how do i convert my yamaha 9.9hp to 15hp

[rodbolt17]

some years back I did my personal 1999 9.9.
before changing the reed assy it would turn test wheel YB1619 to about 5100.

after changing the reed block assy it turned about 5300 with the same test wheel.
I really don't think I gained much on my 14ft jon boat but it did increase the power level.

took me an hour or so to compare the major part numbers.
engine block,CDI and induction system and ex tuner and gear ratio.

mostly I did it just to see if it would work.

[Yamahataos]

I have a 2010 9.9 hp 2 stroke bought it new ran it. For about 5 hours for break in and as others have said went to the yamaha parts online and compared all parts on the 9.9 to the 15 hp and the only mechanical part difference ( besides for the stickers )is the reed valves as someone mentioned the reed stoper alows the reed valves to open more on the 15 hp so not to mess with things I ordered the 15 hp reeds and *** what a difference, to compare the improvement , to go the same speed from the original reeds(9.9 hp) I had to open the throttle aprox 1/3 more, and top speed is better with the new reeds (15 hp) I can't remember the GPS speed difference as I did compare the two when I did this four years ago . Oh and there was not adjustments to carb ,timing, nothing just replace the bolt on reeds valves , the carb jets are the same for both engines


Don

[tri_fishing]

recently made the change, best money ever spent. Took 15 mins to change and you can feel the additional power, at least 50%, lol.
In my opinion this is a case of taking a 15 and limiting it to the legal in some areas 9.9.
Reason I say this, I am very familiar with read cages/valves from bikes and PWCs. The 9.9 reed cage is very strange compared to any I have ever seen, ie, not normal, much smaller opening.

[Wotnoplane]

Hi everyone, I'm new here and loving the forum already.

After much internet searching I came across this forum and thread, it has confirmed what I had suspected.

I have a 13.5hp 2003 engine. Local restrictions limit it to this strange HP rating (Spain) so it is a ”sin carnet" engine, or "without licence." I do have my boat handling ticket, but the boat/engine/trailer was such a good deal I couldn't pass it up. It was an ex rental boat, but the previous owner thinking it would be good to tap into the unqualified renting market was horrified to see how much danger these clients were putting themselves in and took it out of the water mid season.
So I have a relatively unworn engine, even if the boat is a bit tatty. It's a 3.7m polyurethane hulled RIB styled boat which planes wonderfully with one onboard but struggles to plane with two, unless they're very light. He did say the engine used to run better before it was layed up for some years, so I took a gamble.

So after cleaning, flushing and tweaking everything to do with the fuel system, I now have the engine running sweetly, but it lacks the power it once had. The compression is great, the carb is opening fully (now, linkage problems) and the ignition advance is operating properly. So being a two stroke it can't be much else, can it? Reeds I thought, too late now that I'm back in the UK for the winter and my boat is 900 miles away!

I too noticed that all the relevant engine part numbers were the same for the 9.9, the 13.5 and the 15hp variants, except for the reed valves. So that is what I shall do, change the the reeds as I am suspecting the ones fitted could be worn anyway. A 1.5hp upgrade doesn't seem like a lot, but since the engine is lacking torque at the top end and probably only behaving probably like a 9.9hp, I hope to get a nice increase when I do the upgrade.

I get 16 knots now with one onboard, I don't expect more speed, but it would be great for the boat to plane and do that speed with two onboard. At present it tantalisingly reaches 9-10 knots on a flat calm and the engine revs are just not quite there for it to just push it through the barrier, no matter how we balance the boat, so close, as 11 knots is the transition speed to planing.

It's a great engine, and is very economical. It will be a few months before I get out to Spain again, but I shall post on this thread to update on the performance change. Fingers crossed.

Simon.