1988 merc 90hp can't break 4000rpm or plane

[k_loy]

Also have replaced the prop hub already so shouldn't be any chance of a slipping prop either

 

[Silvertip]

If there is no compression on #1 the engine is making only 2/3 of its power. What do you expect? Instead of 90 up you actually have only 60 and the two remaining cylinders need to drag a "dead body" along. Keep running it that way and you will totally destroy the motor.