motor performance

I have a 15' truline fiberglass vee hull boat with a 1984 70hp evinrude outboard. I just got it a couple months ago. I took it out a couple weeks ago and gave her a run. Was very disappointed in it performance. Also the motor cut out and would only do 6mph. Got back to the house and checked the plugs, they didn't look too bad, but they were champion plugs, I don't like champion plugs. Changed the plugs to NGK and put some seafoam in the tank. Took it out today, the motor did not cut out, but all I could get out of the boat was 28mph, according to my gps, I don't know what the compression is in each of the three cylnders and should probably have that checked and don't know what size the prop is on it. My locations is Boise Idaho, the boat came from Randall Washinton, not much difference in elevation. Does anyone have any ideas for me to look into???

Re: motor performance

I just took a brush to the prop and the size is 13 3/4 x 15 aluminum

Re: motor performance

first the motor was designed to use champion plugs, QL77JC4. you need to do some trouble shooting before thinking a new prop. compression #s, spark test on all 3 cylinders. when were the carbs last cleaned and rebuilt, that is a very common and popular prop. you don't say what year the boat is, or the condition of the boat, it could be carrying 500 -700 lbs, extra weight in saturated foam, under the deck.

Re: motor performance

I believe the boat is a 1989 model year. The guy I bought the boat from rebuilt the floor, new ribs, new foam, new floor. The hull is in great shape. Am not sure what type of foam he used thou????
The boat seems to be heavy, it takes the chop really well while running.
I am not a mechanic.