TOPIC: 20hp mercury pull start

hi,i am trying to help a friend troubleshoot a 1981 20hp serial# 7100084. It is a pull start and when you pull the starter rope it pulls hard and when it tries to fire it wants to pull the rope out of my hand. I have gotten it to start but haven't run it past a fast idle. Could it be a timing issue? I pulled the starter off and everything looks good there, the clutch dogs are working as they should. compression is 90 pounds in each cylinder. Any ideas?