I got a Hyster H120XM, K005D, GM V6 with the impco system equipped, with a no start situation while it's hot. It turns over and if you play with the primer on the vapourizer, then it will catch and fire up.

I've verified that the ign system is working ok. Checked that the lock-off was letting fuel in, inspected the diaphram in the mixer, and someone else put a vapourizer on it already.

Now this truck has whats called the "Open-Loop" fuel system on it. The spring side of the diaphram in the vapourizer is sealed off, and has two vaccum lines; one going to the governor, with a normally-open, power supplied-closed switch to close that vaccum line off. The other line @ the vapourizer goes to the clean air intake, with a normally-closed, power supplied-open switch to open the diaphram to atmospheric pressure.

Anyway, when I take the atmosphere supply switch out, the thing starts no problem. The power being supplied to these switches are
-for the governor switch: 0V with key off, 6V with key on, 10V cranking, and 12V running.
-for the atmosphere switch: 0V key off, a momentary 12V with key on, witch goes back to 0V. ( I mean momentary, to quick for my meter to read) 12V cranking and 12V running.

My resistance at each coil in the switches is 24ohms and 27ohms respectivly.

I suspect that the "atmosphere" switch (I'm sure thats not what it's called, but thats what I'm calling it.) coil is getting weak when it heats up, closing off the supply to the vapourizer, but if you get the truck running, then it will continue to run.

It seems to me this system is just acting like another lock-off for the truck, supplying vaccum to the vapourizer, or closing it off from the atmosphere, basically shutting the truck down. What is the purpose for this system?

I think I'm just gonna cap the line @ the governor, open the one at the vapourizer, and call it a day, but if you've run into, or have any info. I'd appreciate it.