1978 1150 prop

I have a 1978 mercury 1150 sn 4859218, its on a 1978 Ranger not sure of the model, its 17ft and weights around 2300lb with the trailer. The reason im telling you this is I have a 17 prop on it and it sems to be running to high of throttle when the tach worked it would read over 6000 Rpm's, since then the tach has quit working, rectifier I think (anyone know which one I need from radioshack). But it seems to sound like its cutting out or something I just cleaned the carbs, new timing belt, new trigger, and new switchbox the timing seems correct, if its off alittle would it sound like that? you can hear and feel the motor rpm's jump around it will speed up and slow down roughly 35 to 40 and back again, and it keeps doing that. The more I try to trim it out the more often it gets worse. any help is greatly apprecitaed
I would like to thank you for any help I recieve
thank you

Ryan

Re: 1978 1150 prop

Sounds like you got yourself a 1978 Ranger Aztec. I would say the 17 pitch prop is letting your motor turn to fast (good hole shot but no top end). I think every pitch is about 300 rpm. If I were you I would find myself a 19 pitch prop.