It’s supposed to be 6-7lbs.
When I first fire up it wavers around as high as 9 then settles down to 6 . Went to a show up the road from me yesterday, popped the hood and felt the lines which were hot. The pressure has been hovering around 4-5 lbs and drops if I get on it.

 

sounds like your pump could be failing as well.

 

I’m beginning to wonder…
I tried to look up symptoms of a failing fuel pump and all I seem to find is zero pressure… no kidding .
Getting the feeling most people don’t see a problem until it won’t move.

 

Sounds like a failing pump. I had one like that on a Hurst/Olds some years ago and it did the same thing.

 

Ok I’m still having the issue but have 2 more pieces of the puzzle to work out. A friend noted that the can fuel filter that is mounted in the rubber line between the hard line from the tank to the fuel pump is within 2-3” of the exhaust manifold and as such is probably heating up. It had a generic plastic fuel filter there when I brought it home and it was filthy. Not comfortable with the cheap plastic filters and that’s why I replaced it with the metal can filter. I’ve since bought a proper Holley 100 micron pre-filter which I’m going to install back by the tank. I held off doing that because since then I took a drive to the shore and stopped for gas. Opening the cap there was a loud rush of air into the tank showing a vacuum had developed. Apparently the vented fuel cap was no longer venting. Could this be at least part of the problem ? I’m thinking yes so I ordered a new vented cap and with it installed I‘ll take it to a show tonight and see how it goes. Either way I’ve got to fabricate a bracket to bolt the fuel filter too by the tank. Going to try to utilize existing bolt holes to secure the bracket to the rear frame. Once thats in place I’ll remove the filter by the pump.

 

Having no vent in a gas tank will create al lsorts of issues. I had a motorcycle that would stall until I took off the cap to vent it...thats a big issue

 

That low oil pressure is something that you should check further. My 427 does not get that low, ever. I have less than 20k miles on my engine since it was built but even with the higher compression and high temperatures the oil pressure doesn't drop that low. You might want to try a slightly thicker oil and see what it does in the heat. Before anything I would put a mechanical oil pressure gauge on the engine and check the gauge. Harbor Freight has a nice oil pressure testing kit that has many fittings to allow you to make a good solid connection allowing you to verify the real pressure numbers

Cooler Fuel was mentioned, this really helps prevent detonation if you have a high compression engine. I have 12.25-1 compression ratio in my L88 clone and it survives normal driving on the highways with 93 octane fuel. The Cold Air from the L88 Hood Intake system and the the cooler fuel keep me from having any major issues. I blocked off the original fuel pump location and switched to an Aeromotive Fuel System but then upgraded to the Holley Stealth Sniper that is currently on the Corvette. One of the items on my car is a MSD Digital retard Box and it works with my MSD6AL in the ignition. Back in the early days I would run the car on full timing of ~38-39* and IF a knock was heard I could pull the timing back manually. I have four switches that would each pull a few degrees totaling ~20* you could retard. Then I installed the Water/Methanol Injection system which activates at specific vacuum settings. This system is amazing as it allows me to run all the timing I NEED and not have any issues with my engine being unhappy.

My Fuel lines are insulated and have a Fire-proof cover from the throttle body back to the fuel pump output at the rear of the car (in-tank fuel pump). The lines that supply my water/methanol are insulated as are the heater box hoses. My fuel never gets exposed to the heat by going up to the front of the engine and then up behind the radiator and into the carburetor. I currently have the fuel lines coming up over the bell housing feeding the throttle body from the rear and this is a big step for making cooler fuel. With the Holley Stealth Sniper I am in the process of setting the EFI system to control the water/methanol injection system activated by both temperature first and vacuum setting second. I am very happy with the Holley EFI control software as it is very versatile and can be used in many ways.

The 427 engine with the three carburetors look great! I have always liked the design but had my scares earlier in life in a buddies 1964 GTO Convertible with it's 389 and a six pack. The few times he opened up the GTO he had problems with sticking linkages and all sorts of issues with the carburetors leaking. I had access to a L71 intake and carbs but chose to go single four barrel as it was easier for my wife and I to drive. This is why when I re-built the 427 I went nuts and built a L88 engine since I had access to parts from a wrecked 1968 L88 Corvette.

The L88 engine is truly an awesome Engine to have in any car.

 

Took the car to a show that was about a 1/2hr drive away. Idle did drop to about 500rpm but didn’t see the fuel pressure drop and it wasn’t threatening to stall. When I got parked I cracked the fuel cap and saw no issue with it venting.
The new cap is doing its job. I think that the old cap not venting was at least part of the problem.
On the ride home I got on it letting the outboard carbs open and did see the fuel pressure drop from 6 to 5 momentarily while I had my foot in it. I’d bet that NAPA fuel filter before the pump is restricting it. That’ll be out very soon.

 

I had the same issue after my engine swap. They forgot to connect the wire for the choke and it would close a little off and on.

So while looking into that make sure your choke spring is in good shape.

 

mine will still occasional want to drop idle coming to a stop at a light / stop sign if I have been gettting after it and am slowing quickly but only in that instance so it would be a fuel bowl filling and not having enough head pressure to sustain an idle but popping the throttle lightly it goes back to 1000rpm. So mine is fuel slosh and possibly air in the idle circuit. Maybe your circuit has varnish in it or a chunk of something. You could disassembly it completely and soak the main body and your fuel block and then blow it out and maybe get some welder tip cleaning rods and try to make sure the passages are all clear.....maybe check all your linkages, choke idle speed cam settings....,maybe something was sticking and your idle was set with that interfering and now is out of the way occasionally...its always something with these carbs. Maybe your floats arent as high as they used to be or need adjusting

Ive had sticking butterflies, warped bottom plates, warped main bodies both at the botttom and the fuel block faces. (My research through carb rebuilders have stated that the aluminum that was used for the main bodies melts around 300 degrees so the castings get soft easily. So thats always a vacuum leak issue.) Ive had main bodies that were had the idle air bleeds on the top drilled way to large and had to fix those. My last main body had so many issues I just gave up and altered a brand new holley center carb from the small block setup. I havent had an issue since other than coming to quick stops and that is obviously a me issue.

 

Engine mounted fuel pumps do draw a slight vacuum on the supply side while operating. But they are not very good at that and do not do it very well.
Any sort of restriction on the supply side should be avoided.
The Non-functioning vent in your gas cap would cause too big of a vacuum in the tank for the pump to pull against.
So would a filter before the pump. I would avoid that.
Vacuum, or even low pressure, in fuel lines, with NJ ethanol containing street gas, could cause vapor formation, and increase your problems, especially on the supply side of the pump.
Putting a filter at that location increases the likelihood of that happening.
I would not run any filter between the tank and the mechanical pump other than the in-tank sock.
Good fuel filters are designed to pump thru, under pressure.
In your case I would add an AC filter, with threaded couplings, in front of the cylinder head.
It looks pretty factory since it was on some 427 cars, 390HP IIRC.
There is a version with the 3rd vent, and one without.
I would hope the NCRS guys would consider it a driveability modification for street gas.
Not sure where you are measuring your fuel pressure at, but it should not drop but a few psi, even at WOT, even after a couple gears.
It frequently shows up in drag cars in sustained 3rd gear, near the end of the track.

 

It's not a vacuum issue, it's not a fuel supply issue, it's a idle tuning issue that is common to tripower 2 barrel carbs. It will take tuning the the Idle circuit and the transition circuit to fix it by modifying the Idle Air jets and the fuel metering block...or just up the float bowl level halfway to 3/4 of the way up the site plug hole. You need an extra plug with a hole drilled in it to get that level or just guess until it's spilling out the vent then back it off...
Don't use those plastic site plugs, the fuel will dissolve them in a few seconds. Hopefully you get all of the threads out without taking the bowl off
These things will drive on the fuel bowl at idle for a hundred yards or 100 or so feet if you are driving it....so no. Other than the tank pulling a vacuum on the line so that you are literally starving the fuel bowls, you actually need to work on it and smell of gas

 

Figured I’d Follow up on what I’ve done and where I’m at with the issue.
I removed the NAPA can filter (10 micron) that was installed inline just before the fuel pump. Note I know they are supposed to go after the pump but my car has the 3 two barrel tri-power set up and they dont have that filter after the pump rather 3 small filters in each carb inlet. Not thrilled with that and don’t want to mess with them. The car came to me with a plastic filter before the pump that was filthy so I elected to replace it but with a metal can because of its proximity to the exhaust manifold. This was to be temporary until I knew all the crap had been run out of the system.
Ok so theres no filter there now like the factory had it. I did mount a “pre”-filter made for this purpose by Holley back by the tank. That filter is a 100 micron, is freer flowing and is designed to be installed “before” the pump. I had also installed an Autometer fuel pressure gauge on the the steering column to monitor the pressure before I changed the filters. Note the fuel pressure gauge is electronic with a sensor mounted on the line, it is not a mechanical gauge.
I’m happy to say this all but cured the problem.
I did have a moment when the rpms at idle dropped from 750 to 600 after some slow beach traffic so I think the lines might still be heating up some but the overall fuel pressure is much better and steady at 6lbs when cruising on the highway. At initial startup its wavers between 7-8lbs but settles to 6 after running for a bit. I haven’t seen it drop to 3 lbs like it did with the can filter in place either.
Another thing to note is I used marine clamps and NAPA / Gates new fuel line.
I am considering buying a thermal fuel line wrap for the where it runs thru the engine bay.
Here are some pics of whats been done.

This what was there when I brought the car home and was replaced with a metal can filter. Safer but still wrong…


This is the fuel pressure gage installed…


i removed the factory line and bent a new line to mount the sensor. I utilized the same brass T-fitting used by the carbs and wrapped its wiring in cloth tape to give it a period look…



The cap on the car wasn’t venting so I replaced it. Did it help? Well it didn’t gasp for air when removed like the old cap did but merely changing the cap did’t seem to help the idle issue.



I fabricated a bracket to mount the new fuel filter that utilized existing bolt locations in the rear frame.


The filter installed…



This is the thermal wrap I’m considering using for the lines in the engine bay…

 

that thermal wrap should help...I run a line out to the front of the radiator and have a passive fuel cooler out there. I want to install a small one with a fan eventually to keep the fuel cooled efficiently. I have a similiar setup with a fuel shutoff at the tank in front of the rear filter so I can remove it to empty the filter and another in the front after the the pump so I can keep the fuel front syphoning out the of the bowls when I clean the front filter

 

Well done!
You were probably drawing too much vacuum on the supply side of the pump, and bam it went to vapor.
You got rid of two restrictions there, the 10 micron filter, and the no vent cap, good move.
You still have a 100 micron filter there, but that is not considered a vacuum restriction.
10 micron filters go on the pressure side of the pump.

I like your elec fuel psi gauge setup. Nice!

Now do you have a 10 micron filter in the system after the pump?
That should be there to keep debris off the needle & seat.
If not, you may get occasional flooding, or idle issues if you get debris on the needle & seat, either cold or hot.

I like this AC Delco GF-90 screw-on version for single fuel line cars:




It looks fairly correct mounted vertically in front of the water pump.

Some of the the Q-Jet cars and some of the AC cars used this one including the 390HP 427 cars. GF-432 or similar, with a vapor vent: in the same spot.


Those also had two fuel lines, the second was for the vapor return line.
On some cars the vapor return line was built into the fuel pump, and those did not need this vented filter.
The Holley Carb'd cars typically did not have this vented feature at all, and ran only one line.

The symptom the vent and the second line cures is this. After engine shut-down, the heat goes into the trapped fuel between the pump and the carb. On a no return line car there is no way for the fuel to get out. It is still under pressure. More heat adds to the pressure and / or some of it vaporizes. It expands and the greatly increased pressure overcomes the needle & seat, and it dumps the excess fuel down the carb. It bubbles out the main venturis. That becomes the pressure / vapor vent. The engine floods. And it is extremely hard to start after being parked a short time.

If the engine has a vapor return line, and either the pump is vented, the gas filter is vented, or you add your own .060" orifice vent in the line, it does not matter. Immediately after engine shut-down, the orifice / vent bleeds off all the pressure in the pump to carb line and it goes back to the tank in the 2nd line. If it flashes to vapor, same thing. No engine flood. No hard to start when parked hot.

2000 and up ethanol containing gas has a much lower vapor pressure than the stuff from the 70s. The stuff from the 70s was designed for carbs, and low psi fuel systems. The later fuel is designed for EFI and designed to be under 40psi pressure all the time. So it has issues with carbs and low psi if everything is not just perfect.

Many of these old carb'd cars need small adaptions to be user friendly to today's fuels.

 

Mines the L68 with three two barrel carbs so the filters are in the carb inlets. I think you called it for the vacuum buildup before the pump causing it to vapor lock. Start up now hot is dramatically better as you’ve said. I think the next step is to wrap the lines and it’ll be good.

 

Bottom line if you set the idle when the choke is not completely off you will be setting it again
believe me.