85 hp max rpm

[bashr52]

I have an 85 horse power head on my 1979 (1980 model year) 75 horse lower unit on my 17 foot tri-hull. I finally got a working tach installed last weekend, and took the boat out for one last trip before pulling it for the winter. Max speed was 35, but after i re-did the floor, stringers, and water logged foam, I was able to get 37 out of it. Trimmed out and at full throttle I was only able to get about 4900 RPM's out of it. I have a re-built 17 pitch prop on it, and am wondering if I may be better off with a 15? I know the carbs are a bit rich since I get smoke at start up that does not immediately clear up, and plugs seem to be running a bit black. Timing and carb linkage are adjusted to spec, so the only other things I can think of are to big a prop, and carbs to rich.

Suggestions?

 

[Frank Acampora]

Re: 85 hp max rpm

4900 is almost at the center of the operating range of 4500-5500 and only 100 off the horsepower rated rpm of 5000. AND, 37 MPH is not too shabby for that engine.

If you go to a 15 pitch prop, you will most likely gain another 400 RPM BUT-----top speed MAY drop a bit. You need pitch to go fast.

If you can borrow a test prop in 15 pitch, try it, but I would not buy one just to test it.

 

[bashr52]

Re: 85 hp max rpm

OK, that is good to know! I'll just concentrate on getting the carbs dialed in and double check everything else (timing, linkage, etc) and run it!

Should I adjust carbs for best mixture at idle, or at speed?

 

[Frank Acampora]

Re: 85 hp max rpm

BEFORE you play with the carbs, search and read my post: Synchronizing carbs and timing. Everything is in that one.