If that is a 28A you HAVE to buy the manual to change the impeller. Also special tools needed. This site has everything you need.
http://www.johnsmcintoshvintagemercuryoutboard.com/
You need an open air spark tester also. Spark needs to jump a minimum of 7/16". Without a manual to tell you what the specs are your DVA test may be useless.
Did you leave the motor in gear when you shut it off? There is a recoil lock on that if you leave the throttle advanced. That motor has the shifter in the tiller handle?