Thinking out loud… I have 1995 Johnson 88spl (basically a 90 - same block, carbs, etc.). It yanks my 17’ HenryO up on a plane nicely but tops out at about 27 mph at around 4500 rpm. So, technically, she is slightly over propped.
The 88 runs great. Idles nice. Starts easily. No foot unit issues. Just wish I had a little more top end and would like to get the wot max rpm up in the appropriate range.
If I had $10K laying around, I’d be looking for new motor. That not being the case, I’ve stared wondering what it would take to get more hp out of my 88 (90).
I think the block and foot unit are the same as the same model year 115 hp. If so, might it work to swap my 90hp carbs with 115hp carbs? Might be some linkage issues. Ignition should work. I could probably find 115hp carbs fairly cheap and clean/rebuild myself. I’ve reworked my current carbs twice over the years.
Time to start comparing part numbers between my motor and the same year 115hp. But, thought I’d float the idea here to see if anyone new of any obvious deal breakers.
they are identical
you can even find a set of older 140 carbs
you will need to change the exhaust tuner to the “bubble” exhaust from your current flat exhaust
also, do yourself a favor and install a set of carbon fiber reeds
i think you have some other issue
that boat and motor should run more than 27mph and 4500 rpm
what is your current prop size?
you should be turning much closer to 5500rpms
Hey Chris, thanks for the response. 'Sounds like converting to 115 (or, maybe a 140) might make a nice winter project. Or, I might get talked out of it, if you think she still isn’t running right. Maybe 90HP is enough?
Re: current performance/prop. The current prop is an OMC aluminum 13+1/4 -x- 17.
I realize wot of 4500 rpm is low. But, thought if I dropped diameter or pitch I’d get more revs but less speed. 'Slow enough already.
Either way, if she is not running right and getting her right can get her up into the correct WOT RPM range, going for larger carbs would be fun. But, want her to be dependable before fast (or, not as slow).
if you do carbs, you have to do exhaust
if you get more going in, you have to get more going out, or it wont help
based on your numbers and prop size, you have a prop slip value of over 25%
normal is 10%
just getting your prop slip under control you would go up to 32mph @4500
if you had a 15pitch prop and could turn it 5500rpm (which you should be able to do) with 10% slip, you would be running over 35 mph, and a 17 pitch would put you at 40mph