There's a plate and diaphragm on the top of what you're calling a carb, I think Clark calls it a final regulator. A bad diaphragm here will give you problems. But also check that you're getting good LP from the tank and that there is only one black gasket at the Rego fitting. There is also usually a fuel filter at the fitting where the LP hose from the tank attaches to the back of the truck. Good luck with it.