Troubleshooting Quiz: Barracuda Starts Backfiring / Cutting Out @ 1/2 Tank

[Road_Runner]

This is a weird one.  About 6 weeks ago my car started spitting & sputtering at about half a tank of gas.  It hadn't been run in a while so I figure either the gas had begun to go bad, water had gotten in the tank through condensation, or the last tank of gas was bad.  So I topped it off and all was well until I used about 6-7 gallons again & the gas gauge showed half a tank it started again.  I put some Gumout gas & carb cleaner & topped off the tank again.  At half a tank same thing, topped off with premium all was well.  At half a tank it started same thing so I topped it off but this time it did not clear up immediately continuing to spit & sputter all the way home.  This has continued for several more half tanks, but now it has completely shut off a couple of times, back fired LOUDLY a couple of times and now I'm thinking maybe something in the fuel pickup, or vent system is faulty.  I put some Seafoam fuel cleaner this last go-round, but if it does it again I'm thinking it can't still be bad gas unless I have a gallon or two of water floating around in the tank.  Any ideas?  Has this ever happened to you?  If this is bad gas its the most persistant case of bad gas I've ever experienced.  All help is appreciated as I'm really concerned it could cut out at exactly the wrong moment you know.

Thanks, Jim

[burdar]

If I had to take a guess, I'd say that you have a hole in the pickup tube.  When the tank is full, the hole is submerged in gas.  After a certain point, the fuel level drops below the hole and you start sucking air.  You could be running the carb out of gas.

You should be able to pull the sending unit out without removing the tank.  If the tube look OK, then drop the tank and flush it out.

[Road_Runner]

I should add that the Barracuda has a mild/stock 318 2 bbl, rebuilt by the original owner several years ago but then barely driven so its a great running 'low mileage' engine.  Since its a 1973 and the original female owner did zero hotrodding on it, my guess it also has the original low compression.  It has been running fine on regular until this latest round of problems.

[Road_Runner]

If I had to take a guess, I'd say that you have a hole in the pickup tube.  When the tank is full, the hole is submerged in gas.  After a certain point, the fuel level drops below the hole and you start sucking air.  You could be running the carb out of gas.

You should be able to pull the sending unit out without removing the tank.  If the tube look OK, then drop the tank and flush it out.

I wondered if it could be something like that, but couldn't figure out why a hole would appear after its been running fine up til then.  Do you think that could be causing the loud (think 30.06) backfiring?  That's happened a couple of times just recently so whatever it is it seems to be getting worse.

Thanks, Jim

[GranCuda1970]

Sounds like the sending unit. If it happens consistantly at half tank , that would be a good time to pull the tank flush and reseal it.

[Road_Runner]

Sounds like the sending unit. If it happens consistantly at half tank , that would be a good time to pull the tank flush and reseal it.

Agreed, might as well cover all the bases to be sure.

Thanks, Jim

[MEK-Dangerfield]

If you had a fuel pressure gauge under your hood, you could verify the problem. It does indeed sound like air is getting in the line. That would cause the fuel pressure to drop, and when it's really bad...It leans out the carb, causing the backfire.

[Road_Runner]

If you had a fuel pressure gauge under your hood, you could verify the problem. It does indeed sound like air is getting in the line. That would cause the fuel pressure to drop, and when it's really bad...It leans out the carb, causing the backfire.

Sounds like replacing the sending unit is the logical next step.  Any reason I shouldn't go to a 3/8" unit?  Not sure if it came with a 5/16 or 3/8 but both show as fitting the 73.  I'm sure the 5/16 will work for the 318 but at the very least I'm going to build it up or swap in a 340 or possibly even a stroker, so I thought just go ahead with the 3/8, opinions?

Thanks, Jim

[burdar]

73's came with 5/16" lines no matter if it was a 318 or 340 car.  There's no point in installing a 3/8" pickup if you are still running 5/16" fuel lines.

[Road_Runner]

73's came with 5/16" lines no matter if it was a 318 or 340 car.  There's no point in installing a 3/8" pickup if you are still running 5/16" fuel lines.

Agreed, but I was thinking more that the 3/8" might provide more fuel down the road if I went with a bigger engine.  Though if 5/16" is more than enough for a healthy 340 that's all I'm likely to go with in the future.

Thanks, Jim

[MEK-Dangerfield]

Jim,

   I tell my story, so you can glean what you want from it.   

I have a beefed up 440, but I still am running a 5/16th line with a hi-volume mechanical fuel pump. I think upgrading would be overkill unless you plan some track time. You would have to replace all the clips holding the line to the body to accommodate a 3/8, so it's just another pain in the butt.

Some genius(major sarcasm there) took the fuel line off to repaint the underbody. When they put it back on, they found it was easier to cut the line in half right in front of the rear tire on the passenger's side. Well, after a few decades, this little rubber hose that was holding the two halves of the line together, turned to petrified wood...With a nice little crack on top of it.     I would have normal fuel pressure at start up, but when I drove it a while, it would take a dive when air started getting in there. I still think your problem is in the sending unit, but this is an example of how air can get in your fuel line. So you might want to check your fuel line before you take the sending unit out, just for kicks. 

[Road_Runner]

Jim,

   I tell my story, so you can glean what you want from it.   

I have a beefed up 440, but I still am running a 5/16th line with a hi-volume mechanical fuel pump. I think upgrading would be overkill unless you plan some track time. You would have to replace all the clips holding the line to the body to accommodate a 3/8, so it's just another pain in the butt.

Some genius(major sarcasm there) took the fuel line off to repaint the underbody. When they put it back on, they found it was easier to cut the line in half right in front of the rear tire on the passenger's side. Well, after a few decades, this little rubber hose that was holding the two halves of the line together, turned to petrified wood...With a nice little crack on top of it.     I would have normal fuel pressure at start up, but when I drove it a while, it would take a dive when air started getting in there. I still think your problem is in the sending unit, but this is an example of how air can get in your fuel line. So you might want to check your fuel line before you take the sending unit out, just for kicks.

Sounds like 5/16" it is, and that's great advice about checking the fuel line.  I haven't done a thorough end-to-end inspection on the underside yet.  I took a quick look when I was getting tires put on and found several things that I've since repaired but didn't have time to check everything.

Thanks again, Jim