First you could have a stuck fuel lock that is not shutting off also check the fuel pressure at the top of the Nikki converter there is a nut with a Phillips slot in it remove the nut and install a propane gage it should get approx 5psi with key on and hold steady when running could go up a few pounds with key on but should come back to 5psi. You could have bad o ring as stated by other post on main injector. if every thing checks out and you do not find anything wrong.the last thing that could be wrong is inside the main injector housing there are two popite valves that could stick if that is the case l would replace the main injector housing hope this helps