235 johnson outboard.?

Question

just had the motor rebuilt... they say to keep it at 5750 rpm's, but I only got it up to about 55 mph that way. I use a three blade prop. it seems to me that it has potential to go 65 or at least 60 mph. What can I possibly do to increase topend speed?

Answer 1

chang the prop pitch to get more top end but you will loose some take off power out of the hole...try another pitch and see if you loose to much at take off if so you may wanna invest in a jack plate for the motor or some engine mods..

a jack plate gives ya more adjustments on your motor trim by moving the motor up and down unlike the trim,,this will put theboat on a better plane and make it somewhat faster on top end but try the prop first..theres a certan pitch for that motor that make s it perform the best at both ends but you can add to either end but not both with a prop change

Answer 2

If this is a fresh motor-you must be now running double oil --Right? Did the guy who built it tell you to run it 5750rpm? Or some salesman? I'm sure the mechanic would have told you to stay below 4000 and only go there for short periods of time. A fresh rebuild from a shop will require some break in time before you can go run it like a new motor. Hold off on all the "go fast" stuff till you have 20 or 30 hours on the motor and are back to regular oil mix.

Answer 3

Over 6,000 is overreving -- you want a prop that will get you to 6000 at WOT, with a normal load on board.

If you get to 6000 and still have throttle, you're going to have to push it up to full throttle for a short time, to see how many RPMs she can make.

Every inch of additional prop pitch will put more load on the motor, limiting the top end RPMs by about 200.

Depending on what year, your motor probably has a gearcase ratio of 1.65 to 1.  Means that the prop turns once for every 1.65 crankshaft revolutions.

With an 18-pitch prop, then you'd would go 59 mph at 5750 RPM (5750 x .606 x 18 = 63k inches per minute = 59 MPH).  But the prop is pushing water and some of it gets out of the way instead of pushing back, so you get some inefficiency called "slip".  If your actual speed is 55, then you're experiencing 8% slip, which is in the normal range (6% to 10% at hi speeds, usually greater at low speeds).

Now, if you could push it up to 6400 RPM at WOT, you'd be going 61 MPH if slip didn't change.  Going up to a 21-inch prop limit the revs to 5800, but the bigger prop would push the boat 64 MPH.

You need to find out your actual gear ratio and prop pitch.

This is not something to be doing until AFTER the break-in period.  

Make sure you use the recommended spark plugs, surface gap champion UL-77V.