You can visually check whether the lever on the engine is hitting the stop when the control lever at the helm is at maximum. Achieving 3600 in neutral as little as with no load it does not require much fuel. Is the 2000rpm under way or tied up to the pontoon? Quite normal not to reach high rpm in the latter, although it should be higher than 2000. Two main causes of low rpm under load are insufficient fuel/air mixture and overloading for example because of the wrong propeller or a heavily fouled propeller.
