Converting 9.9 Merc Two-Stroke to 15hp

[dutch45]

awesome, im gonna give it a shot. Thanks for the quick responses.

[Sunspot Baby]

I went to a bigger dinghy (my old one was stolen) so I needed more power. It definitely made a difference. With our Advanced Inflatable 11' catamaran dinghy we do 15 kts with the two of us (about 375 lbs total) in perfect conditions. Can't plane with 4 adults.

George

[Roy M]

Chuck, I was being cute about the decal.

[Alan Wheeler]

We also have 9.9 HP outboards in NZ that are mainly produced for the Ozy market. In Oz, you have to have a licence or registration or something like that for 10Hp and up. So hence 9.9Hp.
There is not a lot of point in making a dedicated 9.9Hp when there are already 8Hp motors. It's also the reason why the weight difference between 8Hp and 9.9's are such a big step.
Reject or replace the carb and away you go with a 15Hp.
One could ask why the price should be different between the 9.9 and 15 though. Surely they should be the same.

[wpg_snowboarder]

hey sorry for bringing up an old thread but i have a 1988 merc 9.9 and i've heard to convert it to a 15 you need a carb, possibly intake manifold and exhaust tube/manifold can anyone confirm this for me

[David M]

Originally Posted by Sunspot Baby

Sonny:

The reason I pursued this was that in another life I had an Evenrude 9.9 that could be converted to 15 simply by changing carbs. A prop with more pitch took advantage of the 50% increase in power. I lived on a private lake with an HP limit of 10. Being able to make a stealth 15 out of my 9.9 was a very appealing option. Driving a 16' pontoon boat that extra 5 hp made a big difference. Still the little john boats with 8 hp ran off and left me.

George

Given the cost of outboard parts, would it have been cheaper to buy a 9.9 engine cover and put it on your 15? I would imagine its exactly the same size. I went and priced out the difference...its only $145. I cant imagine getting those parts for less than that.

As far as the original question goes, I would be more inclined to sell the 9.9 and buy a used 15. The difference for two new engines is only $145. I bet the parts are going to cost more than that. Your time to do the conversion is also worth something as well.

[wpg_snowboarder]

trying to find a good used 15hp shortshaft im my city is almost imposible and theyre quite a bit more than 9.9s i got a minty looking fresh rebuilt 9.9 from a small engine mechanic down the street from me for a good price. he's done the conversion on omc motors and its just the carb but he's not sure about the merc. i can get used parts for almost nothing and being a millwright i love tearing stuff apart and workin on it so doing the work on the motor is no problem

[Celestialsailor]

Originally Posted by David M

Given the cost of outboard parts, would it have been cheaper to buy a 9.9 engine cover and put it on your 15? I would imagine its exactly the same size. I went and priced out the difference...its only $145. I cant imagine getting those parts for less than that.

As far as the original question goes, I would be more inclined to sell the 9.9 and buy a used 15. The difference for two new engines is only $145. I bet the parts are going to cost more than that. Your time to do the conversion is also worth something as well.

This is quite true and if you buy a 15, it should be a 4 stroke especially if you are on a lake. The pollution of a 2 stroke is devastating to the fresh water environment. I would imagine one of the reasons to limit hp to 10 is for pollution reasons as well as speed.

[wpg_snowboarder]

im not going on any lakes that have horsepower limits on them so im not worried about it making look like a 10hp i would love to run a 4stroke motor but my little boat wont handle the weight, my 4hp 4stroke kicker on the big boat weightes the same as my 9.9 so i 4stroke would weigh way to much to make the boat safe

[flipit]

Yes you can do it. I have a 92 merc 9.9 that has been half way converted. the carb has been changed but i need to change the exhaust tube (which I am having a hard time finding because of the age) the carb is around 200 but the tube is cheap, like 30 bucks or so. on mine the 9.9 tube is 1/8 of an inch smaller than the 15. converting makes a big difference. for us folks with little boats 4 strokes are not an option they are just too heavy.


If anyone has any tips on where to find that part i would appreciate it. the part # is 41344a-3

[Sunspot Baby]

Just to restate the original need. I had a dink with a max HP rating of 10. I bougth a 9.9. Went with 2 stroke for the weight advantage.

That dink was stolen. The new one was rated at 25 HP max, but I didn't want to pay the $$ for a new engine and felt that if I could bump my 9.9 to 15 it would be money well spent.

I bought a new carb and gained significant HP increase. Money well spent.

Local parts guy researched part numbers of both engines and found no difference on exhaust side and/or ECM. Carb only.

Still running that little hummer and happy as heck.

[elmcken]

Originally Posted by Sunspot Baby

I bought a new carb and gained significant HP increase. Money well spent.

Local parts guy researched part numbers of both engines and found no difference on exhaust side and/or ECM. Carb only.

I have the same motor and was hoping that I could just replace the jets and get the same result. I may be oversimplifying: more fuel = more power?

[Sunspot Baby]

The part number on the carbs are different. It could be only the jets, but it could be venturi or throat diameter which I suspect. You need more air, not just more fuel in the air you have.

George

[elmcken]

Originally Posted by Roy M

My assigned motor was a Mercury 9.9. The difference is the jet in the carburetor. All the other goodies are the same engine.

Sorry to reply to an old thread, but I was wondering if you think I could just change the jets in my 2002 Mercury 9.9 2-stroke carb to get more power. I posed the same question in another thread and Sunspot Baby thought the carb itself might be different allowing for more air intake.

[kddunn]

I'm looking at doing the same thing to my 9.9 but was told it woudn't give any more power. Also it would seem to me that a new prop would be in order. Where did you find the Merc parts diagram?