I have just one tank, the fine in cab unit.
I can fill up on Friday, let the truck sit all w/e, fire it up Monday morning and be missing 1/3 of a tank! So, approx. 5 gallons... gone.
I figured some of the kids in the neighbourhood with gas scooters got smart and were borrowing from me, so, on went a locking gas cap... to no avail.
I can sometimes smell gas fumes in the cab, other times not.
I find no gas stains on the ground where I park.
I see no leaks in the cab... well, one on the top of the tank where the fuel line comes out, but, it is minor minor minor... so minor that the stain never grows.
No leaks at the carb.

Anyone been down this road and have advice?

 

Check your oil.
If it smells like gasoline, you have a bad fuel pump leaking it into the oilpan.
If that is not it, I would guess you have a pinhole leak in the tank.

 

5 gallons is a lot of gas going somewhere. If it is going in the crancase you would be overflowing. Has some bright youngster figured out he can pull a gas line underneath to fill his can?

 

I will check the oil tonight.
Dang youngsters... dunno...
Is it a bad idea to run all hard lines to the carb?
Currently I have many splices of flexable line.

 

The most probable spot Eric is thinking they might be stealing from (if they are doing it all) is the line which goes through the floor and under the cab. You need a rubber line under the cab there to handle flex between the cab and frame.
The other location would be the line between the steel and the fuel pump. Flexibility is needed there too to allow for engine movement.
Anywhere else, they would have to pop the hood, and wouldn't get much any way because there isn't enough height difference.
If those two locations don't show tampering, you are probably OK from the kids.

 

I once had 3 tanks on my 72. One of the locks could be opened with almost any key. They aren't very well made. Maybe your gauge isnt working right?

 

Do you have any teenagers? A friend and I used to "borrow" his dad's Ford truck in the middle of the night. But we would at least "borrow" some gas from someone else before we returned the truck. Check your odometer. Weekends were when we would "borrow" the truck. Since it was his dad's truck, I would have the honors of pushing the truck down the street before we'd start it. Those were good times!

 

I wondered why there was flexabile lines in those locations. Thanks for the explanation.
The truck is not being borrowed. LOL, that is a funny story.
Our yard has a slope to it, and to back it in it has to be under power... and, the dog goes crazy at the sound of the truck, so, she'd let us know.
The leak/evap/whatever has my milage down to ~8mpg. I was getting 10.5-11mpg before new carb/elec. ingition/coil/dizzy/plugs/wires etc.... now ~8mpg.

I filled up last night for the first time in a while, and the smell in the cab was terrible.
When I got home, I noticed the thing on top of the tank damp all around it. Sooooo, I thghtened down all those 1/4" bolts, and they were fairly loose, but, I could not wiggle it.
There is a gasket underneath.
What do I ask for at NAPA to get that gasket?
I did not see any signs of leaks/tampering underneath along the lines, or at the pump itself.
Oil leaks and power steerings leaks are in no short order however.

At any rate, I'll be heading to work in a bit and once on flat ground I'll see if the tank leaked any overnight.

Oh ya, the guage works... when it hits 'E', it takes dang handy to 16gal to fill her... every time.

 

Great descriptions! I would check the following:

1. The oil, for a gas smell. Just in case.
2. The gas tank cap may not be sealing completely. If there is any side angle to the slope of the truck, and it is parked with the fill tube leaning down from the rest of the tank, (that is, pointing downhill), it is probably running out the gas cap also.
3. The gasket may not be available. Although RTV isn't supposed to be used to seal gas tanks, I used it successfully on the sending unit in my son's '72. It isn't immersed in it all the time. I used a thin bead, just enough to seal it, reassembled it, then let it sit overnight before filling it with gas again. Fixed the leak just fine a year ago, and still doesn't leak.

 

Most local hardware and auto stores sell gas friendly gasket material in sheets.. cut to fit

 

well...
no gas smells in the cab thru the day.
guage read the same this morning as it did last night, and did not seem to leak any thru the day.
I guess the real test will be as it sits over the weekend...

Oh ya, no gas smell in the oil, and, it's down about 1/2 a quart... about perfect... lol.

I find it hard to believe that much gas would evaporate/leak out the top of the tank... but, given the Az summer conditions, maybe it's not out of line?

I'll get some sheet gasket material tomorrow and replace what's there just to be extra safe.

 

If you can't find the gasket material the Permatex Aviation Form-a-Gasket in brush on cans is resistant to hot fuel. I use that stuff to seal the ends of gaskets etc instead of RTV. Every time I needed RTV the tube was hard so I quit buying it. I have had a can of that Permatex for 20 years.

 

I had this problem and I thought somebody was stealing the gas but a little poking around found that where the gas line goes into the fuel filtter was loose , very loose. I tapped it and it fell all the way out and I got a face full of gas. next time I was on empty I pulled it out and flared the tip a little bit and tapped it in with a hammer. it has not happened again.

 

Yeah I have had that dissappearing gas problem, Its really embarrassing when you are in public, like at the movies...... But seriously, I had the same prob. as #29, with leakage at the fuel filter. Lose fittings,extremely lose as in finger tight.

 

On a sloped driveway like you described, those in cab tanks are notorius for leaking. Since you periodically smell gas, it's my guess that is your problem. Although 5 gallons is way more than can be expected. My question is, when you are reading 5 gal. low are you reading that on your sloped driveway too?