I have a 10 HP Briggs (28B707) in a rear engine Toro. It was running fine then started to loose power and after shutting it off it would not start. Now its blowing air out of the carburetor. Applied air pressure to the cylinder at TDC and air comes out of the intake. Pulled off the head. Put a flash light into the intake port and can see light around the intake valve seat. The intake valve opens as it should on the intake stroke but doesn't completely close on the compression stroke until the piston is at TDC. Since the exhaust valve seems to be working ok and the camshaft is driven by a gear on the crankshaft, why would the intake valve not be closing properly?