i have a 1989 merc 115 with a 90 hp lower unit on a 17 foot bowrider.i was running a 24p s/s lightspeed prop and the rpm,s are still in the 5400 to 5600 rpms.i bought a 26p and it won,t get out of the hole.should i make the vent holes bigger or what. it acelerated fine with the 24p. any sugestions.