We have a truck with a Nissan H20 engine and Impco LP carburation. The engine backfired and now the truck is extremely hard to start. The compression is 140 psi on all cylinders, there is good spark. We have removed the front cover and checked to make sure that it didn't jump timing and it is perfect. We removed the head, checked the valves, cleaned and relapped them. The lock off/filter, lp regulator and mixer are all know good components and have been checked on other trucks. The engine has good vacuum as well.
What happens is when you turn the engine over it will fire and die and the more it is turned over the more times that it will fire and eventually it will start. Once it starts it runs fine and accelerates well. We're stumped. Any help would be apprecitaed.