Hi, I will try to help you out. I think it just needs adjustment, probably the high speed mixture needle located on the bottom of the carb. Get it running at high speed and try to turn the screw in 1/8 turn increments. It sounds as if it might be flooding out. Turn in (clockwise) 1/8 turn and wait a little, it takes a few seconds before you hear the difference. If it seems to run a bit better try that again, if worse turn back to original position and turn it the other way to add more gas to the mixture. If a bit better try that again and listen for the difference. If that that does not work at all try the low idle mixture screw which is located on the side of carb, it has a spring on it just like the high mixture screw. Do the same as previously described and see what happens. Some might say the low needle has nothing to do with the high speed, but it does have some effect as you will notice. Bring to idle and adjust for smoothest speed.
Other things that could cause that is the fuel tank cap vent hole might be partially blocked. (if you remove fuel, air must be able to enter so a vacuum does not stop the flow of fuel). Something that happened to me is that the shut off valve under the tank gradually closed with the vibrations causing low fuel flow. The carburetor might need cleaning or the fuel filter might be plugged. The gas tank might have dirt in it limiting fuel flow. The governor might need adjustment. With the more load you put on the engine, the governor should feed more fuel to the engine to compensate. Is there an air filter on this engine, if there is check it out and clean or install a new one.
That is about all I can think of right now. Without hearing it run it is difficult to give you an exact fix. All the best and hope this is a bit helpful to you. Good luck. Will gladly answer any other questions.