Yes, original fuel solenoid from the BEAM system.

Is there something special about the IMPCO VFF-30 vacuum lockoff that the Model J needs it ??

I think Mr fixit is closer to the mark here, not being familiar with this truck model myself I can say I'm am familiar with beam regulators and impco fuel systems. When old beam systems where used they had an inline fuel solenoid and filter combined these would eventually get filled with the heavy ends and clog up. Most impco systems I've come across don't have a fuel lock of solenoid but a vacuum lock off unit and filter that feeds the fuel regulator. Have you left the original fuel solenoid in the line when you changed to the impco units.

Regards Titus

"Could the regulator mixer be defective out of the box?"
Not really that likely. Be careful of your terminology in describing things. Mixer is the part that is sometimes called a carburetor.
Carburetor is incorrect because it is metering a gaseous fuel instead of atomizing a liquid fuel.
If you did the lean out test properly by spraying an alternate fuel into the mixer intake when the stalling began.....and you noticed an improvement in the running while spraying.........then fuel deficiency is the culprit, not ignition.
Not much of a practical way to actually test the regulator/vaporizer for fuel flow volume.
I wonder if 1/2" copper is large enough to operate the J model secondary valve in this application.
If you could substitute a larger diameter vapor hose from the regulator/vaporizer to the mixer that would solve that question.

No hose between the reg and mixer......brand new 1/2" copper line. The prev. Beam setup also used a 1/2" copper line.

I am going to closely go over the ignition system again, maybe swap coils also.

This seems like a very simple fuel system and very simple ignition system.
Very frustrating symptoms that make no sense.
There is nothing else to replace with new parts.

Could the regulator mixer be defective out of the box?

Yes did that this morning first thing....seems like plenty of fuel flow for 4-5 seconds and beyond.

The cap, rotor, points, condenser all have less than 30 minutes run time on them. Will hook timing light to coil and watch that....
Would be handy to have an automotive scope!

OK, if you are disconnecting the liquid hose BEFORE it enters the regulator/vaporizer you should get LIQUID fuel out when the lockoff solenoid is energized. That part of the hunt is not a defective condition. The fuel is supposed to be in liquid state when it enters the regulator/vaporizer.
What kind of hose do you have that leads the vaporized fuel to the mixer (part you are calling carb)?
Unless that hose is rated for LPG service, the inner hose liner layer might have swollen and is now unable to flow the volume of fuel needed at higher RPM and power demand.
A constricted vapor hose will also cause fuel deficiency because the engine induction vacuum (that is supposed to unseat the regulator secondary valve) will be greatly reduced inside the regulator secondary chamber.
For this engine, the vapor fittings at the mixer and at the regulator should be 5/8" size.

It's got to be running out of gas due to lock off, hoses, or tank and coupler. Do that test again that you take the hose off and energize the lock lock and let it stay on for 3 seconds, outside and carefully.

Model J regulator, CA100 mixer.

The bulkhead fitting has less than 25 hrs on it and the filter appears to be as new.
Perhaps it's impossible to visually tell if the felt filter is clogged?

Liquid fuel out of the end of the hose fitting just before the regulator (where it goes INTO the fuel solenoid). Also liquid fuel out of the solenoid valve when energized. And for several seconds, not just a split second. I wanted to be certain there was plenty of free flowing fuel available up to the regulator.
It blasts out the end of the hose.

No VFF-30....there is a vacuum switch in the circuit with the fuel solenoid.....which is bypassed for testing.

I am positive it goes lean now because I had my hand almost completely over the air intake and it ran fairly well till it got so lean it died. It seems to run 30 seconds or so the lean out and stall. It actually sat there at high idle and then quit more than once.

Also tested with carb cleaner and it runs OK and steady on that, as long as I'm spraying it in the carb in a steady stream.

Weird....

I can check the ignition also, but it really seems like fuel delivery problems.

Any way to test the regulator??

I will try a different tank too!

Thanks
dk

May I suggest that you keep in mind that we are trying to help you- being short & demeaning with the hand that feeds you (hats off to Swoop 223 for taking his time & talents in trying to help you) isn't the best way to get the help you require.

The proper Impco fuel system for your application would be the VFF-30 vacuum lock off, Model J or Cobra regulator/vaporizer & CA100 carburator/mixer.

Perhaps if you explained yourself more clearly our answers would be more on target. Your symptoms can be caused by a few different issues, proper diagnoses & comunication can get us to resolve this, if we work together.

Have you concidered the ignition system- have you checked for spark when the unit is begining to die? As mentioned- have you checked for a clogged fuel filter at the bulkhead fitting? Have you checked the connector at the fuel tank? And lastly, have you tried using different propane tanks? These should be the basic tests being done before diving into the fuel system components.

And for the record, I personally have never seen a pnuematic governor exhibit the symptoms you have.

You said, "I tested the fuel solenoid WITH THE TANK VALVE CRACKED OPEN and the fuel hose disconnected completely from the regulator.....liquid fuel came out...... "

Are you meaning that you disconnected the fuel vapor hose that leads from regulator to the mixer? and liquid fuel pours out of the regulator at that hose nipple on the regulator?
Tell us WHICH Impco regulator and mixer you installed.

The regulator and carb on there now is NEW
There is no fuel leaking in the carb now......read my post carefully.

I tested the fuel solenoid WITH THE TANK VALVE CRACKED OPEN and the fuel hose disconnected completely from the regulator.....liquid fuel came out......

dk

if you are hearing a hissing at the mixer when you turn the key on even while the engine is not running this should not be happening. the diaphragm in the mixer is what regulates the flow of vapor into the engine but only when it is running, it opens and closes using the engine vacum from the engine via a vacum hose coming from the intake manifold and should only open the mixer under vacum while the engine is running. If you are getting a hissing sound just by turning on the system i would make sure the diaphragm is not damaged. Pull the cover off the mixer and remove that diaphragm and check it for any holes, cracks and pay close attention to the center button on the diaphragm itself and also inside the mixer where it seats, this is what actually seals the vapor. A dirty or bad seat on the diaphragm button can allow vapor to pass when it is actually supposed to be closed and can make the engine run badly or not at all.

Hmmm. Maybe try this test just to prove it is a fuel problem. Rev it up and get it to stumble and ready to stall, then spray a shot of WD-40 in the carb and see if it wants to take back off and rev. It sure acts like it is running out of gas. How about the fuel tank coupler? If that had a problem it would still give you a shot of gas out of the disconnected hose because the hose was full but maybe its not coming out of the tank properly.

I had the hose to the PCV valve plugged
And I took the high pressure fuel line off the regulator and
momentarily bumped the key......plenty of liquid propane came out of the hose.

???

There is no diaphragm that can cause a vacuum leak. Bad gaskets could. There is a spring in there, that if it breaks, the throttle plate will close too easily and won't rev up. It sounds like maybe the fuel filter, located between the tank and the lock off solenoid, could be plugged enough to be restricting the fuel flow. The flow may be good enough for idle but when throttled up the flow may not be good enough and the pressure starts to drop in the hose downstream of the filter and after 20-30-60 seconds it goes lean and stalls. Also, I would check the PVC valve.