Carburetor or Fuel System Problem

[AJC]

My engine runs worse after filling the car with fuel. The idle is lower and seems like its going to die. It has a stumble on acceleration and all around acts weird. The car runs great whens there is almost no fuel in it.

Any ideas?

 

[Davea Lux]

My engine runs worse after filling the car with fuel. The idle is lower and seems like its going to die. It has a stumble on acceleration and all around acts weird. The car runs great whens there is almost no fuel in it.

Any ideas?

Those are classic symptoms of a clogged tank vent. If check to see if someone has replaced the fuel cap with a non vented unit. If this car has evaporative emission control, you likely have a kinked hose someplace between the tank and the carbon pot or the crankcase breather, depending on which system you have. If the fuel tank has severely rusted, this will sometimes cause the vent nipple on the tank to get plugged.

Dave

 

[detmatt]

Venting issue maybe?

 

[detmatt]

Ha! You beat me by seconds Davea!

 

[AJC]

I did just reinstall the tank after having it fixed. There were 3 lines 2 came from the sending unit, 1 the tank. The fuel filter has 3 ports so I'm guessing that 1 port is a return. Another line is cut off at the firewall. It probably should go to the carb.

Would removing the gas cap prove this problem?

 

[Davea Lux]

I did just reinstall the tank after having it fixed. There were 3 lines 2 came from the sending unit, 1 the tank. The fuel filter has 3 ports so I'm guessing that 1 port is a return. Another line is cut off at the firewall. It probably should go to the carb.

Would removing the gas cap prove this problem?

I would run the engine until it starts to run like crap. Then see if you get a hiss when you remove the gas cap, if the tank shows vacuum, that is likely the source of your problem. What year car and which engine?

Dave

 

[AJC]

76 with a 440. Has a thermal-quad with a number of vacuum lines plunged.

 

[Davea Lux]

76 with a 440. Has a thermal-quad with a number of vacuum lines plunged.

You should have a carbon canister mounted on the right side of the engine compartment. There is a purge valve located on top of this canister, there should be a large vacuum hose from the purge valve running back to the fuel tank, another large vacuum hose from the purge valve should be teed into the line for the PCV system. Finally there should be a small vacuum hose that activates the purge valve that will be teed into the line for the vacuum advance. All of these need to be connected to get a vent to the tank.

Dave

 

[AJC]

There is no carbon canister, I have looked. That large vacuum hose is just open to the atmosphere.

 

[Davea Lux]

There is no carbon canister, I have looked. That large vacuum hose is just open to the atmosphere.

Then, someone has probably disconnected and tossed it. Hook a splice fitting to the open end of the hose and attach a longer section of hose to it. Remove the fuel cap and see if you can blow air thru the hose, it could be plugged with dirt. If air will blow thru the hose and into the tank, you have a tank vent and your problem lies elsewhere.

Dave

 

[75LandYacht]

Take some pictures for those of us that need visual stimulation.. lol I have a 77 NYB that’s had the return lines disconnected as well, so I need to reconnect.. and pics would help.

 

[75LandYacht]

Although! My 75 has the canister.. I’ll take some pictures and post here as a reference.. I think they’re the same as your 76 and my 77.. No?

 

[Davea Lux]

Although! My 75 has the canister.. I’ll take some pictures and post here as a reference.. I think they’re the same as your 76 and my 77.. No?

From '73 on they were all pretty much the same, later variants no longer had the purge valve and instead used a purge port. See
www.allpar.com/fix/fuel/evaporation-control.html

Dave

 

[AJC]

I will do that in the morning and get back too you.

 

[detmatt]

You should have a carbon canister mounted on the right side of the engine compartment. There is a purge valve located on top of this canister, there should be a large vacuum hose from the purge valve running back to the fuel tank, another large vacuum hose from the purge valve should be teed into the line for the PCV system. Finally there should be a small vacuum hose that activates the purge valve that will be teed into the line for the vacuum advance. All of these need to be connected to get a vent to the tank.

Dave

Are they plastic and about the size of a large coffee can?

 

[75LandYacht]

Are they plastic and about the size of a large coffee can?

Mine are size of a plastic coffee can.. Sorry AJC! Lol

 

[detmatt]

I’ve got this one available. After a bit of research this one might be 1972 only.

 

[Stern70]

This one is available, and

out of a 76 fury 400 lean burn.

 

[AJC]

So the idle is the same with the gas cap off. Need to get a piece of hose to check the vent line. The carbon canister should be by the windshield washer and overflow tank right? My vacuum line diagram shows 3 ports like the second carbon canister pictured.

 

[Davea Lux]

So the idle is the same with the gas cap off. Need to get a piece of hose to check the vent line. The carbon canister should be by the windshield washer and overflow tank right? My vacuum line diagram shows 3 ports like the second carbon canister pictured.

View attachment 284183

View attachment 284185

If pulling the cap did not help, the problem is not vacuum in the tank. You are correct, the carbon canister should go next to the overflow tank. I am noticing a police alternator and oil fill tube for the power steering, this a PK series vehicle (police). Filling the tank should not affect the performance of the vehicle if the vent was plugged, the car should run correctly with the fuel cap removed. Check your fuel tank mounting straps and the frame rails next to them to be sure the tank is not sagging because of a loose mount and pinching the fuel line where it comes out of the fuel tank.

The various emission controls on this vehicle are pretty much all missing of unhooked, that makes a performance related diagnosis difficult. Unhooking all that stuff usually messes with the mixture settings for the carb and can also affect timing. Check to see if the EGR valve is still on the intake manifold, it is unhooked, but it could be stuck open which will make the engine run like crap. Next check the basics, plug gap and timing to eliminate tune up issues. Check your fuel filter for rust accumulation, as the filling of the tank could be stirring up rust and plugging the pick up sock on the fuel sending unit. Check these items and report back.

Dave