carburetor is my question?
Lead Driver
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The spring force has nothing to do with the fuel pressure. The spring has to be strong enough to keep the cam follower snug to the lobe. After the high point passes the follower the suction stroke is finished. The follower stays snug to the cam. If it doesn't it's gonna slap. Slapping causes damage so the spring has to be at least that strong. Any stronger than that makes no difference on the delivery pressure because the stroke is cam follower limited. It's volume differential per stroke and strokes per second that creates the pressure.
You need 5 psi at idle and a volume of 1 pint in 20 seconds..
You need 5 psi at idle and a volume of 1 pint in 20 seconds..
Lead Driver
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If there's a diaphragm spring is it helping on the suction or discharge stroke?
Lead Driver
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I looked, it does both. More like a dashpot.
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From: Northern Kentucky
In your initial post, you state that the fuel pump has a fuel return line. Why does a carbureted 351W have a fuel return line? I thought that was only on the hot fuel set-ups. School me here.
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Trailering
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Apparently the 80's vintage F-250 351W (and maybe the 460 as well) with the >8500 lbs. GVWR option utilizes a fuel return line even with a mechanical pump, presumably to handle extreme heat conditions.
Lead Driver
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If this truck has a return fuel line then the over pressure at the carburetor would be even less likely. Pointing to the float needle valve as the likely issue there.
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Trailering
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Finally back on it after a weather delay and (drum roll please) fuel pressure is fine at about 6 psi. If I clamp off the fuel line just before the carb the engine will go from a rough lope to a nice idle for a while, until it runs out of fuel. After restarting nice idle for a bit, then rough, so certainly seems confirmed that float bowl is over-filling.
So good news is that I can leave the pump alone, bad news is that I guess I get to learn about Holley carbs about 50 years too late. But YouTube is my friend, looks like the needle/seat assembly is very easy to remove.
One thing I've noticed is that after I shut down and remove the fuel level sight plug, no fuel comes out until I rock the vehicle -- which is what I think indicates a correct level. Under the circumstances I kinda expected fuel to come pouring out(?)
So good news is that I can leave the pump alone, bad news is that I guess I get to learn about Holley carbs about 50 years too late. But YouTube is my friend, looks like the needle/seat assembly is very easy to remove.
One thing I've noticed is that after I shut down and remove the fuel level sight plug, no fuel comes out until I rock the vehicle -- which is what I think indicates a correct level. Under the circumstances I kinda expected fuel to come pouring out(?)
Lead Driver
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Trailering
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Lol. You really wanted me to have to replace that pump again, didn't you? 
Ok, running like a top now (well a 38 year-old abused top anyway), but the secondary problem was not the float bowl needle valve, or at least not the expected one. I removed it, seemed fine, cleaned it anyway, reinstalled and set fuel level and... no difference. But the symptoms screamed overfueling so I thought about it a bit and considered that this truck (exclusively a snow plow truck as mentioned above) sits a lot, and I bet those secondaries never even open. So, pulled the sight plug on the secondary float bowl and fuel came pouring out. Hmm. Removed the needle valve and it was grunged up. Cleaned, reinstalled, set level and problem solved. So yes, the secondary problem was a secondary problem.
Man this job brought me back to my teenage years -- no fault codes, no scan tools, just basic diagnostic skills. It was almost fun.
Thanks again for everyone's help.
Ok, running like a top now (well a 38 year-old abused top anyway), but the secondary problem was not the float bowl needle valve, or at least not the expected one. I removed it, seemed fine, cleaned it anyway, reinstalled and set fuel level and... no difference. But the symptoms screamed overfueling so I thought about it a bit and considered that this truck (exclusively a snow plow truck as mentioned above) sits a lot, and I bet those secondaries never even open. So, pulled the sight plug on the secondary float bowl and fuel came pouring out. Hmm. Removed the needle valve and it was grunged up. Cleaned, reinstalled, set level and problem solved. So yes, the secondary problem was a secondary problem.
Man this job brought me back to my teenage years -- no fault codes, no scan tools, just basic diagnostic skills. It was almost fun.
Thanks again for everyone's help.
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