Does it spin a few times, then stop? If so, chances are you don't have the best connection. Did your friend use his cables, or yours? If he used his, chances are his cables are better than yours.I have a Wizard lawnmower with a 14.5 hp Briggs, and I can't get it to start. When I turned the key, it would crank until it started to build up compression. Then it would stop.
Does the engine spins less than twice and stops? And if you wait a few minutes, it will do it again? Is it an OVH engine? (Over Head Valve) If so, the valves may need adjusting, because the clearance may be too great and won't allow the Compression Release to do its job. If adjusting doesn't help, then the compression release may be bad.