Get a impco mini kit , you will leave old vaporizer in housing plug water ports
The mini kit comes with vaporizer, fittings hoses and lock off carb and fittings and hoses except tank and tank brackets that's what I did when I ran across thise I did it for years
I retired from Clark dealership after 32 years in field service

Impco

low coolant or thermostat

Bill make sure your problem is related to a propane problem if you buy the kit you will have to do some conversion to make it fit the old zenith went into the thermostat housing the you will have to make water lines to hook to the emco from the engine block also you will have to mount the emco converter to the frame of the truck also make sure it will fit to your manifold measure bolt to bolt on the old carb to the new carb you could call centuryfuel in Chicago and give them the model and serial number or Nash fuel they should be listed on the web they both should sell conversion kits for this old Clark if you need help don't hesitate to get back to me.

make sure you have coolant in radiator or thermostat is working or just remove thermostat. have to have coolant to heat lp system or it will freeze up.

Scott, I am having the same issue as arnold_d Clark C500 runs for like 5 mins and then stalls. I have to leave it sit for 5 mins & then i can repeat the cycle of use for a few minutes & then it stalls. After seeing this i think i might be able to fix it with the J&S 525k Impco Replacement. It is on Ebay for $370...... was hoping you might be able to confirm that it would work before i buy it. Hoping you see this soon. Thanks, Bill

The correct pressure is 5to7lbs if my memory is correct

No problem hope you get it working as for the zenith converter it was in the thermostat housing had a bolt that went in the backside of the housing that held the converter in. If you get a converion kit you leave the old converter in the housing the kit you want to install is emco it should have carb and emco converter with water lines and bracket to mount unit you can use electric fuel lock or vacuum lock zenith had electric so might be easier to in stall if you call century fuel ask for Harry if still there he is very knowledgeable hope l was of help if they can't help get back to me

Thanks for your information. I wish you were still working on Clark lift trucks since I haven't found a knowledgeable mechanic yet.
Was the old zenith converter in the water jacket? The water jacket fits over a hole on top of the engine connecting it with the radiator.
I have the steel body carburetor with choke cable.
I looked at century website and will contact them for a conversion kit. That seems like to only way to go. I hope the conversion is easy. Any tips for the conversion?
You gave me lots of good information. Thanks again for your help.

I read your post l worked on lift trucks for 37 years the system you have is very old it used a zenith converter in the top water housing it put out approx if l recall about 11 psi there is a Allen screw in the converter top if you remove you can check the pressure there it bad will drive to apply 30 psi also there were to style carbs that were on that truck one was steel body with a chock cable and also later and allumium body with and electric primer they had problem with them if you check the pressure at the converter and it is good then the carb is bad l do not think they are available any more you may be able to get the zenith converter LG it checks out bad. the other thing you could do would be is to change the whole system to emco you could call century fuel and ask them for a converion kit also l recall the converter went in the thermostat housing there was a metal ring that held the thermostat in if it moved over the water hole fitting the converter might freeze and give the same problem you have hope l shed some light for you