18.5' Bassboat, 1974 Evinrue 115HP, 13-3/4X 15 Aluminum prop, The boats gets on plane supper quick and at about 1/2 throttle it will cruise on plane at about 20mph if I go WOT it just makes more noise but doesn't go any faster, every now and then it seems to kick in, sound better and begins to pick up speed and then gets louder and slows down. It also sounds like a slight miss when its running loud. Plugs are good, compressing good.

Thanks

Joe

Explain louder:

Exhaust

RPMs

What?

Give us a clue

Sorry, say you get on WOT to get a hole shot, you know how the motor screams and then you get on plane back it off and it sort of just Hummms. Mine will humm at 1/2 throttle and scream at WOT but doesn't go any faster. 

Are you able to verify the RPM's are increasing when you go from 1/2 throttle to WOT, what is the increase in RPM's form 1/2 to wide open. Is your prop damaged? Are you sure you have spark to all cylinders? Is your prop blowing out?

Check the hub. Mark it with some paint and go run it. I bet it comes back out of alignment.

I had a 85hp Johnson on this boat with a 13 X19 prop that I busted up, I got this 13-3/4 X 15 for $10.00 at a yard sale, it push my boat around 30mph. The 115 Evinrude was my sons motor, I have a SS 13 X 19 I'm going to try. According to Michigan Wheel it should have a 13 X 19, I'm trying all the free things first, I have a tach on my boat but I wasn't watching it, I will next time I'm out.

Joe

You really need a tach reading. I would think the 19SS to be too much prop, how many rpm was you turning when you had it on you motor? That 74 115 is about the same as as your 87, 85hp on the back, provided it's a good motor. The 115's became the 90's when they quite rating off the flywheel and started rating off the prop shaft. They just droped the compression for the crappy, low octane gas the oil companies had gone to.

I haven't been out since I had the problem, I do have a tach, I just wasn't watching it, I will next time. The 85hp Johnson I had on the boat is a 1975 not a 1987 but that pushed my boat 32mph with the SS13 X 19 prop, it's the only other prop I have except for the 13-3/4 x 15 that's on there now. I'll keep you guys posted.

Thanks Again

  Joe

The two most important guages on a boat:

Tach

Water Pressure

I'm clueless why you would not watch a tach, especially when you are having issues.

Sounds like either your hub is slipping or you're cavitating. Take fishfordollars' advice.  You'll find out in a hurry whether or not your hub is slipping.