Hi guys I need some help .
had plans Saturday to hit PNP , woke up with a bad stomach flu so stayed home , woke up today felt normal so figured I'd head out at 7:30am got about 5 miles from my house and truck started sputtering
when i stepped on gas , figure i was low on gas (new tank) so stopped and dropped 16 gallons of mid grade in . she fired right up got about a block sputtering again ,sheet ! so headed back towards home . i noticed if i just barely touched gas she ran ok . if i stepped on gas at all she sputtered , figured maybe my vent cap wasn't venting . took it off no go same deal .

got home driving her the slowest i ever have , lifted the hood and noticed the glass fuel filter between carb and stock pump had air pocket in it (normally full) ok so maybe the stock pump can't suck the fuel from all the way back under bed . ran to local Autozone picked up a Mr gasket 7psi electric pump . mounted it right by tank ran wires and a safety on/off switch on dash .
i left the stock pump as the Mr gasket said it could be used as a secondary pump along with the stock pump .

STILL SPUTTERING so i gave up for the day as it's 100+ out .
time to think about it a little .
thanks in advance guys

oh truck only driving about 10 miles since Mustang tank installed but it ran fine then . all new 3/8 metal lines run

 

So the only thing you've done is the fuel tank?
No restrictions in any of the lines? All lines are pretty much straight no loop-d-loops? (Loop-d-loops is a technical term used in the advance studies of automotive repair, must have at least a Masters Degree in order to use it) Now back to the problem.
I went back and looked at your build, this is the only area that concerns me.

That line should be straight it looks like a flat spot on the metal line. Can you do away with the Loop-d-loop?

 

Thanks Robert , when i was under there doing the Electric pump I saw that as maybe a problem too , so i cut the loop out and re double flared the line .
I am going to take the stock mechanical pump out of the loop to see if that's it , only other thing i can think of is vapor lock ?
when i first put tank in and test drove ambient temperature was 85 ish now 100+
i'll try driving her about 7-8pm tonight when temps drop a little .

 

Is the tank vented?

 

NO tank not vented
but cap is , i even tried taking cap off to see if vents in cap were blocked .

 

Ok, looks like you already ruled that out.

Are you sure the accelerator pump in the carb is good?

I had a Chevy years ago that exhibited similar symptoms and would only move if you feathered the gas pedal lightly.

Turns out the accelerator pump had a torn diaphragm and was starving for fuel until the high speed circuits kicked in.

Just a thought.

 

carb is about 1 year old (new ,not rebuilt) truck ran perfect before tank relocation , so I am thinking something i changed is cause .

 

Maybe the '68 mechanical pump didn't have enough "suckage"? Dunno if Dentside mechanical pumps are internally different since they were offered with rear-mounted tanks. Larger internal diaphragm?

On the electric pump, they do a better job at pushing fuel than pulling it. I'd look to mount the electric pump as close as possible to the tank like on that crossmember.

 

I'm betting on the stock fuel pump not having the head pressure to pull the fuel up to your carb. The stock tank behind the seat helped the old style pump with gravity feed. The mustang tank is not only way back there, it's way below the carb so the pump is working too hard. Hio Silver is correct. Mount the electric pump as close to the tank as possible. They don't suck very well but they'll push the fuel 10 feet in the air!

 

Just curious, what carb are you running?

 

holley 3310s
thanks guys going to do some testing tonight . I'll let you know
mech pump was an Autozone one for a 67 tbird 390 , so maybe it took a Sheep ?

 

Why are you certain it's a fuel issue?
I'm running a mechanical pump with a Mustang tank, also. I have no problems getting fuel.

 

seeing air and bubbles in glass fuel filter between carb and mech fuel pump .
i don't remember ever seeing air and bubbles before

 

If you are seeing this and there are no leaks on the output side of the fuel pump, then it looks like you have a bad connection on the suction side, anywhere from the fuel tank to the fuel pump. I would check this first before moving anything.

 

thanks 84espy
will check , no leaks but maybe it's sucking air in . all new rubber and metal lines .