Converted my 1999 2 stroke 9.9 to a 15hp

[merc16]

Hey guys! Just here to share what i did to convert my 9.9 yammy to a 15 hp (the only changes yamaha uses) so first off i got the 63W reeds (15hp reeds, the 9.9 are part numbered starting with 63V) I have a pic below you can see the difference (the 9.9 reeds are on the left) and the second thing was the prop! Both are 9 1/4 diameter, but the 15 comes stock with a 10 1/2" pitch as the 9.9 has a 9" pitch.

First impressions were the motor ran great at idle had to adjust idle mixture a bit but its dialed in great, it trolls a bit faster (1.8 mph) i could prolly turn it down a bit but i dont troll anyways, but the main difference is the WOT baby! I was averaging 22.5 mph in my 14 foot princecraft scamper with a 20lb anchor and fishing gear, now im averaging 25.5/26 mph with the 15 conversion done, so all it is, more air allowed in to turn a more aggressive pitch prop, thing runs like a dream at WOT though, took her on a few long runs and no issues! Anybody have any questions shoot!

 

[Sea Rider]

What is exactly the reed assy difference between both, as per the spare parts manual both shares the same exact reed valve part, what's the physical difference, has slight wider, taller reed ports, the position of the reed valves ? If you install a less pitch prop definitely will see a hole shot and speed improvement, the issue is at full throttle grip at which rpm numbers is the motor revving at independently of the speed achieved. Is it revving around middle, max wot rpm range factory stated which it's important to know to get the max out of that motor.

Happy Boating​

 

[boscoe99]

What is exactly the reed assy difference between both, as per the spare parts manual both shares the same exact reed valve part, what's the physical difference, has slight wider, taller reed ports, the position of the reed valves ? If you install a less pitch prop definitely will see a hole shot and speed improvement, the issue is at full throttle grip at which rpm numbers is the motor revving at independently of the speed achieved. Is it revving around middle, max wot rpm range factory stated which it's important to know to get the max out of that motor.

Happy Boating​

Different part numbers. Different parts. Different valve opening.

 

[merc16]

What is exactly the reed assy difference between both, as per the spare parts manual both shares the same exact reed valve part, what's the physical difference, has slight wider, taller reed ports, the position of the reed valves ? If you install a less pitch prop definitely will see a hole shot and speed improvement, the issue is at full throttle grip at which rpm numbers is the motor revving at independently of the speed achieved. Is it revving around middle, max wot rpm range factory stated which it's important to know to get the max out of that motor.

Happy Boating​

As you can see the reed stops on the 15 are twice as tall as the 9.9, i was sitting around 5200 rpm with the 10 1/2 pitch prop which is perfect

 

[scout-j-m]

Was there a restriction plate behind the carb?

 

[boscoe99]

Was there a restriction plate behind the carb?

There is not one. The reed valve assembly is acting as an air restrictor.

 

[boscoe99]

As you can see the reed stops on the 15 are twice as tall as the 9.9, i was sitting around 5200 rpm with the 10 1/2 pitch prop which is perfect

Only twice?

 

[merc16]

Only twice?

Maybe even more tbh the 9.9 reeds are barely open

 

[99yam40]

Only twice?

some may not know how to compare .7mm to 6.0 mm

 

[boscoe99]

some may not know how to compare .7mm to 6.0 mm

Seems like a hellava difference.

 

[Faztbullet]

Mercury does the same exact thing on the inline motors...a 90 has has less reed opening than a 115. A smaller pitch prop will improve hole shot but rarely adds increase in speed.....

 

[Sea Rider]

Sorry, was referring having the same leaf metal plate part number for both 9.9 & 15 HP with different reed valve assys and heights adjustments. Are you interested in speed with less hole shot, or better hole shot with less top end speed ? at which rpm is the motor revving at full throttle with a 10 1/2 pitch prop ? If not mistaken the wot rpm range for said motor is 5000-5500, at 5200 the motor is revving 300 rpm under its max, one less prop pitch will bump the wot rpm towards the max or slight over the rpm range.

Happy Boating​