I bought my 250AS new in 1989, and, honestly, it has NEVER been right on starting. I had the starter rebuilt once, attached an extra ground wire., rewired the switch once...everything I could think of, but for the last 3 years, I have had to start the tractor with a screwdriver across the two large posts on the old starter.
Ebay seller CALTRIC sells the reduction drive starter for the 250AS for $155 with free shipping. Got mine in last week, and the results are astonishing.
As an aside, A few weeks ago, my original equipment alternator locked up, so I replaced it with a Delco 1-wire for under $60 from Auto Zone. The Delco pulley is set back almost exactly one inch, so I removed the original mounting bracket and re-drilled the holes 1 inch forward. Only two holes will catch now, but these studs are very large and the alternator weighs less, so all is good. I also lengthened the top slide rod adjuster by two inches so the alternator has somoe adjustment.