Long Post-Fuel Issues - Too much Pressure?????

[71C10SWB]

Guys,
I am running out of answers.
I'll give you a rundown of the truck and recent upgrades:
71 C10 SWB, 350, TH350
Rebuilt 350 1 month ago.
New Fuel Pump
New Holley 600 w/adjustable Fuel Bowls (I'll explain later.)
New 3/8" fule line from tank to Carb
New Fuel Sender installed about the time trouble began.

Here's the deal: I've had the truck since March. It had a whipped 350/Powerglide combo in it. The engine had an Edelbrock 600 Carb on it and it leaked all over. The fuel sender didn't work and it was originally a 6-cylinder so it had 5/16" line from the tank to the carb.
Back in July, the Edelbrock carb started running worse than normal and got to the point whee it didn't want to run. I had a 750 Barry Grant Street Demon in the garage and slapped it on there to see if it was any better (I knew it was a little big, but wasn't too worried about it). It ran great for a day or two and then started leaking too (it had been sitting for quite a while so I wasn't surprised if the seals were all dried out.) I rebuilt an old Holley 600 that I had and put it on there. It ran good for a day or twom, but it didin't want to idle and the truck was gradually running worse.
I got sick of jacking with my old carbs and went down to the local Autozone (more due to convenience than the joy of shopping "The Zone". ). I bought a brand new Holley 600 with electric choke and thought my problems would be solved. Then I realized that it didin't have adjustable floats. It didin't take long before it was running like crap too. I knew the engine was really whipped and I figured I thought maybe the vacuum signal was so bad that it might be affecting the operation. since I couldn't keep it running, I decided to go for broke and do it right. Especially since I don't want to be stranded this winter.
I had my buddy's shop rebuild the engine. It has a mild cam (perfomer-like), new Performer RPM intake, new fuel pump, water pump, HEI distributor, etc... I also bought a new aluminum radiator and swapped in a TH350 that I had.
Everything ought to run ,ike new, right? Wrong!
The carb is giving me fits. I swapped in the rebuilt fuel bowls from my old 600 with adjustable floats becuase I thoguth maybe the float level wasn't right on the non-adjustable one's.
Here's what I have observed:
When I pull the fuel line to the carb after it's been running, it seems to have a lot of pressure built up in the fuel line. It actually spews out of the line when I start loosening the clamp.
Last night, I could see that I was getting a little bit of a leak around the sight plug on the rear bowl of the carb. I pulled it out to put on a new gasket and fuel came spewing out like it was under pressure.
I replaced the fuel line from the pump to the carb becuase there was an area that I thought might be questionable. When I pulled the line from the pump, there was fuel running out of the pump. I expected that to happen for a second or two, so I went and bent up a new line. Whan I came back, it was still trickling out. I've never seen that. It's almost like it's bypassing something in the pump.

I guess my main question is: Where is all of this pressure coming from and why????

[rwolf9653]

Is it possible that you need to fix/clean the vent for your gas tank? Just sounds to me like there is a constant suction from the tank that needs to be broken. Do you have a vented cap, or any other way to vent the tank?

[71C10SWB]

Well, I thought about that. It is a '71, so it had the charcoal canister system and the line is all still there and hooked up. I can't guarantee that it doesn't have a mud-dawbers nest in it somewhere. the cannister has been removed, but the line is still there and the end is vented to a dry area under the hood.

It would seem like a vent problem would cause a fuel shortage instead of a high pressure situation though.

On a side note, I thought it might be vapor locking, so I covered the steel line between the pump and carb with 5/8" Heater Hose. Seemed to help a little, but I'm not sure.

[hayhauler71]

Was there any dirt or rust in the fuel bowls when you removed them? Or possibly try a fuel pressure regulator and set it at about 6 to 8 psi. Also Holley carbs are famous for sticking floats and will not pass trash through them so make sure your filters are clean or use the clear plastic ones to monitor for trash.

[71C10SWB]

I have the clear plastic filter on there and it's been really clean.
Basically, with the exception of the tank itself, the entire system is new.

I understand the whole float sticking issue. That was my first thought. I just don't get this high pressure build-up. When I pulled that sight plug out last night the fuel didin't just run out, it spewed out like there was pressure.
It's just a basic chevy pump in a standard system. It's got me pulling my hair out.

I've worked on cars for years and this is the first time I've seen this.

The other thing I thoght might be happening right after the rebuild was maybe the aluminum radiator was producing a lot more heat and cuasing the fuel to boil so I covered the line. that sounded logical till I pulled the line loose last night and it appeard to have a pressure build up in the system.

[mclairmo]

Originally Posted by CutlassSMan

IWhen I pulled that sight plug out last night the fuel didin't just run out, it spewed out like there was pressure.

This comment really concerns me because your hoses and lines are designed to run very low pressures, less than 8 psi and you clearly have very high pressures, like an EFI system. This could lead to a major leak and an engine fire. Are you running an electric pump or very high pressure mechanical pump? Gas should not spew out the sight plug at all. The fuel pump is what produces pressure; problem must be with the fuel pump somewhere somehow. Focus on that. I would try replacing it with a standard store bought normal pressure fuel pump and make sure it is installed correctly with the correct length fuel pump push rod.

[71C10SWB]

It is a new, standard store bought (Car-Quest), Chevy, mechanical pump......nothing special.

I'll get a pressure gage on it and see what that says.

[rwolf9653]

Originally Posted by CutlassSMan

New Fuel Sender installed about the time trouble began.

Unless anyone elses suggestions help you figure it out, I would go ahead and try another sending unit, just for s&*Ts and giggles. You kind of said that this is where the problem began. I dont know what could be wrong with it seeing as how the are usually work/dont work, but it is worth a shot.

[mclairmo]

I would install a fuel pressure gauge between the pump and the carb and see what it reads. Should be no more than 8 psi for a Holley to run right. If it is more then you know the fuel pump is off and should be replaced. If 8 psi or less, then you know that you do not have a pressure problem. I got mine from Summit racing for cheap as part of their hard fuel line kit for my Holley 3310. Works awesome and lets me know what kind of fuel pressure I have. Also, what kind of fuel pump are you running? Make sure it develops no more than 8 psi.