I've seen a shortened P38 (shortened to make even better at off-roading, I forget the name of the mod 'Hog' or something... I'm sure someone will know) with extremely poor running to the point it wouldn't start on petrol even with a brand new engine because of a fuelling issue. Part of the mods on this particular example was fitting of a smaller petrol tank in the (very much shortened) luggage area. The fuel pump on this petrol tank had been sealed with some sort of silicone, it seemed this silicone had made it's way through the fuel system messing up injectors and the fuel pressure regulator. It was trailered to me, I was tasked with converting it to LPG by the owner but simultaneously tasked by the engine builder (who I know) at proving the engine he built was a good one just that the problem was with the petrol system... There was a dispute between the owner and engine builder because the owner reckoned the builder had made a dud engine, they agreed to let me find the real situation. I fitted the LPG system (mixer type) and the engine fired straight up and ran great on LPG, much to the engine builder's relief! The customer then had the option of starting and running on LPG only or fixing the petrol system, he went with running on LPG only. I could have fixed the petrol system but would probably have involved swapping out most of the bits (pump, pressure regulator and injectors) and flushing all the lines.