I own an 18 ft 1985 StarCraft. 400 stringer 3.0L, 140HP. It runs strong, and planes out fine but the engine tops out at 3400 RPM. It has an electronic distributor, new plugs, wires and the compression is good. I changed to a higher pitch prop that improved the hole shot but the engine still topped out at 3400 RPM. The carb is working perfect and the plates are wide open. My question regards the exhaust flow. I noticed when I stated it the exhaust appears to be coming out around the outdrive where the sterndrive drops into the upper case when the drive is lowered, not down through the prop. If I understand the system correctly the exhaust should be going down through the sterndrive and out through the prop center hub. I am thinking that the exhaust back pressure from a plugged exhaust passage may be creating a lot of back pressure and limiting the RPM. Am I understanding the exhaust design correctly? Is there an easy way to check the exhaust passages for debris or a fault that would cause this?