I am good with engines but not on Johnsons . My buddies engine starts and red lines in Neutral. What causes that and how do i fix it?

First thing is to take the air silencer off and make sure all four butterflys in the carbs are fully closed.  Then check the mechanical advance and make sure it's coming all the way to the stop and that the stop hasn't gotten turned in too far.   If the carbs are fully closing, the timing should only take it to about 2000 rpm so even if the mechanical advance was sticking, it would still have to be getting air to hit max rpm.   So, if the carbs are fully closed and the mechanical advance to coming to the stop, you probably have a major air leak somewhere in the intake system.

that make sence thanks