The engine surging may be a clue for you, it will do this when there is a shortage of fuel, this may be due to incorrect setting of the main Jet or this jet becoming restricted with crud.
To set the main jet, it is not enough to set it like you did for hard work in the field and it will overheat. That setting is OK for light work and to get it running before you set the carb.properly. Get the engine hot, that means it must have been running at least 20 minutes, open the throttle fully and adjust the main jet,start to close it until you hear the engine note changing and then open it to about 1-1/2 turns, close the throttle and allow the engine to idle slowly about 450 rpm ONLY when it runs at this speed can you set the idling. The engine idling speed screw is at the back of the Carb on the side nearest the air cleaner. Now turn the air screw to the right (close) until the engine exhaust note changes and then unscrew (open) the air screw SLOWLY until the engine runs smoothly. You will need to turn the screw a little one way and then the other to find the sweetest spot for the adjustment.
If you can't get it to run smoothly, you should carry out a service of the ignition and set it according to the book for this engine, unless you do this it won't run properly. When you have set the Air Screw, open the throttle quickly and the engine should speed up without blowing black smoke or spluttering. If you get black smoke, turn the main jet in-wards 1/8 turn and try it again, if it splutters, turn the main jet out 1/8 turn. Do all adjustments slowly with the engine running, you need to allow a little time for the Engine to change when you have adjusted the Carburettor, especially if it is idling. You MUST have the engine idling at 450 rpm before you can set the Air Screw and you must have the Engine HOT before you can adjust the Carburettor.
I saw somewhere in this post that it was overheating, this can be due to either incorrect ignition timing or the Carburettor set too weak, but don't forget that although you may have the Carb set 1-1/4 turns open it can be that this jet is restricted and it is not getting the full flow like it should do. If, after setting the Carburettor and it still surges at full throttle, it can be an Engine Governor adjustment that needs checking and adjusting. Overheating can also be caused by a blocked or restricted Radiator or a pressure leak on the cooling system. Look for rusty water marks on the outside of the Radiator and Engine. Make sure that the Radiator Cap it marked at 7lbs on the top, this is the pressure in the cooling system. it allows the engine to run hotter before it boils and is important. If you are using a Bush-Hog, are the blades sharp and are you cutting too close to the ground, it is harder to cut the lower you cut.Have the back of the Hog set 2" higher than the front so that the cut grass will escape faster and not absorb engine power.
If you need more info about all this, email me and I will send it to you...John(UK)...firstname.lastname@example.org