I'm having a similar problem. Earlier in May, she acted as if the batteries were dead although they were full charged. Jumping the solenoid seemed to work. Then she sat for a while. Today she started acting the same way after a bit of mowing and she died. Opps needed a bit of oil. Ran some more, then turned her off for a break then hitched wagon to her. She would not start. I could see the engine trying to spin then quit. Then the solenoid began to smoke! I replaced it. I took her a short distance to pick up branches, and she's doing the same thing (though I quit before smoking this new solenoid). Perhaps then it's the starter? Yes, mine is a YardMan 20hp also. (1985 or so model)
Katherine, April 2007