I have inherited a service problem, my customer has a Cat GP18K forklift with very low power, its an LPG engine, it stars instantly, tick over is smooth and quiet, but when the accelerator is pressed very little happens. when you engage drive in either direction it barely moves. I have never dealt with lpg systems before so any help would be appreciated. it has an Impco model 100 gas control unit (not sure what its called)

cheers in anticipation zzzzzzzz