1920 in need of rebuild?

i recently bought a Deutz-Allis 1920 ultima. it has the 20 hp kholer Magnum engine. when i am mowing it seems to dog down in some of the thicker grass i mow in the yard. also when i shut it off or it stalls out i have to let it sit for about 10 minutes before it will restart, and when it does it blows out black smoke on start up which i am thinking is raw fuel because the same thing happens when i pull the choke while it is running. when i first start it it starts right up. any help on this would be appreciated as i am not familiar with the magnum engine.


Jason