Help trying to fix vapor lock run into other issues

[Dlm11]

72 c10 350 mechanical fuel pump. I’ve been fighting vapor lock for awhile. I recently installed an AC delco gf423 fuel filter with a vapor/fuel return line. To the best of my knowledge it has an orifice in the return side of the filter. Also has an under bed tank. It starts,idles, runs fine in the driveway. When I drive it I get about 1/2 mile and it starts feeling like it’s starved cutting out all but shutting off and sputtering and I barely make it home. I pull in the driveway way and it runs fine. My fist thought is is the filter running to much fuel back to the tank and the second is that the filter is 5/16 and all my fuel lines are 3/8 between that and the fact it’s returning fuel to the tank do I need to raise the level in the fuel bowls and possibly adjust the carb? Any help will be greatly appreciated.

[geezer#99]

What carb?
What intake?
Do you have headers?
What choke?
When first started cold, how long do you run it before driving?

[RustyPile]

One possibility is the fuel pump is weak and can't provide enough fuel delivery, or the filter is returning an excessive amount of fuel and there's not enough left to feed the carburetor.. If memory serves me, the '72's fuel pump provided return through a 3rd line that is plumbed back to the tank. An "extra" return line is not needed.

[Ironangel]

I agree with Rusty, the 72 fuel pumps have a 1/4" return barb for the return. I havent used the return pumps or the return lines in either of my 72's for years. I use the 3/8" lines in their factory locations with a 1/4" thick base gasket under the carb. I've never had vapor lock either and I run headers. I do run a canister filter between the pump and the carb as well as that tiny paper filter in the Quadrajet. So, I'm thinking you may very well have a low float issue with a return line issue compounded by that weird filter. The 71-72's also had a 5/16" vent line that may confuse you that terminated down there with the other two lines. The vent line used rubber vacuum hose from the steel line to a charcoal cannister behind the battery. The 3/8" steel lines are the gasoline lines, the other two smaller lines a return and a vent line. There's a sketch of the lines here. https://www.lmctruck.com/1967-72-che...-and-clip-sets You may also be building a vacuum in the tank if the cap or tank vents become clogged. Loosening the gas cap and listening for whoosh of air getting sucked in or out reveals a clog in the venting...

[Dlm11]

Originally Posted by geezer#99

What carb?
What intake?
Do you have headers?
What choke?
When first started cold, how long do you run it before driving?

Holley style carb made by summit 600cfm
Aluminum intake also a summit don’t know much about it
Headers I recently wrapped
Electric choke
I always let it get up to temp

[Dlm11]

Originally Posted by RustyPile

One possibility is the fuel pump is weak and can't provide enough fuel delivery, or the filter is returning an excessive amount of fuel and there's not enough left to feed the carburetor.. If memory serves me, the '72's fuel pump provided return through a 3rd line that is plumbed back to the tank. An "extra" return line is not needed.

I replaced the fuel pump recently when I did the tank relocation it’s just a supply and a discharge. It’s always possible it’s weak cause they don’t make good parts anymore

[Dlm11]

I should mention prior to the filter change it ran dood other than boiling the fuel after it was shutdown for awhile and have hard starts. In the summer it acted up while driving on hot days and in SC we get a lot of thise

[geezer#99]

Simply get a different filter and put a heat shield under the carb like this one.


https://www.classicmuscle.com/p-1970...holley-3969835

[Dlm11]

The new tank has a bung for a return which is what I’m using now for the filter return I didn’t use it on the pump because I assumed that line was metered through an orifice in the original tank return line and I don’t know of another way to meter it other than a valve or pressure regulator. I’m going to check the bowl levels tomorrow and take that return line loose and see how much is going back to the tank. Thanks for the replies so far

[Dlm11]

Originally Posted by geezer#99

Simply get a different filter and put a heat shield under the carb like this one.


https://www.classicmuscle.com/p-1970...holley-3969835

Thanks for the link I’ll check it out

[RustyPile]

Originally Posted by Dlm11

The new tank has a bung for a return which is what I’m using now for the filter return I didn’t use it on the pump because I assumed that line was metered through an orifice in the original tank return line and I don’t know of another way to meter it other than a valve or pressure regulator. I’m going to check the bowl levels tomorrow and take that return line loose and see how much is going back to the tank. Thanks for the replies so far

Referencing your statement in bold.. That's an incorrect assumption. The return fuel is "metered" through an orifice located in the return fitting, a 1/4" nipple on the fuel pump. The orifice is actually visible inside the nipple.. No need to add additional hardware to control the flow rate..
also, no need to take the line loose to see how much is going back to the tank.

I might also add that it's a bad idea to block the return fitting one of these fuel pumps.. The pump is designed to develop a certain fuel pressure with the return line installed and operating.. Blocking it off could cause the fuel pump to develop excessive fuel pressure.

With all due respect Dlm, and with no intention of me being insulting or disrespectful, you are overthinking this project.. Overthinking or attempting to "redesign" can lead to the creation of additional problems.. I suggest you install a proper fuel pump with a fuel return fitting. Install a 1/4" steel line back to the tank.. Vent the tank properly and you won't need a vented gas cap. Install a 1/4" thick gasket under the carburetor and chances are pretty good you won't have vapor lock conditions..

[Dlm11]

I’ll check the fitting. I don’t take insult I probably am overthinking it. That’s why I jumped on here. I appreciate the feedback.

[nightrider388]

Sounds more like air lock in the tank. There are several mentions of having to drill a tiny hole in the aftermarket fuel caps that only let air out and not in.

[my67c20]

How much fuel is in the tank? Is your pickup tube near the bottom of the tank? Do you have your fuel lines plumbed to the pickup tube in the tank and not the return line? Do you have an additional filter after the fuel pump? Did you replace the rubber lines or could you have one collapsing? How does it run without the fuel cap on? Your symptoms read like its sucking the fuel lines dry when you get on it and the pump can't pull more fuel from the tank. Did you remove the line from the carb and put it into a can and see if the pump is pumping while cranking? Have you thought about going to an electric pump?

[RichardJ]

The Turbo Charged Corvair used a similar filter. 180 HP from 164 cu. in. The Carter YH 1bbl sidedraft works well with 3psi. The surprising large amount of return is actually reduced by the small return line that connects to the tank at the front of the car.

https://www.youtube.com/shorts/_nsuF6sctYA

Don't be confused by the extra filter the guy is using at the gal gas can. The fuel pump is at the back of the engine, next to the Alternator, above the oil filter.

[Dlm11]

Thanks for the help. The has a good vent on it. I ended up adjusting the level in the bowls. I raised the level because I noticed I was barely getting any fuel to splash up in the sight glass when I shook it. The only thing I can think was when it was dead heading it was good but with that little bit of recirc from the filter it dropping it just enough. Knock on wood it’s running fine now and the hot restarts are much better. When I shut it off after a few minutes you can hear the fuel and vapor that would flood the carb going back to the tank. The filter is an AC Delco gf423 and it has a a .060 orifice in the return. I also am probably going to go the electric fuel pump route in the future