The description by mrfixit is spot on - exactly as we have on our one, with the exception that it has never had a dual fuel set-up, even though it has a carb. I do not know its full history, but it looks to have been factory converted from new. All the gasolene connections etc. are blanked off and there looks to have never been a tank and fuel line. It used to run perfectly with its previous owner, who then started to have this problem. We took it over - thought it would be a simple problem to cure, but is proving more difficult. At the moment have the lpg feed connected straight into the solenoid on the top of the Impco, bypassing old pipes and filter, and are now looking at the bottom solenoid valve to again check its operation - if this is working OK we can see no reason why at least it should not idle properly.

I stand corrected on the choke issue- thank you. Would it be a good option to just replace the carb with either a C-50 or C 100 Impco unit. Do the duel fuel setups run as good as the standard LPG carb?

I got a good look at one of these today. Impco regulator with a electric lock off. The fuel hose goes to the top of the carb. There is a "T" in the fuel hose with a small diameter (vacuum line) hose that goes to the base of the carb, like where a vacuum hose would hook up. In the "T" is a valve that you can adjust how much flow the small hose has. This is the idle mixture control, like the pilot circuit on a gas carb. Just under the base of the carb is the 1/2" water hose that goes to the Impco with a thermostat in it. On the very top of the carb is the gasoline inlet. I would think on a duel fuel system you have another electric lock off on the gasoline line somewhere by the carb

Has this truck ever ran satisfactorily in service since you acquired it or have you been trying to resolve this issue ever since you got it?
Has the exhaust system been checked for blockage?
We once had a Komatsu exhibit somewhat similar symptoms and that turned out to be where the partition wall (baffle) inside the muffler broke into pieces and a large piece stuck against the muffler outlet.
The engine would idle but would die if you tried to accelerate.

it might be a water hose
if there is a plate that hose goes to with one hose in and one hose out(one looked as if it was in a T fitting off the block and the other a T fitting to the bypass) then it may be a heaterblock.
I have seen those on some older lifts. While looking at the diagram i couldnt determine for sure but it did seem that there were some hoses that looked like they ran from the block or intake over to the carb but it didnt show the picture very well.
A very crude drawing unfortunatly
And its been a few years since i've even layed eyes on one close enough to remember they layout.

Are you sure that lower hose from the base of the carb to the Impco is not a water hose?

after looking through the information i have found on this there has been like 6 different revisions done on this fuel system.
What information i could find is mainly parts info, not much on service information. Since it is a dual fuel system the 2 solenoids you see are for LP lockoff and most likley gas lock valve. The one you see on the mixer is probably an idle solenoid? Since the mixer/carb is playing a dual role here. Also should be a switch located somewhere in the same area for switching it from LP to gasoline, the solenoids are controlled by this switch, its function is to switch circuits to energise either the LP solenoid(s) or the gas lock valve for gasoline operation.
Lets not forget about the LP safety switch valve, a flat pancake looking switch with a vacum line. If this switch is not working properly the truck will not run for long or at all.
I guess you have tried in both modes LP and gasoline and get the same results?

Forgot to mention, there is a vacuum lock-off which we have bypassed for purposes of testing.

Ours is old and has the dual fuel carb modified for lpg. It is a very simple system - two solenoids on the Impco, one for starting?? which feeds into the base of the carb with just a small pipe, and one for main supply which feeds to a mixer ring in the carb body. It has the good old-fashioned choke on the top of the carb. One thing I am not sure about is that there is a link pipe between these two feed with an adjustable valve in it - is this for idling and could it affect starting and running? Also not sure how the starting?? solenoid connected to the small pipe operates.

The ones that I work on all have the duel fuel carburetor with a regular old choke and a Beam regulator. There is a mixture contol valve right in the inlet hose to the carb.

on some of the earlier models(when peugot engines were first introduced in these trucks) there was a choke cable and butterfly assy that is inline with the airhose if i remember correctly, this was specific to the LP conversion trucks only if i'm not mistaken.
Later on they got away from this as the LP systems developed into better setups with improved components.

But it sounds to me like your issue might be in either the LP vacum switch or the LP lockoff solenoid. There is a slight possibility it could be in the converter/vaporizer but i'd check the other two things first.

good luck

You say it will start only on full choke? To my knoledge Impco systems don't have a manual choke- as they self choke while cranking. Describe the fuel system on the truck. If the carb has a manual choke, then the carb is incorrect for the impco regulator

It sure sounds like you have a problem somewhere from where the fuel goes in the Impco back to the tank. Did you crack open the hose fitting that attaches to the lock off valve solenoid and see LIQUID propane? It could be the lock off valve itself not allowing good fuel flow. Maybe hook the tank right to the Impco, bypassing the lock off, to see what happens.

What type of fuel lockoff does it have on it a VFF30 Vacuum lock off or a electric type lock off? Also what type of air mixer and governor are on it? Could be any one of those 3