[NeighborsComplaint]

Just got my car out of the body shop having had a new trunk floor installed and fixing some 30+ year old body work that finally started showing some rust flowers. I wound having the rear quarters, trunk opening and filler panel stripped, metal patched and the car painted from the doors back and restriped. Car looks good once again.

The drive (65 miles total) was uneventful until just after filling up. I started smelling fuel and pulled off. It was leaking off the corner of the gas tank onto the ground. I knew the tank had been removed and I thought, a hose clamp might have been missed but knew it was all in steel lines in that area. I needed an oil change anyway so I pulled into a shop and looked under the car. I found the unused vent on the corner of the tank missing the rubber plug I had put on. Made sense since it started to leak just after filling up. I figured the cold gas expanded in the tank, blew off the plug (which I hadn't clamped on) and ran out like an overflow. I plugged it with a vacuum cap and hose clamp, got the oil changed and off I went.

About 15 minutes later, the car began to vapor lock, bucking like a bronco like turning a switch on and off but not stalling. I switched on the my Holley red "pusher-pump" momentarily and it went back to normal as traffic began moving so I switched it off w. no further hiccups the rest of the way. My engine temp was 200 in stop and go and 190 on the highway and my pickup/sock/supply and return lines/pump probably have about 500 miles on them, but it definitely tried to vapor lock.

Another good reason to have that inline electric fuel pump.

Next on the agenda is the Sniper EFI install.

[JLMounce]

Car looks good!

Vapor lock was the number 1 issue that drove me to my FiTech. Not a hint of it ever again, even in triple digit heat.

[Formulajones]

Todays pump gas isn't going to like 200 degree engine temps with 6 lbs. of fuel pressure for a carb.

We've talked about this in depth before when the subject comes up.

No sense in worrying about it now if going EFI. 60 PSI fuel pressure solves those problems.

[ZeGermanHam]

Originally Posted by Formulajones

Todays pump gas isn't going to like 200 degree engine temps with 6 lbs. of fuel pressure for a carb.

Yep. I couldn't believe the struggle by brother had with vapor lock in his 461. He started to hate driving his GTO because every time he turned the car off when hot, it didn't want to start again for 15-20 minutes. He then began using ethanol-free gasoline and boom, problem solved.

[NeighborsComplaint]

Originally Posted by Formulajones

Todays pump gas isn't going to like 200 degree engine temps with 6 lbs. of fuel pressure for a carb.

We've talked about this in depth before when the subject comes up.

No sense in worrying about it now if going EFI. 60 PSI fuel pressure solves those problems.

Yep, just a brief annoyance.

I had goosed it at 40mph under a bridge to hear the exhaust note and with the kickdown to low, the car sidestepped like I was on dancing with the stars. It was running so good I was actually thinking "why am I going to EFI?".

A few miles later, the vapor lock smacked me with a dose of a reality and I thought "Yep, that's why.

[Formulajones]

I'm betting you'll like the EFI

[Kenth]

You can not plug the tank vent and expect the fuel to flow properly.
Fix the tank ventilation factory style and add an electric fuel pump (Carter P4594) on floor pan above rearend. Remove the mechanical fuel pump and cover the hole with a CBB plate.
Job done.

FWIW

[steve25]

So let me confirm this with you, you say you have a mechanical pump drawing through a electric one that you only turn on when you need it?

[steve25]

Do you realize that when the electric pump is not running that the pump is then a great restriction and that if the fuel is hot enough at that point that the difference in pressure can make for vapor lock taking place right there at the pump?

[NeighborsComplaint]

Originally Posted by steve25

Do you realize that when the electric pump is not running that the pump is then a great restriction and that if the fuel is hot enough at that point that the difference in pressure can make for vapor lock taking place right there at the pump?

That's an old wive's tale. I tested mine before installing. Free flow is not restricted. When the pump is off, the rotors will freewheel and rotate to the point where the fuel flows around the vanes.. Fuel flow moves the vanes away from the inlet and outlet ports and flows through the unrestricted path. It is no different than a surge tank with a plenum inside. It naturally assumes this position when fuel is flowing and the electric pump is not running. The suction from the mechanical pump is sufficient to make the fuel flow through the pump on a dry start too.

[JSchmitz]

Originally Posted by NeighborsComplaint

That's an old wive's tale. I tested mine before installing. Free flow is not restricted. When the pump is off, the rotors will freewheel and rotate to the point where the fuel flows around the vanes.. Fuel flow moves the vanes away from the inlet and outlet ports and flows through the unrestricted path. It is no different than a surge tank with a plenum inside. It naturally assumes this position when fuel is flowing and the electric pump is not running. The suction from the mechanical pump is sufficient to make the fuel flow through the pump on a dry start too.

Just because it is able to pull fuel through it doesn't mean that it's not a restriction. Doesn't take much pressure drop to make high vapor pressure fluid boil.

[Joe's Garage]

Originally Posted by steve25

Do you realize that when the electric pump is not running that the pump is then a great restriction and that if the fuel is hot enough at that point that the difference in pressure can make for vapor lock taking place right there at the pump?

Originally Posted by JSchmitz

Just because it is able to pull fuel through it doesn't mean that it's not a restriction. Doesn't take much pressure drop to make high vapor pressure fluid boil.

^^^ THIS ^^^

Carter makes an electric fuel pumps that is designed with the bypass (#4602RV). I think it is used mainly on motorhomes.

ANY other electric pump is a restriction, and it does NOT take much restriction with the crappy fuels currently offered to get the fuel to flash to vapor.

If I had this problem, and were using the later-style tank with multiple vents, I'd plumb all the vents together and keep them open. Think about it a different way - your tank built up enough PRESSURE to push off the cap you had installed over that extra vent nipple. That means it would also build up VACUUM when the tank is being emptied. It might need that vent to be open to atmosphere to function correctly.

It will take you about ten minutes to install a foot of fuel line with some kind of filter/sponge in the end of it and tie it up above the tank and go for a test-drive.

Good luck!

[NeighborsComplaint]

Originally Posted by Joe's Garage

If I had this problem, and were using the later-style tank with multiple vents, I'd plumb all the vents together and keep them open. Good luck!

I get what you're saying, but the 3rd vent is not needed unless the evaporative emission system is installed. Early '71's came with a 2 vent tank and no emission system. Later production added the third vent as part of the emissions control. '68-'70 and early production '71 all used the same 2 vent tank. Both were positioned on the passenger side. Securely capping the 3rd vent (driver's side) does no harm and functions the same as '68-'70/ Early '71.

All replacement tanks for '71- '72 have the 3rd vent to eliminate early/late production confusion on ordering. If you have a two vent breather, you plug the unused vent on the driver side vent to match the early configuration ... or plumb it into the larger line. I just failed to secure the cap, my bad.

[Tom Vaught]

Originally Posted by steve25

Do you realize that when the electric pump is not running that the pump is then a great restriction and that if the fuel is hot enough at that point that the difference in pressure can make for vapor lock taking place right there at the pump?

Holley pumps for sure need the fuel "by-pass" circuit (LIKE SOME CARTER FUEL PUMPS OFFER), because the moveable vanes move outward,stick, and then block fuel flow. It only takes one or two sticking vanes to restrict the fuel flow to the mechanical fuel pump. Same deal with any pump that uses Vanes that move outward when the pump is on. Even with the vanes retracted, there is a restriction. The BY-PASS CIRCUIT solves that issue. I use bypass valves with a large "sealing ball" to reduce back flow when the electric pump is on.

As was mentioned, you can buy that bypass system with the pump in some carter pump applications. Not sure of their by-pass flow area.

Tom V.

[NeighborsComplaint]

Originally Posted by Tom Vaught

Holley pumps for sure need the fuel "by-pass" circuit (LIKE SOME CARTER FUEL PUMPS OFFER), because the moveable vanes move outward,stick, and then block fuel flow. It only takes one or two sticking vanes to restrict the fuel flow to the mechanical fuel pump. Same deal with any pump that uses Vanes that move outward when the pump is on. Even with the vanes retracted, there is a restriction. The BY-PASS CIRCUIT solves that issue. I use bypass valves with a large "sealing ball" to reduce back flow when the electric pump is on.

As was mentioned, you can buy that bypass system with the pump in some carter pump applications. Not sure of their by-pass flow area.

Tom V.

I could see that possibility. My mechanical pump draws through my Holley vane pump without issue though. I don't drag race the car but on short bursts of WOT, it doesn't run out of fuel with the mechanical pump alone. I do have a dual bowl Holley carb which could possibly compensate with its larger bowl capacity.

Before I installed the Holley pump, I put a reducer on the inlet side and ran some low pressure compressed air through simulating a stream and really didn't note any difference in flow out of the pump vs the supply. Not entirely scientific but a test at least. I just decided to plumb without the bypass and see. It has worked just fine without the bypass.

Perhaps if I were a drag race and running at sustained WOT on a Qjet with limited inlet needle and bowl capacity, I might notice a difference but the current setup has worked just fine since installed. The recent onset of a vapor lock condition was the first time the car ever stumbled. Todays fuel of course continues to evolve.

["QUICK-SILVER"]

Fresh out of body shop from having work done on back of car.

I'd be checking for over-spray (dirt in general) on that vent filter.
And vent hoses for kink or pinch after tank R&R.

Poor vent shows up faster and worse the more gas you have in the tank
Clay

[Joe's Garage]

Originally Posted by NeighborsComplaint

I could see that possibility. My mechanical pump draws through my Holley vane pump without issue though. I don't drag race the car but on short bursts of WOT, it doesn't run out of fuel with the mechanical pump alone. I do have a dual bowl Holley carb which could possibly compensate with its larger bowl capacity.

Before I installed the Holley pump, I put a reducer on the inlet side and ran some low pressure compressed air through simulating a stream and really didn't note any difference in flow out of the pump vs the supply. Not entirely scientific but a test at least. I just decided to plumb without the bypass and see. It has worked just fine without the bypass.

Perhaps if I were a drag race and running at sustained WOT on a Qjet with limited inlet needle and bowl capacity, I might notice a difference but the current setup has worked just fine since installed. The recent onset of a vapor lock condition was the first time the car ever stumbled. Todays fuel of course continues to evolve.

Thirty years ago, we installed a Carter high-performance mechanical pump and a Holley blue pump on a 1973 Formula 455. With the qjet, it would sometimes stumble under acceleration if the electric pump was off AND on a long drive on the highway. We would eventually have to turn the electric pump on.

With the Holley 800 spread bore DP, the stumble under acceleration was lessened but, eventually, we had to turn the electric pump on if driving down the highway.

With the Holley the problem was less noticeable under short bursts of acceleration than it was with the qjet, probably, as you noted, due to the larger fuel bowls.

[Formulajones]

Wow, all these replies and it's really not that hard to make a stock system work and work effectively without the need for added on doodads.

I've raced my Formula for 25 years, and daily drive it, with the stock Q-jet and stock fuel system in place. Have gone as quick as 12.76 at 106 with a stock 400 RAIII....PS legal. Just 91 octane pump gas at any chain store, no hint of fuel starvation, no hot start issues, no vapor lock, and it gets pretty darn hot here in Arizona. Really all the fuel system needs is a well thought out stock setup and a tank that is vented properly.
I have some tricks in the carb with a larger needle/seat, a marine style filter in the carb which is basically a screen, and the brass fitting opened up as far as I can without affecting the flare nut seal, all in an attempt to keep the carb fed.
Stock AC Delco large body fuel pump with factory return line. My 70 Formula is California built so it has the Charcoal cannister system. I vent the tank as intended through this system, all hooked up and functioning.
The steel lines I run glass bead through them at high pressure as an attempt to extrude hone the lines, a trick I learned from the PS racers to possibly increase flow but it does in fact increase capacity, I've CC'd a couple lines after the process. Again all in an attempt to keep the engine fed with fuel.
I've never bothered to block the heat cross over because I do daily drive the car and prefer the divorce choke to work.

It's really just simple tricks like that, doesn't take anything really special. It does have it's limitations however. I've gone as quick as low 12's at 108 mph in another car with similar fuel system setup before I kind of hit a wall with it, and that car weighs 4100 lbs so the stock fuel system could possibly go 11's in a lighter car.

None of that really matters though as the OP is going EFI anyway. As long as you put together a solid fuel system to support it with a pump in the tank you'll enjoy that EFI.

[dataway]

I wouldn't think a mechanical pump would like any restriction at all on the suction side. I think some electric pumps have a by-pass, not sure how much restriction they create.

[JSchmitz]

If you're going to run a pusher pump, that will be off some of the time, you need to run a bypass with a check valve.