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Converting a 1990 Johnson from 9.9 to 15 hp

R

reiddo1

Member
I have read on some posts that a JOHNSON 9.9 can be converted to a 15 hp by doing just a carburetor swap only. Is this all that would be required for a 1990. This engine is a long shaft and I would like the increased power since I intend to use it as a kicker on my 1979 Glastron Sea Fury v216. Any comments are welcome.
 
Yes it can be done, But in your case it will make no difference at all in power. The 15 hp makes it's power at 6300 rpms the 9.9 at 5500 rpms. Because your motor on that boat will never see those Rpms, the 9.9 carb is a better choice than the 15 hp carbs. Because of the lower RPMs your motor works at the smaller venturi in the 9.9 carb works better.
 
I run a 15 hp kicker on my 16'ss starcraft, that motor runs to about 4000 rpm with a 10" pitch prop. So if I need another 2000 rpm at the theoretical 1" = 200 rpm, I need a 0" pitch prop. That motor on the OPs boat is always going to lug. You cannot get around the fact that sometimes the motor is so underpowered it can't be fixed. I actually converted my 15 to a 9.9 carb this year because it runs better at an idle and trolls much better, I saw no loss in speed. My top speed is about 6-7 mph with my 15/9.9
 
??----Look and see if you can find the " sailboat " prop for that motor.----10" X5" pitch.-----Available right on this here " top notch " site.----------Slow boat means low pitch !!
 
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flyingscott said:
Yes it can be done, But in your case it will make no difference at all in power. The 15 hp makes it's power at 6300 rpms the 9.9 at 5500 rpms. Because your motor on that boat will never see those Rpms, the 9.9 carb is a better choice than the 15 hp carbs. Because of the lower RPMs your motor works at the smaller venturi in the 9.9 carb works better.
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So are you saying that even with the potential 5 hp increase, my boat size will prevent the engine from reaching the maximum speed achievable with the bigger carburetor. I have attached from Leroy’s Ramblings his estimate of rpm increase.

[FONT=&quot]The piston diameter for both the 9.9 and 15 hp motors from 1974 to 1992 are 2.188" with a stroke length of 1.760" which equals 13.2 cubic inches, or 216 cubic centimeters. Full power operating RPM for the 9.9 is 4500 - 5500. The 15 hp motor is rated at 5500 -7000 RPM according to a 1978 Johnson sales brochure. This higher RPM is achievable by using a different carburetor which uses a larger throat, which gains a higher fuel/air mix.


Thank you for your comments![/FONT]
 
flyingscott said:
I run a 15 hp kicker on my 16'ss starcraft, that motor runs to about 4000 rpm with a 10" pitch prop. So if I need another 2000 rpm at the theoretical 1" = 200 rpm, I need a 0" pitch prop. That motor on the OPs boat is always going to lug. You cannot get around the fact that sometimes the motor is so underpowered it can't be fixed. I actually converted my 15 to a 9.9 carb this year because it runs better at an idle and trolls much better, I saw no loss in speed. My top speed is about 6-7 mph with my 15/9.9
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I have been running an Evinrude 9.5 Sportwin as my kicker and at wot the boat speed is 6 mph according to my GPS, so what you are saying makes a lot of sense. I was just hoping that a different newer 9.9 converted to a 15 would give me higher speed.
 
There will be no 5 hp increase because you will not be able to spin the motor fast enough. If you were running a light hull yes the difference is there. Do what racer said and re-prop your motor and try to maximize it. But the 15 hp carb will be a waste of time and money.
 
Have you got the carb already? If so, try it out and give us a report. I got 2 carbs here from Joe Reeves, retired grandfather of Marine Engine Forum. They made substantial performance gains on my 9.9's. I did once "detune" a 15 for a customer and a smaller boat. He loved it and praised the economy benefits as well as the sweet purr at idling speed.
 
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timguy said:
Have you got the carb already? If so, try it out and give us a report. I got 2 carbs here from Joe Reeves, retired grandfather of Marine Engine Forum. They made substantial performance gains on my 9.9's. I did once "detune" a 15 for a customer and a smaller boat. He loved it and praised the economy benefits as well as the sweet purr at idling speed.
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Appreciate the offer timguy. My thoughts are winter ones for a winterized boat. I believe earlier comments on the enigine at 15 hp for my boat are correct. I did have a 35 JOHNSON as a kicker just for a trial and the engine fell far short of full rpm. It makes sense to me now. It seems I should leave well enough alone. Thanks
 
The OP said he wanted it as a Kicker, which in my mind reads to a trolling motor OR a backup to the main motor. If used as a troller, RPM may never be over 1000. As a backup motor, Yes as much HP as possible. I used to have my articles posted here under Frequently Asked Questions. I have been gone for a number of years due to health problems & now can not find those links. My expertise was 9.9/15HP OMCs. One of many of my articles addresses this conversion. http://www.leeroysramblings.com/Johnson 9.9_15.htm

Just the carb change does not really give all the potentials, but used parts for those old motors are getting scarce.
 
99% of all kickers are seriously underpowered on the boats they are on. They are only used as backups in case of emergencies. They are not meant as primary motors. A 15 hp on a 14 ft boat is not the same as a 15 hp on a 21 ft boat.
 
Welcome back, Machinist. Hope to learn more from you. I have extensive experience with the aforementioned motors, they are among my most treasured and trusted "bush motors". They are still shoulder carry for me, (I am no longer a "young man"), and excellent power to weight ratio. In NW Ontario, we have to be "portagable". These 9.9 and 15's, in mostly longshaft, are perfect for our 14 16 18 footers. We often run 2 of them side by side.....when freighting with the 18 footer, have even run 3.
 
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The Machinist said:
The OP said he wanted it as a Kicker, which in my mind reads to a trolling motor OR a backup to the main motor. If used as a troller, RPM may never be over 1000. As a backup motor, Yes as much HP as possible. I used to have my articles posted here under Frequently Asked Questions. I have been gone for a number of years due to health problems & now can not find those links. My expertise was 9.9/15HP OMCs. One of many of my articles addresses this conversion. http://www.leeroysramblings.com/Johnson 9.9_15.htm

Just the carb change does not really give all the potentials, but used parts for those old motors are getting scarce.
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Here’s the treasure trove of Leroy’s Ramblings:
http://leeroysramblings.com/Outboard Motor Related/OMC outboard related articles.html
 
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