Johnson 9.9 hp to a 15 hp conversion question??

[Jagzilla]

I hear that the 9.9 and 15 horsepower Johnson's are the same except for the carb, and maybe the ignition. Does anyone know exactly what needs to be changed to convert a 1981 9.9 hp Johnson Seahorse into the 15 hp model? Most things I read state the carb is the only difference, but the local boat mechanic says the ignition is different as well. Any ideas here?<br /> Jag

 

[OBJ]

Re: Johnson 9.9 hp to a 15 hp conversion question??

Hi jag....the ignition systems are the same. The carbs and exhaust tubes are different. The 15hp has a so called "tuned exhaust" which is round in comparision to the 9.9 square tube. The carbs are different to. 15 with the larger throat and high speed jet. Reed shims are in both.

 

[Jagzilla]

Re: Johnson 9.9 hp to a 15 hp conversion question??

With the exhaust difference between the two motors, can it still be beneficial do just do the carb exchange? Thanks for your info, <br /> Jag

 

[chilifriedpork]

Re: Johnson 9.9 hp to a 15 hp conversion question??

Can you switch over exaust housings?<br /><br />Because in 1981, when the marine industry went to prop rating, the johnsons, mercs etc were 10% under rated. So the Johnson and Evinrude had a new exhaust housing to up the rpm's and make it flow better.<br /><br />Is it possible to change the old one for a new one?<br /><br />Thanks<br /><br />Before 1981, you could change the carb on a 9.9 to a 15, and you would gain rpms, and hp. After '81 the exhaust housing was changed

 

[Tacklewasher]

Re: Johnson 9.9 hp to a 15 hp conversion question??

Uhmmm. Can I let you know in a couple of days? I have an early 80's Johnson (I really need to dig up the model # to find out what year it is).<br /><br />Put the "other" carb on it two nights ago but it won't see water until the weekend. Still have to fix the rope starter. The carbs are definatly different looking down the throat.<br /><br />Two weeks ago I had the boat out with my GPS and got 16 mph sitting in the back and ~22 mph when I moved forward and planed better. I will see what I get this weekend assuming calm water.

 

[charliesused@cs.com]

Re: Johnson 9.9 hp to a 15 hp conversion question??

In the carb...74-early 80's its the jet. I took a micromometer and measured the difference in the <br />front and back openings in both of the orifices and that was stupid cuz they are the same Of course the cards say they are the same but all of these rumors make you wonder. From years of garbage ive heard changing the jet and reed plates behind them will change your HP down or up whichever way you want as far as the exhaust tube there are different part numbers but from what mechanics ive talked to for years say it doesnt matter the HP / thrust will go up or down if you just change the reed plates and the carb jet. Again just advice....

 

[Mr. Bwok]

Re: Johnson 9.9 hp to a 15 hp conversion question??

In the carb...74-early 80's its the jet. I took a micromometer and measured the difference in the <br />front and back openings in both of the orifices and that was stupid cuz they are the same Of course the cards say they are the same but all of these rumors make you wonder. From years of garbage ive heard changing the jet and reed plates behind them will change your HP down or up whichever way you want as far as the exhaust tube there are different part numbers but from what mechanics ive talked to for years say it doesnt matter the HP / thrust will go up or down if you just change the reed plates and the carb jet. Again just advice....

Please Please Please Ignore this guy. It is NOT just a jet. Yes the openings in the front and back are identical, but if you open the choke and look in, you will see a huge difference. The 9.9 hp carburetor?s INTERNAL THROAT is .625 dia., 15 hp internal throat is .875 dia. up until late 1987 when they went to a plastic topped carburetor. Please notice the word INTERNAL THROAT, not front and back openings. There are 3 different stages converting to a 15 hp and it depends on year and how much work you are willing to do. The common factor and the single largest piece of the puzzle that will increase power regardless of year is the CARB. Here are the requirments for conversion with corresponding years. (1) The 15 hp from 1974 through 1978 change carburetor only with impressive results, it is amazing what a 50% power increase will do.....lol, however trolling is a little rougher (may just be my engine). (2) 1979 through to early 1981 change Carburator and if you are ambitious add a 0.0125" shim to the reed assembly. This not necessary, but does seem to allow a bit more air flow
(3) 1981 through 1987 Carburetor, shim and now they have a tuned exhaust tube. (more trouble to change that what it is worth in my opinion)

Realistically if you are looking for an increase in power, but not concerned with dyno numbers matching exactly 15hp...........Change the CARBURETOR only. Do not get confused with people saying the intakes are different, or the head is different or the ignition is different.........THEY ARE NOT. 90% is all CARB. The shim and the exhaust will add a bit, but are not necessary to make that old 9.9 an old 15.

 

[jmendoza]

Re: Johnson 9.9 hp to a 15 hp conversion question??

See my response to your other post on what I did back in 1978 to my 1975 9.9hp.

Here is something interesting: Initially, the 9.9 and 15 had identical square exhaust tubes. Then the 15 got a "tuned" round pipe. At some point much later however, both engines reverted back to the original square shaped exhaust tube. So I guess it really did not make that much difference or they would not have gone back to it. My best guess would be that at the end of the day it was most likely decided it was cheaper to make one style of exhaust tube.

FYI, if you don't mind working on your engine periodically, replace the steel reeds with .018" G-10 epoxy-glass reeds ( make them yourself) and bend open the reed stops a little. This is what the Class A guys do to the 15SS OMC class A stock outboards, and it's about the only thing you are allowed to modify. The reeds do have to be replaced every 10-20 hours according to OMC, they sat they have a limited life. An easy power trick is run high quality synthetic oil, like Motul or equivalent, this reduces friction and increases the availlable power at the prop.

Because this engine is an old cross flow style with baffle crown pistons, it's not reccomended to increase squish/compression very much as you will melt the piston crowns...it's old school designed for longevity and reliability and due to that it's way over built for it's power rating.

 

[Bob_VT]

Re: Johnson 9.9 hp to a 15 hp conversion question??

CLOSED This is a 2003 thread.