[Pierre1967]

Hi

I'm thinking of buying a 550hp Robbmc and using its return line port option.

But before selling my actual Carter M6907, I'm wondering if I could simply add a 5/16 "T" just before the carb inlet. Actual pressure is 6 psi and that is perfect for my 400hp+ engine.

Doing that, how can I control the pressure drop caused by the return line ?

Thanks for your help

[Tom Vaught]

I would suggest that you rear some articles on how a carburetor and the fuel line systems work.

I really hate to see people spend money installing equipment that may not cure the root cause of the problem.

I suggest that you describe exactly how your vehicle is built, Fuel system wise, even though we know you have a M6907 mechanical fuel pump on the engine currently.

Also please describe in some detail the vehicle, model year, current engine, options, stock or modified, etc.

Thanks Tom V.

[Formulajones]

You can make your own return with a "T" if you want to experiment but much rather prefer to do it with the pump or a good regulator.

With the "T" you'll have to restrict the flow to keep from dropping too much fuel pressure with the small pump. Solder the inside of the nipple you'll use for the return and drill about a .060 hole is enough. Cheap way to experiment but ultimately I'd recommend the Robbmc pump with the return option if you want to stay mechanical pump.

[Brentco]

X2^

Ideally you’d want a pump that put out a few extra psi, and then use a bypass regulator to get the pressure down to 6 psi for the carb.

Also, FYI, Robbmc’s site says you shouldn’t use a return regulator with his pumps, but it works just fine.

[Pierre1967]

Thanks to all for your advice, I really appreciate.

Tom, car is a Firebird 67 : 406 cu, stock KRE heads, xe274 cam, 700r4, 3.55, Cliff carb, electric fan, alum. rad.

In my case, after having tested the fuel system with a temporary return line, vapor lock issue completely disappeared. Now I want to fix it permanently using a new mech. pump or keeping the M6907.

Surely a new Robbmc is the way to go for me.

[Brentco]

Return lines do wonders. If you don’t need the additional fuel flow the RobbMC provides, you may be fine using your current pump. Maybe start by adding a bypass regulator after the pump (output from pump to regulator, then out of the regulator one line to carb and one line to return). No need for a return port on the pump itself.

IIRC, Q-Jets aren’t particularly fuel pressure sensitive, so it may be perfectly fine with that set up. If it starts nosing over when you’re hard on the throttle, then you’ll probably want to upgrade to a RobbMC pump.

[Singleton]

Originally Posted by Pierre1967

I'm wondering if I could simply add a 5/16 "T" just before the carb inlet.
Doing that, how can I control the pressure drop caused by the return line ?

I did that on my tri-power set up by using a carburetor jet as a restriction inside the return hose. In the picture, the red arrow is the fuel flow, the green arrow shows the brass nipple. Inside the brass nipple is a holley jet, (a #80 I believe, which is about .090” hole). Pressure went from 6lbs to around 4lbs, which is perfect for a tri-power.
This keeps the fuel moving at idle and low engine speed, and a side benefit is that during engine-off hot soak periods the hot fuel in the line has a place to expand to, (the tank), if it starts to boil. Before this mod the hot fuel would sometimes blow past the carb inlets and flood the engine during heat soak.
I’m using a stock replacement type pump btw, can’t remember the brand.

As stated previously, a bypass type regulator would work better than a fixed orifice, as it is an adjustable restriction. My method is just a cheaper way to go

[78w72]

Originally Posted by Pierre1967

Hi

I'm thinking of buying a 550hp Robbmc and using its return line port option.

But before selling my actual Carter M6907, I'm wondering if I could simply add a 5/16 "T" just before the carb inlet. Actual pressure is 6 psi and that is perfect for my 400hp+ engine.

Doing that, how can I control the pressure drop caused by the return line ?

Thanks for your help

the robbmc "return line port" is strictly a vapor return, it doesnt function as a true return line that returns unused fuel to the tank like an electric pump set up. as brentco mentioned, robbmc doesnt recommend a true return line with their pump, he explained to me that it will reduce the psi too much on a mech pump, im sure it might work for some applications but shouldnt be needed for most situations. if you have vapor lock, use the vapor port option that is designed to help that issue, it only reduces pressure about 1psi.

you mentioned 6psi from the carter pump, the robbmc 550 runs right at 6-7psi also & for most mid to late 70's q-jets they can easily handle that much psi, cliff has told me they can even handle 8-9 for short periods & i have done that at the track a few times & left it that high for a short time on the street, no issues at all.

i run the robbmc 1100 on a 500+hp E-head 467 stroker with a 1978 cliff built q-jet, the 1100 pump is adjustable & when turned all the way down is at 7psi, i leave it turned down for normal driving & turn it up to 8-9psi for drag strip use... no vapor or fuel return line & it never has any vapor lock or other fuel related problems. if it did show any problems related to vapor lock i would add the vapor return, not a fuel return line based on their recommendation.

also can you describe your "vapor lock" symptoms? many times its not actually vapor lock that causes some symptoms like hard starting or engine cutting out/bogging etc.

[jhein]

Originally Posted by 78w72

the robbmc "return line port" is strictly a vapor return, it doesnt function as a true return line that returns unused fuel to the tank

Not to hijack the thread, and hopefully this is not too dumb of a question, but whis the difference in function and plumbing for a vapor return line vs a true fuel return line?

[Tom Vaught]

Since it is a "Cliff Carb" this will be my last post on this thread.
Let the Q-Jet Experts handle it.

Tom V

[78w72]

Originally Posted by jhein

Not to hijack the thread, and hopefully this is not too dumb of a question, but whis the difference in function and plumbing for a vapor return line vs a true fuel return line?

a vapor return line is usually very small, 1/4-5/16" & is intended to return vapor only or very minimal amount of fuel, its more of a vent for vapor than actually recycling unused fuel.

a true return line like used for electric pumps & fuel injection cars requires a large return line, usually at least 3/8" but some go to 1/2" & say the return should be as big or bigger than the feed line. its designed to constantly return the unused raw fuel to the tank to keep it cool. on high psi elect pump/FI cars most the fuel pumped is returned to the tank since the engine uses very little fuel at idle or light cruising compared to what the pump can & is pumping out. & for these older classic cars that requires running a complete new fuel line vs trying to use the little vapor line they came with.

[jhein]

Originally Posted by 78w72

a vapor return line is usually very small, 1/4-5/16" & is intended to return vapor only or very minimal amount of fuel, its more of a vent for vapor than actually recycling unused fuel.

a true return line like used for electric pumps & fuel injection cars requires a large return line, usually at least 3/8" but some go to 1/2" & say the return should be as big or bigger than the feed line. its designed to constantly return the unused raw fuel to the tank to keep it cool. on high psi elect pump/FI cars most the fuel pumped is returned to the tank since the engine uses very little fuel at idle or light cruising compared to what the pump can & is pumping out. & for these older classic cars that requires running a complete new fuel line vs trying to use the little vapor line they came with.

Thanks. The vapor line still goes back to the tank? And if you have an existing return line that's bigger than 1/4-5/16 that can still be used, I assume.

[PunchT37]

Originally Posted by 78w72

a vapor return line is usually very small, 1/4-5/16" & is intended to return vapor only or very minimal amount of fuel, its more of a vent for vapor than actually recycling unused fuel.

Correct. And usually with a restrictor in the already small 1/4 port.

[Formulajones]

Originally Posted by jhein

Thanks. The vapor line still goes back to the tank? And if you have an existing return line that's bigger than 1/4-5/16 that can still be used, I assume.

It usually goes to a vapor cannister somewhere in the back of the car.

You technically can use the existing vapor return line for a fuel return setup for what you are trying to accomplish IF you take a few things into consideration.

1st, if you are using a stock style mechanical pump and PSI isn't outrageous.

2nd, make sure the line terminates in the tank.

3rd, you have to restrict the flow of fuel going back to the tank, either the way I mentioned it earlier, or the way Singleton mentioned it. The fuel has to be metered in small amounts. Too much and the stockish mechanical pumps will lose too much pressure.

Decades ago I used to do this mod back when I would ditch the stock mechanical pump for an external electric back at the tank. Usually a Mallory 140 back then. At that time I used a cheap regulator with no return, but I still wanted to return fuel to the tank just to keep things circulating. So I simply used the stock 1/4" return with a "T" down by the frame near where the original mechanical pump used to be (where the vapor return terminated) and I would solder the return in the "T" shut and drill a .060 hole.

This way it was all hidden, and I had a functional return. I actually rigged dads car this same way back in the 1990's and both cars were running 11's at the time. It worked well for what it was.

These days I just prefer it one of two ways depending on the HP goal......

Either a stock style pump with the vapor return and hook things up the way they are supposed to be. I've gone mid 12's with stock setups, roughly 400-450hp.

Or I just run a big electric pump in the tank with a regulator at the engine, that has a return line feature, and run a big return back to the tank, and put enough pump in it to support the HP. Trouble free setup that will last years and take anything I throw at it.

[78w72]

Originally Posted by jhein

Thanks. The vapor line still goes back to the tank? And if you have an existing return line that's bigger than 1/4-5/16 that can still be used, I assume.

YW. yes still goes all the way back to the tank & connects to a vent line inlet port.

each set up is different, so depends on the size of the existing return line, most factory vapor return lines on these classic cars wont be big enough for the average electric pump set up. some guys run elect & mech pumps deadhead with no return at all but its usually best to have a good return for an elect pump. my robbmc1100 is set up deadhead & ive never had any vapor lock or fuel venting related issues, i use a simple tank vent with a rollover check ball & a vented gas cap.

[78w72]

Originally Posted by Formulajones

It usually goes to a vapor cannister somewhere in the back of the car.

just to clarify, the vapor return lines on most these 70's cars goes direct to the tank, the charcoal canister is located in the front engine compartment, not rear of the car... the charcoal canister is what gets the vented vapors from the tank via a separate line. at least thats how all my mid 70's 2nd gens are set up from the factory.

[PunchT37]

The idea on this pump piss today is to keep it properly vented and as cool as possible. New piss with lower boiling point and higher vapor pressure than the old days makes these old carbed cars harder to work with vs back in the day.

[Formulajones]

Originally Posted by 78w72

just to clarify, the vapor return lines on most these 70's cars goes direct to the tank, the charcoal canister is located in the front engine compartment, not rear of the car... the charcoal canister is what gets the vented vapors from the tank via a separate line. at least thats how all my mid 70's 2nd gens are set up from the factory.

There is also a vapor cannisters on some cars. That's what I call them anyway. If it has one, it's either mounted behind the seat or on the floor of the car above the rearend.

It just depends on what we are working on. Some cars have both, like my Chevelle for instance. My Firebird only has the charcoal cannister, which has a different purpose. Dad's GTO has a factory return line but doesn't have either cannister, no vapor cannister and no charcoal cannister. GM was funny about that stuff, I guess depending on what the EPA was requiring at the time and/or what plant the car was built or what state it was destined to be sold in.

[78w72]

Originally Posted by Formulajones

There is also a vapor cannisters on some cars. That's what I call them anyway. If it has one, it's either mounted behind the seat or on the floor of the car above the rearend.

It just depends on what we are working on. Some cars have both, like my Chevelle for instance. My Firebird only has the charcoal cannister, which has a different purpose. Dad's GTO has a factory return line but doesn't have either cannister, no vapor cannister and no charcoal cannister. GM was funny about that stuff, I guess depending on what the EPA was requiring at the time and/or what plant the car was built or what state it was destined to be sold in.

right, but only the early year cars had that EEC standpipe on the rear seat, my 72 had that but i eliminated it.

charcoal canisters on most or all 73/74 or so & up years were up front in the engine bay & used a separate line than the vapor return line off the pump.

[78w72]

Originally Posted by PunchT37

The idea on this pump piss today is to keep it properly vented and as cool as possible. New piss with lower boiling point and higher vapor pressure than the old days makes these old carbed cars harder to work with vs back in the day.

yes thats the idea & some have issues with it... but myself & many others on here & out there in the world dont have any problems with todays "piss gas". i normally run 91 premium with no ethanol in my higher compression stroker motor but it will run fine on mid grade 89 with 10% ethanol too... i have huge super comp headers & the fuel line passes within 1" or so as it passes by that area of the engine, never any vapor lock or fuel heat related issues in any street driving situation or at the drag strip. same for other pontiacs i own/owned with ~400 or more HP, all stock fuel systems. yet some need to add all kinds of crazy mods & tons of heat wrap just to get down the road to the dairy queen. guess im just lucky.