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To start, truck was running great before I started replacing stuff.
79 f350 with 351m
I put in a new rear tank
new sending unit
new stainless fuel lines
new clear inline filter
new tank switch
(holding off on saddle tank as i have to fabricate a carrier for it. )
truck starts strong and idles at hi or low idle strong .
at some point it starts to sputter and shuts down.
When i first start the truck, the fuel filter fills up fast, over time it starts to empty. Maybe its getting TOO empty. But why?
my new tank has no emissions vent, so to be sure this wasnt the problem I disconnected the filler neck vent. I wanted to make sure i wasnt creating a vacuum.
is it possible that the new pickup on the sender is so clean, and the new fuel lines are so clean that its messing with my fuel air ratio?
The air you see in the filter is normal, it's trapped above the exit to the carburetor, and it lives at the same pressure as the fuel does. The fuel pump pushing fuel into the filter will do that. If a metal filter, you'd never know it has air trapped in it. You also do not need two fuel filters in line, once the first one stops flowing, the second one gets no more fuel. If the older second one is already dirty and plugged, adding a new one in front of it does no good.
What size is the sending unit 90* out port elbow? It is 5/16 or 3/8? And what size of rubber line did you use, it looks awful big. X2 run it on a 1 or 2 gal pony tank setting on the ground by the tire. X2 on ditch (the metal solid) double filter set up. And that header is in close proximity of the steel and rubber fuel line. Just saying, heat soak fuel boil.
Usually mechanical fuel pumps either work or do NOT work. I have never seen a intermittent one. Maybe carb rebuild time? X3 the clear filter will not stay FULL of fuel.
What size is the sending unit 90* out port elbow? It is 5/16 or 3/8? And what size of rubber line did you use, it looks awful big. X2 run it on a 1 or 2 gal pony tank setting on the ground by the tire. X2 on ditch (the metal solid) double filter set up. And that header is in close proximity of the steel and rubber fuel line. Just saying, heat soak fuel boil.
Usually mechanical fuel pumps either work or do NOT work. I have never seen a intermittent one. Maybe carb rebuild time? X3 the clear filter will not stay FULL of fuel.
i replaced everything with identical parts (except for the emissions vent on tank)
all line and hose fittings are 3/8” except for the line from pump to carb which is smaller.
so in theory everything is the same. Is it crazy to think that maybe the pickup filter on the sending unit is so clean its allowing more fuel and im running to rich?
No the too clean pick up tube theory does not make sense to me. No matter the amount of fuel available (in the tank) you (should) get a set amount delivered from the mechanical fuel pump every rotation of the eccentric cam.
After you run it on the pony tank that will rule out your new fuel delivery system (tank lines). And it sounds like a mech fuel pump change is in your future.
You say you’re running a non evap tank w evap fuel fill neck and cap? If so, that would be my first suspect. The non evap allows air back in the tank from the vented cap whereas the evap does so through the charcoal canister to the vent in top of the evap fuel tank. Try running w/o the gas cap on. If that helps, you’ll need to change the fill neck to non evap or replace the evap tank. A good trick, if running an evap tank but removing the charcoal canister, is to put a small engine fuel filter on the vent line for the tank. That will keep dust and dirt out of tank and let it breathe.
You say you’re running a non evap tank w evap fuel fill neck and cap? If so, that would be my first suspect. The non evap allows air back in the tank from the vented cap whereas the evap does so through the charcoal canister to the vent in top of the evap fuel tank. Try running w/o the gas cap on. If that helps, you’ll need to change the fill neck to non evap or replace the evap tank. A good trick, if running an evap tank but removing the charcoal canister, is to put a small engine fuel filter on the vent line for the tank. That will keep dust and dirt out of tank and let it breathe.
before i swapped tanks the evap hose was cut and just open to atmosphere.
after the swap i tried running with the filler neck vent hose disconnected so there would be no vacuum.
I would recheck the metal line size as that hose (3/8?) Looks too big.
Take a open end wrench 3/8 & 5/16 and see what one fits over the line bet 5/16.
My 81 F100 has 5/16 line and I tried using 3/8 hose with 2 clamps, from pump to a gas can sitting on the inner fender and could not pull fuel.
5/16 hose and all has been good for 3+ years on the road.
Dave ----
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