I try starting my lawnmower (Craftsman DLT 3000, 18.5 HP), the engine starts to crank and locks up. I remove the spark plug and engine keeps cranking. When I put the spark plug back in the engine begins to crank and locks up. I can manually turn back the pully cover on top the engine and the engine begins to crank and then lock up. Any ideas?

Hi,
I will assume that this mower has a Briggs V-Twin on it. The problem is most likely the valve adjustment, particularly the intake valves as these are the ones that the decmpression works on. The valve covers are each held on by four bolts and must be removed. The rockers arms nearest the top are the intake. Thelocking set screws within the pivot nut must be loosened(T-20 Torx) and the adjust made to 0.005 in. with the piston of that side 1/4" past TDC.
Tighten everything down and reassemble and crank it up!
Peace,
Clarke