I have a 17 HP Kohler engine on my riding mower. It all of the sudden started loping. If I take the air filter off anf hold a gas can by the carb, it straightens out just from the fumes. Why do engines lope?:(

Sound like a carburetor problem. I would start with filter, jets, choke circuit(a good cleaning). Don't just turn Idle or air screws to avoid correcting that later. Fuel. Water in fuel?

Causes here could be is a partial blockage of the main jet. That's for bowl type carburetors only. Stop the fuel flow to the carburetor (pinch off the line )

On kohlers the carb jet is attached to the center post of the carburetor. Some are cast in and others are screwed in. Blow this hole out and blow up the center post to clean the main nozzle. Use compressed air or carburetor cleaner.

Another culprit here could be a leaking intake gasket.

See if carb is loose and the manifold connection at the block.