Evinrude 9.9 to 15hp tweak.


Well-known member
12 Feb 2010
Glasson Dock
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My auxiluary outboard is a 20 year old 9.9 Evinrude 2 stroke. I understand that this engine is actually a detuned 15hp as Evenrude/Johnson did not actually make a 10hp and in some countries certain boats with "Under 10hp" were allowed less rectrictions, this is why its a 9.9 and not 10.
Certainly in the 600 page Workshop Manule which I have bought on CD for peanuts off Ebay it shows the same engine/carburettor.
So what is it that converts it to a 15hp, perhaps different carburretor jets as once converted it wont rev any faster. Perhaps a different camshaft.
I need to take the engine in for a 20 year service as I dont think the impeller has ever been changed so if I can find out what the modification is I would have it done at the same time.
"Thinks" It couldent be somthing simple like a propellor change could it ??.


Well-known member
13 Jul 2002
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Practically every thing you need to know about OMC 9.9 and 15hp engines to be found at http://www.sschapterpsa.com/ramblings/Ramblings.html#OMRA

The electronic parts lists and diagrams at http://shop.evinrude.com/ will enable you to check on differences.

Check not only carb parts, but inlet manifolds and even crankcases as the porting may be different, as well as gear-cases and props

Check also that the cylinder heads are the same. A bit of digging suggests you have to change it to fully up rate to 15hp

Cam shaft though? I did not know they made 9.9 and 15hp fourstrokes 20 years ago!
Last edited:

moonraker 36

New member
20 Aug 2009
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i believe you just need to put the carb and reed block off a 15hp.
check out you tube.. their are example's on their..!
i looked up doing the same to my mariner 9.9hp but on my engine you would need to change the exhaust, reed block and carb.just couldnt locate a used carb so gave up on the idea.
the johson/evinrude is supposed to be the easiest to convert.


Well-known member
23 Aug 2004
A tiny Island, Caribbean
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You may also have to make a minor adjustment to the timing.

The 2 stroke 75 and 90hp are the same EXCEPT for the Carb, the stickers on the cowling and the timing.

I know this as a mercury agent (now no longer in business) sold the previous owner of my boat two 90hp's.

The block had a connecting rod come through the side.

When I quoted the serial # to obtain a replacement block, Mercury told me it was a 75hp.

The B<gger of a dealer had changed the carbs and the stickers on the cowling but forgot the timing, but did not disclose what he had done.

I asked Mercury if they condoned this dealer change without disclosure to purchaser and they went silent. They did come up with quite a few freebees including a new electric trim.

IMHO Mercurys are really freshwater engines and not meant for salt water, so it seems to my boating friends who virtually to a man prefer Yamaha.
I replaced my Mercraps with two 85hp Yamaha's last year.