9.9hp to 15hp carb upgrade

Hello - I have a 1986 9.9hp Johnson that I'm thinking about upgrading to a 15. I know the only difference between the two is the carb, but I don't know what parts to get or where to get them. Do I just get the carburetor from an OMC 15hp or is there a kit? Either way, does anyone know a good source for the needed parts?

Thanks in advance - BOB

Re: 9.9hp to 15hp carb upgrade

Well, what you "know" is wrong, Bob.

It is true that the larger venturi in the 15 carb will allow the 9.9 to breathe better and make a bit more power, but that is not the only difference and you won't get the other differences by changing parts.

Remember, you are talking about a 50% increase in power by changing one part. Not real. That would be like increasing the strength of a chain by changing one link.

Anyway, the modest increase in power will not be worth the cost of even a carb change.

Re: 9.9hp to 15hp carb upgrade

Fair enough! Also explains why no kits are readily available.

BOB

Re: 9.9hp to 15hp carb upgrade

Hey JB, as I understood the differences were a reed spacer, a 15 hp carb and a 15 hp exhaust tube. Is that true, or do you also have to adjust timing and or replace other parts? I have heard stories of guys making the swap and getting crazy speed. Does motor year also make a difference? It seemed the early models, like the 73-76 year models were being mod'd.

any thoughts you have would be most interesting

Re: 9.9hp to 15hp carb upgrade

Cylinder head and bypass covers are different.
You can go to
http://shop2.evinrude.com/
and pull up two different windows in the electronic catalog for 9.9 and 15 HP models and compare the differences.

Re: 9.9hp to 15hp carb upgrade

It's covered in this link. It is doable but probably not worth the effort and $$$. No matter what, if you own a 9.9 or a 15 I believe Mr. Wiesner's articles are a must read. Enjoy. Rick.

Edit: Sorry, I forgot the link.
http://www.sschapterpsa.com/ramblings/OMC/OMC info.htm

Re: 9.9hp to 15hp carb upgrade

Thanks for the Link Rick, it really does explain a lot. Now I'll have to find sometime to hunker in to read all the other pages of this site.

Re: 9.9hp to 15hp carb upgrade

Thanks Rick - very helpful link!

BOB

Re: 9.9hp to 15hp carb upgrade

Your more than welcome. All thanks go to Mr. Wisner for taking such a special interest and the time to make all us lucky 9.9 and 15 owners so much more informed. I read his articles before I started working on my first 15 and then again a year later and I couldn't believe how much stuff I missed the first time through. I've printed it and have a copy in a binder along with my OMC manuals. Rick.

Re: 9.9hp to 15hp carb upgrade

here's a cut and paste from the website.
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Difference between the 9.9 & 15hp : When they designed this series of motors, they designed it as a 15 hp & then detuned it with a different carburetor for the 9.9. When the 9.9hp and 15hp came into being in 1974 and up thru 1978, the only real difference between the two motors was the carburetor. In 1979 they added a shim, part #325038, (one for each cylinder), behind the leaf valves & under the stop plate for the 15 hp, apparently to allow the motor to suck more fuel & air mixture in.

The outside visual dimensions of both carburetors appear the same, unless you look inside the throat. If you look in from the rear, you will have to position the throttle plate to a horizontal position to be able to see the throat very well. 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. The outlet or rear throat dia. is the same on both to match the manifold port dia. The idle jet venturi system is also slightly different between the two. The part number for the the pre 1987 9.9 hp Carburetor Assembly is #388936, while the 15 hp Carburetor Assembly is #338273. This increased throat dia. & larger main jet allowed it to breather better & pull more fuel in, increasing the RPM about 1000 RPM.
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So maybe the earlier ones can be converted by just swapping a carb.

thanks Mr. Wisner and Rick for the information.