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