i just remembered that i had a problem with a clark that had a mitsu engine when i first started working on forklifts

it wouldnt start but when it did it seemed to run fine
after beating my brains in lol another older tech told me to try changing the pressure relief valve for the LPG up on the rad cover
sometimes they go bad and wont let the engine start but if you manage to get it running it will run fine

that was the case with that truck
its a cheap and easy thing to try and it looks like youve tried everything else

(how funny would it be if you just had a square cut o-ring from the tank stuck in the LPG connector lol)

i believe the backfire was part of the issue too but it was like 9 years ago so i dont remember the exact symptoms