I have a Doosan G33p-5 which occasionally backfires in the inlet manifold on starting, this in turn blows one of the inlet valves open and causes the hydraulic lifter to hold the valve off its seat for a few seconds causing the truck to run on 3 cylinders. once recovered the truck runs absolutely fine.

It probably does this 1 in every 10 starts, the problem its causing is it destroys the manifold pressure sensor over time.

Any suggestions what could be causing it, ive had the crank sensor out and cleaned it, fitted new gas vaporiser and replaced the carb diaphragm, the truck has a cam sensor but there is no wiring to it, doesn't look like there has ever been wires to it.

Thanks