TORO Mowers

Anyone knows who to trouble shoot mower problems?

My mower does not start when the rope is pulled. When you pull the rope it is not a smooth pull. When the plug is taken out everything seems to pull okay and ofcourse really easy. Does anyone know what this might be. I have replace the head gaskets the magneto, changed the plug and lapped the valves. I just cant figure it out any suggestions would be a big help. This is become more of a challenge now than a lawnmower fix LOL! I have also replaced the key and checked the gap between the flywheel and coil. The mower is only two years old and a 6.5 hrspower front wheel drive.