does it have spark when it wont start? if not, turn key on and off repeatedly and see if it sparks when you turn the key off. if it does not spark when it wont spark and it does spark when you release the key, at least try the ground cable. even if it sparks but it is weak (yellow and short). if it has good spark when it wont crank let me know if it has a standard HEI distributor or an electronic spark advance distributor. (HEI has a vacuum advance).

this doesnot do that it just cranks over I tried a new lpg impco system, hoses, vapor hose, cap rotor wire plugs new battery jumper direct to coil from battery nothing seems to work no matter what I do after it starts it wil fire on second spin every time let it sit 2 hrs or the next day same problem, will not fire on starting fluid either

if it occaisionally fires after you release the key, (this only applies to spark issues) add a ground cable between the battery and engine. i know it sounds crazy but it is a common problem with some of the vortec engine trucks and other brands have had this problem. you can also do a voltage drop test between the battery negative terminal and engine while cranking and that way you can see how much resistance is there.