ok, thanks for that info
this unit is older and has the Impco system on it.
so what we suggested earlier, the coolant is probably low and not circulating through the vaporizer/regulator. This will cause it to freeze up and run badly.
I would still check the circulation though and make sure all the bypass hoses running to and from the vaporizer are flowing properly and the coolant level is full in the radiator. Also if it is full make sure it is circulating by watching the coolant in the radiator while it's running and hot, it should be swirling or moving around inside the filler neck area. If you don't see the coolant moving then the water pump may be bad.