What is the 3rd fuel line on fuel filter on a carb car for?

[Reeves]

Working on my Dad's new to him 84 Dodge Rampage. I'm replacing the brake lines and some fuel lines. I'm trying to figure out the fuel routing. There is a line going from fuel tank down the left side and to the mechanical fuel pump. I'm assuming that is the fuel supply to the pump. There is then a line from fuel pump to fuel filter, I'm assuming that is the fuel supply to the filter. There is then a line that goes from the side of the fuel filter, back to the tank. What is this for? Then there is a line coming off the top of the fuel filter, to the carb.

Don't know what the line on the side of the filter is for?

[contraption22]

Don't know what the line on the side of the filter is for?

A wet kit.

[shackwrrr]

It has a small orifice to regulate fuel pressure and to keep the fuel lines cool to prevent vapor locking.

[Reeves]

A wet kit.

Good idea.

It has a small orifice to regulate fuel pressure and to keep the fuel lines cool to prevent vapor locking.

Kind of what I thought it was for (vapor lock). I'm wondering if I can get rid of it. Did the guys with race setups use it?

[shackwrrr]

These weberish looking carbs are pretty picky on pressure. I think 3-4 psi is max, any higher and it just pushes the floats open.

[contraption22]

I used a two nipple filter on my old carb setup, but I had to add in a fuel pressure regulator to keep the carb happy.

[Reeves]

So 3 lbs of fuel pressure at carb is about right?

[contraption22]

I think so. I never used a gauge. Just used one o these super-fancy fuel regulators set to "3"

http://www.ebay.com/itm/Purolator-55...-/230968870641

[ATaylorRacing]

On Matchbox I currently run a bit under 3 psi. There were two different style three prong fuel filters on the carb cars that I know of. Matchbox came with the little one that looked like a regular one but with an extra line on it....worked fine when new and was a mechanical pump back then. THEN there was a recall for vapor lock...they took off the little filter and put one on that was a lot larger and looked like some type of canister. They also pulled of the mechanical and put an electric at the tank...when they did the different filter...AND added a Holleyfuel pressure regulator at the back...but it had a pointed part where the after market ones have a hex nut, I guess to not allow changing of the pressure. The recall was a big fail on my car. It used to all the way up to red line in every gear with no troubles...but would not make it to the 1/8th without running out of fuel. We kept raising the fuel pressure at the back with only a little bit of progress. We took off the new style fuel filter and I tried to blow some air through it with no luck...got a new one from parts and it was just as restrictive. We ended up sticking the original back into place and then it would cut out at near the finish line. Getting rid of the recall electric fuel pump and putting on a Holley electric red AND lowering the pressure to 3 psi fixed everything. The dumbest thing about our carb cars was that they laid the fuel lines right on the valve cover....I relocated them to the firewall when the car was still not recalled and never had a problem...until 28 years later the Holley electric red was toast.

[Reeves]

So, a resurrection. Different car:
A couple is coming to hopefully pick up the 83.5 Shelby Charger. My Dad mentioned that it was running rough one time when he drove it. I decided to get it out and charge battery, pump up tires etc. It was running like an absolute champ. Super fun to drive and faster than my 01 Neon RT. I stopped at my other garage to get some stuff. Was there maybe 15 min. Car fired right back up and went about 1/2 mile and stumbled badly then died.

Tonight I figured out that it runs on starting fluid just fine. Took off fuel line and cranked it over and got no fuel. Stuck thumb over line and felt no pressure.
Took off suction line and cranked over and felt no suction. Siphoned fuel out of suction line (about a gallon) with no problems (like air holes)
Took off fuel pump. Pushing the lobe it feels like it has suction and pressure like I think it would. Checked lobe on intermediate shaft and from what I can see and feel it looks fine.
Took off suction line and cranked over and felt no suction. Siphoned fuel out of suction line (about a gallon) with no problems (like air holes)
Took off fuel pump. Pushing the lobe it feels like it has suction and pressure like I think it would. Checked lobe on intermediate shaft and from what I can see and feel it looks fine.
This pump I took off looks very new. Something neon green did fly out while I was stroking it (wow...).
Hmm....sitting here contemplating. ...I wonder if the fuel filter clogged could do this? It's after the pump. Would I not feel suction if there is post pump blockage?
I should try blowing through it. Anyone know how these fuel filters work? It's got an in an out and a little line off to the side.
I've got it all torn apart.....pick up and install new $20 pump tomorrow or install old one and test some more?

[mopar-tech]

blockage?
I should try blowing through it. Anyone know how these fuel filters work? It's got an in an out and a little line off to the side.

Its a "vapor return" that "skims off" a portion of the gas and vapor in the line and sends it back to the tank. It was an attempt to prevent vapor lock on these cars which was a rampant issue when they were new (ask me how I know!)

For the worst cases Chrysler had a kit which converted the vehicles over to an electric fuel pump with a regulator mounted on the rear fuel rail. I did about a dozen of them as a line tech.

[83scamp]

I'd replace the fuel filter first. When I was running the stock setup on my Scamp, seemed like I had to replace the filter every couple of years. IIRC, it doesn't take much to plug them. They don't have a lot of surface area inside.

The port on the side of the filter is a vapor port. As was stated above, it's supposed to separate any vapor & bubbles from the warm fuel to help prevent vapor lock. Unfortunately, with the lines running right across the hot engine, it doesn't do much good.

[Reeves]

For testing purposes, can I run an inline fuel filter that doesn't have a vapor return?

[83scamp]

For testing purposes, can I run an inline fuel filter that doesn't have a vapor return?

Sure, just be careful. You could quickly flood the carb. It's easy to watch though, if you leave the air filter lid off. You can look right down into the bowl from the air horn.

Not that you want to put the effort into a car you are selling, but when I restored my Scamp, I eliminated the mechanical pump and lines across the front of the motor. I installed a low pressure turbine pump at the fuel tank, and used the filter that mounts beside the carb. This setup bleeds off any excess pressure back to the tank, and has worked very well for me for the last 3 years. I did it mainly so I didn't have to endlessly crank the motor to get it started after sitting for a while. Turn the key on, let it prime for about 10 seconds to make sure the carb is full, set the choke, and boom, it's off and running. So much better than the factory setup...

[Reeves]

Where is your regulator mounted? Does it use your vapor return line?

[83scamp]

Where is your regulator mounted? Does it use your vapor return line?

I didn't use a regulator. I bought a low pressure pump that doesn't generate more than 5-7 psi, the vapor/return port on the filter does a fine job of returning excess to the tank. I've never had a flooding issue with this setup. I've put over 10K miles on it.

[Reeves]

Oh so you are using the stock fuel filter?

[83scamp]

Oh so you are using the stock fuel filter?

I'm using the stock filter that was used with the electric fuel pump TSB. Fram # G3893. It mounts right beside the carb instead of by the distributor. You can find it on Rock Auto under 1986 Dodge Omni. The pictures online show it upside down.

I can get a picture tonight. My fuel lines run directly from the strut tower over to the carb. This fuel filter sits right in the open spot in front of the carb fuel inlet behind the valve cover, and between the PCV box & air cleaner lid. You end up with about a 3" line from the filter to the inlet barb, and about 12-14" of line from the hard lines for the feed & return to the filter.

It worked out pretty slick, and you avoid most of the heat from the engine.

[Reeves]

Yes please send pics. Very interested, even if not on this car but for my Scamp.

[83scamp]

Yes please send pics. Very interested, even if not on this car but for my Scamp.

OK, I'll take a couple pictures tonight and post them here.

Very interested in hearing about your Scamp!!! I'll have mine at SDAC-26...