I have a YTH 1848 XP with a Kawasaki 18 hp FH531V motor that will not crank over, I have:
1. replaced the starter and solenoid.
2. read no voltage on solenoid output to starter.
3. felt solenoid with hand and feel it "click" when turn the key with brake on and PTO turned off.
4. did not feel...
I had the exact same issue, had changed ignition module and carb with the problem not going away. I changed the spark plug to a NGK BPMR6A per your post and it runs great now - THANK YOU!!