Surging - where the engine speeds up and slows down - is caused by a lean fuel air mixture. The carburetors built after Sep 1997 were not adjustable or equipped with adjustable needle valves. Mixtures can be adjusted with replaceable jets located inside the carburetor. If the carburetor was rebuilt correctly, there are other things to check. If the fuel tank is mounted higher than the engine, remove the fuel line from the carburetor to see if fuel flows freely. Aim the fuel into a jar to see if there is dirt or water present. Flush any debris from the end of the fuel line before reattaching. Another check is to connect a vacuum gauge to the carburetor fuel fitting. This section of the carburetor is actually a vacuum operated fuel pump. Crank the engine to see if suction is present. If there is no suction, check the vacuum hose connection from the the crankcase. Sometimes the hose ends become cracked and vacuum is loss. Is the fuel tank in the rear of the tractor? Assuming the fuel pump has suction, attach the vacuum gauge to the end of the fuel hose to see if the fuel line is good. sometimes cracks form in the fuel line and air is drawn in. Look for cracks or loose clams on all fuel connections. Look for dirt and debris in the fuel tank. If the fuel lines are good, check the gaskets around the intake manifold to cylinder. A lean fuel air mixture could also be caused by worn valve guides - but is not likely on the opposed twin. Most likely there is still some dirt in the carburetor. If the float is the plastic style and the float has warped, the fuel level in the float bowl is too low. If the person who rebuilt the carburetor used fuel to wash the parts, the carburetor could still be varnished. A carburetor spray must be used to clean the internal parts.

Electrical problem with the Rectifier. Lost it's internal ground and can not "tell" the alternator to produce the amps needed to operate much more than the head lights. Simply run a wire from one of the screws that hold the rectifier to the engine block and ground the other end to the frame. Problem solved. Common problem with Command and Courage engines. I have saw countless people waste hundreds of dollars trying fix this very problem when all is needed is a piece of 14 Ga wire.

any idea how many makes/models/engines are out there? Sure might help if you would share what you got....

The usual reason for the grinding noises, and poor forward or reverse, is: Its plain worn out! Time for a new tractor. If ya want longevity in a lawn tractor-buy a standard shift! Of course, if you truly love the machine, well then--go spend as much as a new tractor would cost, and have a new transmission installed!