This old car makes be feel ignorant. Basic Questions inside.

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MattP

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Me* feel ignorant.


I'm hoping you guys in the know can help me out.

Car- 1972 Duster 318

Currently wearing a stock 2bbl holley carb. It's on the way out but may be relevant to helping answer my question.

There are 2 lines leaving my fuel tank headed to the front of the car. They look awfully like a feed and return line for a return style system. One line feeds the mechanical fuel pump and the other turns into the engine bay but was capped off with a bolt by a previous owner. Is that line supposed to connect to the carb to route excess fuel back to the tank?

If too much fuel is added when priming the carb with the old squirt bottle fuel spills out onto the intake manifold from an open port in the carb.

Deduction tells me that the return line is supposed to be plumbed to this port to supply a safer route for excess fuel than dripping on the headers.

Am I correct? If not please explain.

Thanks fabo. Mechanical fuel pumps are a new thing for me. I was born and bred on fuel injection and figuring out this carb stuff is tough.
 
That second line went to a charcoal canister. This is how they dealt with vented gas fumes early on when the vented gas cap was outlawed. The needle and seat in the carb. controls the unneeded fuel pressure.
 
Thanks for the quick reply!

That makes sense with people adding carter electric pumps with no regulator. The carb is acting as a regulator at those low pressures.
 
Roy is right on the nose.
Can you get us a pic of the port that is open, or describe the port and tell us what carb?
 
Unless the previous guy "did something" that capped line is the ONLY vent for the tank. Here, download a free 72 shop manual, and thank AbodyJoe. You have to read a combination of the fuel section and the emissions section in the back. The section on the vent line from the tank to the front is page 25-14. Page 14-2 has some general info, and page 14-76 and onward has more

[ame="http://www.abodyjoe.com/pictures/Misc.%20car%20info/1972%20Plymouth%20Chassis%20Serv%20Man.pdf"]http://www.abodyjoe.com/pictures/Misc.%20car%20info/1972%20Plymouth%20Chassis%20Serv%20Man.pdf[/ame]

This thread

http://www.forabodiesonly.com/mopar/showthread.php?t=183956

The filler CAPS are not truly vented. The caps used on the later cars with the carbon can have a "pressure vacuum" cap which means it has relief valves to let in air or let air out at a certain pressure level. They were meant as a safety, NOT as the primary vent system, this thread:

http://www.forabodiesonly.com/mopar/showthread.php?t=202184
 
The port out the top of the fuel bowl went to that canister too. Never meant to have liquid fuel in it, only vapors. Bowl overfill - get liquid fuel out the vent.
 
I learned the hard way about vented gas cap. As a young man I bought a CST-10 68 327 short bed Chevy and one of the first things I bought for it was a chrome gas cap. This is when they use to have the gas tank behind the front seat! I would drive it and then it would die? I thought the gas gauge was broke so I added some gas. Then it would happen again, and again. Till and old mechanic told me I might have a non vented gas cap. It would run until the fuel pump couldn't pull gas out of the tank, then every time I took the cap off I was re-leaving the vacuum created in the tank. If your car is running with the vent line plugged you must have a vented cap on your Duster.
 
SGBARRACUDA said:
you must have a vented cap on your Duster.
Click to expand...


No Mopar in the 60's --70's ever used a vented cap, which was my point. It has a separate vent, and when the evap emissions (carbon can) system came along, the tank was vented through the can system.
 
SGBARRACUDA said:
I learned the hard way about vented gas cap. As a young man I bought a CST-10 68 327 short bed Chevy and one of the first things I bought for it was a chrome gas cap. This is when they use to have the gas tank behind the front seat! I would drive it and then it would die? I thought the gas gauge was broke so I added some gas. Then it would happen again, and again. Till and old mechanic told me I might have a non vented gas cap. It would run until the fuel pump couldn't pull gas out of the tank, then every time I took the cap off I was re-leaving the vacuum created in the tank. If your car is running with the vent line plugged you must have a vented cap on your Duster.
Click to expand...
\

May have had a vented gas cap. Just switched to this http://www.summitracing.com/parts/snn-10572/overview/make/plymouth/model/duster/year/1972chrome Stant cap a few days ago about the same time as adding headers.

Did you just fix my "vapor lock"?
 
67Dart273 said:
No Mopar in the 60's --70's ever used a vented cap, which was my point. It has a separate vent, and when the evap emissions (carbon can) system came along, the tank was vented through the can system.
Click to expand...

Yes I stand corrected. That is why they used that little vent tube on the filler neck on the early models. Thanks for jogging the old noodle.
 
So that line should not be capped? Will I get fuel fumes in the engine bay if I uncap it?
 
MattP said:
So that line should not be capped? Will I get fuel fumes in the engine bay if I uncap it?
Click to expand...

The tank has to have some method of venting. I would say try driving it without the gas cap and see if that cures your problem. If so You can ad a gas tank filler neck with a vent line from an earlier model car. Or ad the canister to your present set up.
 
MattP said:
So that line should not be capped? Will I get fuel fumes in the engine bay if I uncap it?
Click to expand...

Unless you change something else in the system, you need to open that line. Originally, there was a ball check valve in the engine bay end which was supposed to help keep liquid behind it. Download the manual I posted and do some reading.

I would say you have several choices

1--open the line, run a hose/ line from it as high as you can and safely out into say, the fender area to vent fumes, but don't run it too low, or you can create a siphon.

2--Modify the filler so it's vented like the older in the drawings posted in those threads

3--Find a vented cap. The problem with vented caps is that you run the risk of having fuel spill on the rear quarter panel.
 
You cant close the bowl vent.
Early carbs had a small metal disc laying on top of the vent. Everyone had some fuel vapor under the hood.
 
If your interested. I was pulling some parts off a 72 Duster and the canister and bracket is still there.
 
Opened the line and nothing changed. The car will start just fine when cold but after 5 minutes idling it will die and not restart unless I pour fuel down the carb.


I've wrapped the fuel line from the pump to the carb in heat tape. Wrapped the headers in heat tape and it's still happening. This is frustrating. I'd like to find a simple fix and be able to cruise this weekend. Next weekend the carb and instake and possibly the fuel pump are being swapped out.

Help

**Just occurred to me** while messing with the carb a while back I was adjusting the throttle stop screws. I wonder if once it goes to the normal screww I've got it too low and it just dies. There's a thought.

Doesn't change the "what seems like vapor lock" but it could be why it's dying so quickly at idle.

Also: Found this in another thread "Quote" If it starts right up after it cools down I would rule out a fuel problem. If the carb was getting heat soaked it would evaporate the fuel out of the carb. So there would be no fuel for it to start right up. I thinking you may have coil going bad. Coils going bad can do this when warm but fine when cooled down. Electrical devices do not like heat and when they start going bad it usually shows when the item is warm or hot. "End Quote"

Thoughts?
 
So what have you checked, and what have you concluded from those checks

Since this seems? to be an ongoing problem, it seems to me you should be able to make some checks.......................

"Rig" a way and carry tools, rags, etc in the trunk so that you can "quickly deploy." When the engine is hot and has been parked, rig your spark test gap and test and evaluate the spark. If this doesn't seem relevent,.....................

before you start it, remove the air filter and check for accelerator pump action without disturbing anything, IE don't crank it. If no fuel, or a "poor squirt," or if the thing reeks of gas, it's probably all boiled out of the carb.

The best things I did with mine before going EFI was

removed mechanical pump, installed a rear mount electric

Installed a fuel return line, use Wix 33040 -5/16- or 33041 -3/8- filter with a built in return orifice, and

installed a heat isolator carb base gasket.

These were not a complete cure, but rather a VAST improvement. I b'lieve around here fuel is about 15% alky.
 
67Dart273 said:
So what have you checked, and what have you concluded from those checks

Since this seems? to be an ongoing problem, it seems to me you should be able to make some checks.......................

"Rig" a way and carry tools, rags, etc in the trunk so that you can "quickly deploy." When the engine is hot and has been parked, rig your spark test gap and test and evaluate the spark. If this doesn't seem relevent,.....................

before you start it, remove the air filter and check for accelerator pump action without disturbing anything, IE don't crank it. If no fuel, or a "poor squirt," or if the thing reeks of gas, it's probably all boiled out of the carb.

The best things I did with mine before going EFI was

removed mechanical pump, installed a rear mount electric

Installed a fuel return line, use Wix 33040 -5/16- or 33041 -3/8- filter with a built in return orifice, and

installed a heat isolator carb base gasket.

These were not a complete cure, but rather a VAST improvement. I b'lieve around here fuel is about 15% alky.
Click to expand...


I really don't think it's spark related. If I pour fuel down the carb it will catch and I have to continue trickling fuel into the carb until it regains fuel prime.

Then it will run a few minutes until losing fuel prime again. I'll mess around with it this weekend and see how it goes.
 
Have you determined FOR CERTAIN that the fuel tank is venting?
 
Just tested it with the fuel cap off and it did the same.

Cranked right up and idled fine. Once it got warmed up it idles lower and lower until it dies.

I checked the carb and the accelerator pumps still had fuel. Pumped the gas a couple of time and it cranked right up. It was just fine for another minute until it did the same thing.

Video
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[/ame]
 
Well.... just removed the fuel filter closest to the carb and it's happily chuggin away out there.

Fingers crossed.
 
and then once I thought I had found it I hopped in the car to go ride down to the parts store for a new filter turned the ac on and shortly after it did the same thing.

Carb is hot enough to boil off the fuel that's coming out of the accelerator pump. Time for a spacer?
 
So much for that. Nobody has a 2bbl spacer in stock. I was just going to mess with it out of curiosity but at this point I'll just wait until the new parts go on. New parts include a spacer. I'll probably go ahead and change the filters while I'm at it.
 
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