Can be done to turn it into a pre-94 1/2 9.9 horse.
You have to change a few things.
First you will need a 9.9 carb - although they went with a larger block later on, the carbs are still interchangeable, so any 9.9 carb will work.
Next you need to replace the exhaust plate on your motor. On the 6 horse it is just a plate with no tube. You need to get the 9.9/15 horse plate and tube (comes as one unit) - you can NOT retrofit the 6 horse plate, the outlet hole is not big enough so the motor wouldn't "exhale" properly - you need the other plate and tube to be able to make the 9.9 horses. You have to completely remove the power head to change the plate/tube. The larger carb allows the motor to breathe in, the plate/tube allows it to breathe out - both are needed for more horses.
The "old style" plates are no longer available. The post 94 plates will "bolt right up" but the water tube hook-up is different, so you will also have to go with a new water tube. The old ones had a flare up at the power head end of things and were held onto the plate with a mount and two screws. The new style is held in place with a grommet (don't forget to buy the grommet as well, don't think it comes with the new tube) - so again, no way to retrofit.
Next, because the new tube only uses a grommet, they put a flare about half way down the tube (between the power head and the lower unit). That flare will not pass through a casting in the leg (exhaust housing) which keeps the tube from being pulled out of the head when you remove the lower unit (so kinda locks it in place between the head/exhaust plate and the flare trapped in the casting part way down the leg).
That little casting (with the hole in it to trap the flare in the tube) is not in the same place on the pre-94 models SO you need to get a hold of a long drill bit (like cable installers use) and drill out the existing pass through hole in the leg (where the current water tube passes through) to allow that flare to pass. This allows the tube to fit through properly but does not provide the support (trapping the tube in place) such that when you have the lower unit off there is a good chance you will pull the water tube from the head - no problem when everything is assembled, it won't come out, but can be a bit of a pain anytime you pull the lower unit - have to make sure the tube is "in place", secured with the grommet before you reinstall the lower unit.
Also, in the process you will likely destroy the carb gasket (between carb and intake/reed cage), the power head base gasket will be toast as will the exhaust plate gasket, so all of them will need to be replaced. If you are careful you can salvage the exhaust seal (rubber, like the seal on a cars window) that goes around the outer edge of the plate.
So, if you can source used parts "cheap", it might be viable.
If you are buying new you may want to check your wallet first and weigh what you will get in return. Going from a 6 to 9.9 horse you will gain about 15% more speed at full throttle (compared to what you are getting now) - so if you are making say 15 mph, with the upgrade you make a little over 17 mph (a 50% increase in horsepower results in a 15% increase in top end speed - because the engine displacement is the same (12.6 cubic inches) you do not affect your lower torque band so you don't get a better hole shot, better time to plane etc, only top end throttle is different).
The cost for parts (2016 Merc part lists) is:
Carb - $388.25 (list)
Exhaust plate/tube - $96.58 (list - post 94 models)
Water Tube - $17.60 (short shaft) $34.67 (long shaft) - Grommet - $3.30
Carb Gasket - $2.10
Power Head Base Gasket - $6.70
Exhaust Plate Gasket - $6.60
So if you have to buy "new" I make it $516.93 plus tax (and shipping maybe) to turn a short shaft 6 into a 9.9 and gain a whopping 2 mph at wide open throttle.
And this is why if you want "more horses" you buy them "up front".
PS: If you live in the US, then it's actually illegal to do the above procedure (in Canada where I am we don't have the EPA and I have converted more than one of the portables - I guess some guys just like parting with their money and I am willing to blow it for them if they so desire)
(the word from Merc)
" EPA regulations prohibit changes to engines that could affect emissions limits. Per the service manual, "The dealer and/or consumer is not to modify the engine in any manner that would alter the horsepower or allow emission levels to exceed their predetermined factory specifications." The only changes permitted are carburetor jet and gear case/gear ratio changes for high altitude operation as detailed in applicable Mercury Marine Service Manuals."
So if you live in the US and are buying parts from Merc, don't tell them what you want them for....
