Change the spark plugs. you can over pressure the needles in the carbs and cause fuel to run out the overflows. a 90 has more carbs to devide the pressure therefor can pump up harder than yours which is 2 carbs. Best case is that you have stopped up carbs and they need to be cleaned. when you open the throttle the carb gives a small squirt of fuel through a tiny hole on the venturi of the carb which helps the motor build torque to get the rpms rising. with no load on the prop that little squirt isn't neccessary. Under load, it is. worst case is that your motor could be low on compression (if it isn't, it will be if you keep rapping it on the muffs). Change plugs, if that doesnt help then have the carbs cleaned, if that doesn't work, check compression.