I have a craftsman 18.0 hp 42"cut mower model #$917.274391. The engine will run if the clutch/brake is engaged but when I let out on clutch the engine dies. Also if I try to engage the blade the engine dies. I replaced the drive belt but that does not seem to help. Any suggestions?

It sounds like the "reverse" switch may be stuck. Check that first. Many modern machines have a switch so when you engauge reverse the blade has to be up to prevent accidents.