I work on a LOT of forklifts at my job and think I've seen it all. I have an early 90's Cat forklift with LP conversion, (Peugot engine, not really a winner in my mind). If you're familiar, these use an Impco Model J convertor coupled directly to the gasoline carb which acts merely as a throttle butterfly and keeps the pneumatic governor action of the existing carb. Its actually quite a clean set up and we have quite a few. This one has a performance issue. Will hesitate and stall or fall flat when accelerated. Pull off the air cleaner hose from air cleaner at the carb and it will rev up! Now, theres not even an element in the filter right now just the empty housing! But, attach that hose and "bog", pull it off and "rev". I've checked ALL the usual. The convertor rebuilt, tune up good, timing good, tank full, and NO vacuum leaks, intake hose clear. What gives?