Just need to swap out the spring in the model J Regulator if that is what ya have. For all the reasons ETS1 posted
http://www.impcoparts.com/product-p/jo.htm

You most likely have a F163 continental engine in this truck. Check to see if you have a vacum switch on the intake manifold above the carb. It needs vacum to work and turn on the LP lockoff switch. Also some of them had a oil pressure switch hooked up through the starter circut. As long as you have oil in the engine and good pressure when cranking it would turn on the LP lockoff switch. In any case check to see if the lockoff valve is getting 12v? Also you can take the lockoff valve apart. remove the coil then the center plunger assembly. Check the little spring on the end of the plunger. It gets loaded with rust from the tank and the spring won't compress to let the fuel in. But like I said you need good cranking vacum to start. If it has a Impco carb on it? You want to check it out too! If the engine is beat you may not have enough vacum as needed. You may want to also take a compression test. Goods about 140 to 150 lbs. Take care ETS.

Thank you for your information, I tried this and the lockoff is working correctly, no gas gas came out from the inlet carb pipe when the engine was cranked, I assume the vapourizer/regulator is bad, I have a new one on order. I will advise when I have installed the new requlator.

Thank you,

Tim

check the lockoff and vapourizer.sounds like you've only got enough vaccum to make them work when the engine is firing.take the pipe off going to the carb and crank it over.see if any gas is comming out.dont do it for too long though,its quite dangerous.