There is a minimum cranking speed, poor contacts anywhere between the battery and starter, low battery can also cause a no spark condition.
Clean all battery cable connections and make SURE the battery is charged and it cranks over well.
I have a 1984 merc 50 hp.model # 06486332
it is 4 cylinder
I lost spark in the ign. system last year
its cranks but no spark
and looking for ideas
would it be stator , voltage reg.,etc
do not have a book but going to
look for one