f its got a M4-121G in it it shouldnt have a CA100 cant remember the model number but think its a CA90 its a smaller thinner carb. Theses carbs were prone to this problem they have a piston with a hard ring on it that wears the carb body and will cause the engine to run like crap under load Hyster used to sell a repair kit for it but if its that bad a new card is prob the only answer. Another thing to check has the distributor got points and a condenser in it??? Ive know a few techs who never think to change them both or even to check them Have a look if it has get them changed and see if it improves things