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Can somebody tell me how is Pinz gas tank vented? I went to flush the gas from the tank, opened the bottom plug, and gas was coming, in gulps. It was obvious that there was no vent between the tank
and atmosphere. Opening the tank cap caused gas to flow normally in a steady strong stream, closing it gulps started again. Is regular Pinz gas cap venting type? If it is, in my case it appears that it is
not terribly effective. The so-called vent-hose connecting the top rear of the tank to the filler tube does not vent anything, and given the large dia of the fill tube, I am wondering why is it there at all.
Lately I have been having issues with a loss of power and there are some indications that fuel delivery might be a problem. If I am creating vacuum in the gas tank, this would obviously be a cause of concern.
If anyone can chip in, I would appreciate....

Perhaps your small vent line is clogged; try running some stiff wire or something down it to see if you can clear it. This would be my best guess. Situations like this are extremely common where I live; dirt-dauber wasps will plug any hole of that size they can find with mud to make a nest.

AFAIK, the tank isn't vented. The "vent" from the tank connects back in below the gas cap, so if the cap seals, then it is sealed. I believe the "vent" just keeps gas from backing up when filling.

How can it not be vented? even a 2-3 psi pump creates a vacuum. need to relieve that some how.

The gas cap is supposed to be vented. The 8mm hose that runs from the top rear of the tank to the filler neck is to vent the rear of the tank due to the baffles inside the tank.

Thank you all for your comments. For test purposes, I will run the truck with cap open, and see if that helps.
The thin sheet-metal strap on the gas tank cap must be hiding some hole. I think permanent fix would be to tee-off another hose from the current "vent" hose, preferably at the highest point in the cab and then run it outside of the body, though the floor. I guess once cap is clogged, it could perhaps be unclogged with an ultrasonic bath, but it does not look as a reliable solution. I remember clearly that cars from that era had a vent
straight into the atmosphere. I once owned Fiat 650. Every time I vent over the pothole interior filled with a smell of gas. Finally I removed the trim and looked at the filler tube. There was a main filler tube, then smaller
diameter vent tube tee-ed off it, also going into the tank, to allow for air to escape as tank is being filled, and then from that tube another clear vinyl tube was tee-ed off, maybe 1/4" dia and that one went straight through the hole
on the floor. In this case, it was pulled into the cab or pushed into the cab from outside and it was venting into
the cab. Putting it back in the hole solved the problem.

Pulling the drain plug allows fuel to escape the tank far faster than the fuel pump delivers fuel to the carburetor. The vent hole is not of a size that would be expected to keep up with that level of out flow. In my military days I had to drain a fair number of fuel tanks, we never left the fuel caps on the tanks as that slowed down the rate of out flow. All that said, can a fuel cap vent become clogged? Yes, however I suspect your problems are not from a clogged vent. May want to Post up more specifics pertaining to your power loss.

edzz wrote:Pulling the drain plug allows fuel to escape the tank far faster than the fuel pump delivers fuel to the carburetor. The vent hole is not of a size that would be expected to keep up with that level of out flow. In my military days I had to drain a fair number of fuel tanks, we never left the fuel caps on the tanks as that slowed down the rate of out flow. All that said, can a fuel cap vent become clogged? Yes, however I suspect your problems are not from a clogged vent. May want to Post up more specifics pertaining to your power loss.

I agree that vent size, wherever it is, is much smaller the the gas tank opening and adequate for normal operation. After all truck run fine before. Regarding the power loss, all I can say for sure is that I have significant power loss when climbing the hill and smaller loss when running in the flatlands. Driving even mild downhill, there appear to be no issues. Timing is OK,
exhaust temp (measured with IR thermometer on manifolds close to heads) on all four cylinders appears to be fairly even, voltage on the coil is 17V. But I cannot run any of these tests while under load. I have RPM module disconnected/bypassed. I have Pertronix installed, opened distributor, everything looks OK.
When I went on the last 60 mile trip, with tank approximately 3/8 full, when I noticed trouble, I was, according to fuel gauge (electronic, VDO, probably not well calibrated) , approximately on 1/8 of a tank. But I still had at least 8 gallons left at that time, so it looked to me that gas level may have something to do with the problem, which would imply fuel pump issue. I can crank engine with no problems, starts right away and runs smooth.
I have a spark-plug gauge on cylinder #3 and temp was not out of normal when climbing the hill. I did notice some what it seems erratic RPM behavior, I try to stay at or under 3500 RPM
all the time, and several times during the climb RPM would jump to 4000 RPM. I was clearly pressing gas pedal more then usual and watching the RPM, never intentionally exceeding the 3500 RPM, but it looks to me now that perhaps erratic fuel delivery would cause that.
I recently did some trivial work on carbs, replaced the choke wheel cover gasket and O-rings on two jets on top and gaskets on pump pressure valve and pump suction valves. Since cover was off, I replaced all vacuum tubing on the engine, including the tubing between intake manifolds with brand new tubing. I also cleaned all contacts on the alternator and made sure they were tight. Cleaned also the Molex.
Lastly fuel pump gaskets were leaking oil, so I replaced them too. I cleaned the fuel pump from outside. repacked the space under the small cover with grease, and re-tightened the the screws on the top. I should have cleaned the strainer in the pump, but I forgot. Fuel filter is clean, no red death.This was the extent of my recent work on the engine. But even before doing that, I was recently climbing really steep hill with Pinz, which I could do before with no trouble and this time, truck abruptly stalled. I switched transfer case to low and finished the climb. But clearly something was not right. I'll do more testing and report on my findings

Regarding the power loss, all I can say for sure is that I have significant power loss when climbing the hill and smaller loss when running in the flatlands. Driving even mild downhill, there appear to be no issues.

Sounds like a fuel delivery problem; enough fuel for down hill, not enough on the flats, and starving on the uphill -- have you checked the fuel pressure at the carbs? You should have at least 2-3 psi, even wide open. I once had a car with similar problems, it had less than 1 psi at the carb!

Red death.

"Fuel filter is clean, no red death." from his post

fuel pressure gauge sounds like a good idea. I'll find one.

og wrote:approximately on 1/8 of a tank. But I still had at least 8 gallons left at that time.

Wow you must have a huge tank, I two would have first thought red death but if you have clear fuel filter and can see that no red bits are in it, and from the soulds of it, you must have an aftermarket tank, so I don't suspect that a painted lining is the problem.

When doing a fuel pump rebuild it is important to not replace the push rod unless it is damaged. The length of it is critical for proper fuel delivery.
A rebuild of the fuel pump and the removal and blowout of the carbs secoundary jets is what I would do.
Also you may want to run to the junk yard and pick up a generic electronic fuel pump and try it. If that fixes your problem I would then buy a new one and use the used one as a backup.

when I bought my 712 I had a problem like that and I went through the same deal. I ended up pulling the fuel tank and found all kinds of plastic and paper towels and aluminum foil that was floating over the pickup tube. As they say. my two cents.

It took awhile for my fuel pressure gauge to arrive, so I am late with the update. Basically, truck seems to run fine. I did not do anything particular, but installed new fuel filter (old one looked clean) and blown the jets.
I installed voltmeter on the coil, and fuel pressure gauge on carb fuel hose and took a pinz for highway/mountain road test, with my buddy looking at gauges and me looking straight. Voltmeter at idle shows 17.3V and when running at about 3500RPM, hwy or climb, it shows around 20.4V.
I do not know is this is OK?. Fuel pressure at idle is about 3PSI, going down to 2.1 in the highway/mountain road test. It only dipped once shortly to around 1.7PSI when I was changing gear climbing up. I could not detect and difference with gas tank cap on or off. Pinz is unbelievably loud without the engine cover I must say.
Today I filled the tank and noticed that fuel gauge (electronic VDO) is only showing tank 7/8 full. This is probably the reason why tank shows empty with almost 8 gal. inside. This looks like a ground problem described on SDP_pinzgauer web site. I'll need to check that.
I got my pinz with electronic gauges, so I am wondering do I still have stock fuel level sender, or is the different one used with the new VDO gauges?