These trucks use the starter relay to signal the alternator to turn on instead of an indicator light. Sometimes when the starter relay goes bad, a novice will jumper the relay & the result is a no shut off due to back feed thru the alt to the ignition circuit. Check the starter relay, back near the left engine door hinge, by the battery.

Apparently you are feeding the ignition system off of a battery hot circuit somehow.
My guess is that the problem is located in the key switch wiring. or that an improper jumper type wire or circuit has been put in place.
Another possibility is that the alternator is somehow energizing the ignition system circuits, and that too might be the result of somebody tinkering.
Do you know anything about electrical systems?

Well thanks for all your help. I got a new system in there and it runs great but noticed when I turn the ignition switch off that it doesn't shut off. I have to turn the propane off till it stalls. I also noticed that when I hooked up the battery that I heard the shut off solenoid open with the key off so I must have wired something wrong. Does anyone have a wiring diagram for this model? There was one wire that was hooked up to the positive side of the coil that ran over near where I was working but wasn't hooked to anything I thought, is this my problem? Thanks

No, you can't use an Impco mixer with this type of regulator because the J&S regulator presents pressurized fuel vapor to the J&S mixer.
Then the J&S mixer meters the low pressure fuel into the intake system.

The Impco regulators DO NOT feed fuel out under PRESSURE.
The Impco system only lets fuel out of the regulator when a negative pressure (vacuum) pulls against the secondary diaphragm in the regulator with enough force to operate the secondary fuel vapor valve.
When that valve unseats, fuel vapor rushes toward the mixer, and away we go.
This is why it was suggested early in the thread to scrap the J&S in favor of Impco components........the 2 brands are incompatible, because they work on different principles.

Can I use a new Impco carb with the rest of the current system?

The problem is that carb was a one of a kind thing that was used on some of the D155 motors. There is a updated carb (1320325) that would work but it is wicked expensive. Maybe a propane fuel systems sales specialist would have an idea. Other than that, try fixing your's or convert the whole system to Impco as suggested. It shouldn't let the fuel flow until there is vacuum by cranking the motor to start.

Thanks mrfixit, I don't think it has a choke. It is an updraft carb J&S model #55 that has a inlet on one end and a diaphragm on the other with an electromagnet on the end of the diaphragm under a cover. Forklift is a Clark C500-45 type G serial# 355 890 2548. Engine plate D155-G spec# 2311180 serial# 225931.

Does that mixer have a choke on it? Are there 2 round covers stacked on top of each other that the diaphrams are in, held on by 6 screws? If so, the part number of the rebuild kit is 882630 and anyone with a TVH account can get that for you. If not, what is the full model/serial number of your forklift.

I did end up replacing the vaporizer after finding the diaphragm was blown and it was full of silicone. I can still hear propane down at the mixer inlet with the intake tube off but doesn't seem to be as bad. Am I supposed to hear propane at the mixer with the key on? I think the diaphragm on the mixer is blown but both the diaphragm and the mixer are not available. What mixer can I get that will work with this system without replacing the entire system? Thanks

Ditto here. BBF is dead on. A couple of posts down you will see Eric had a problem with his old J and S system, carb was not to be found. He emailed me a few days ago- h installed Impco conversion and the world suddenly became a better place:)!

I personally wouldn't rebuild this system but take the opportunity to upgrade to a much better Impco system- IMHO.

tvh.com/newen2/
they might have what your looking for, if not they might lead you in the right direction.

Do you happen to know the best place to get these rebuild kits? Thanks

oh its one of those old things
ya mrfixit is right on that
high pressure can cause it to blow past the mixer diaphragm for sure

That's the J&S with the regulator mounted in the thermostat housing. You can measure the outlet pressure and is it goes up over 15 and climbs then the regulator is bad. That would cause that problem by flooding it. They are available and so are kits for the carb.

It uses an electric solenoid, no vacuum. Are the j&s model 55 mixers rebuildable ?

there are two things to look at
first is the fuel lock solenoid, that opens flow from the tank into the filter and/or evaporator(converter).
Now some LP systems have a vacuum hose running to a filter assy that is hooked inline with the converter, in this case there is probably no lock-off solenoid because the filter is actually the fuel lock valve and opens when the engine pulls a vacuum on it.

secondly there is a diaphragm in the mixer (carb) that also has a shut off (bullet valve) on it.

If you hear it rushing into the mixer (carb) then the mixer diaphragm is probably bad and should be replaced.