From 9.9 to 15 - Converting my Yamaha 2 stroke

[Blankton]

I thought I'd start a new thread just to do with this work. I had read a lot of threads online and today I decided to convert my newly acquired 9.9 to a 15hp.

This is a 2006 engine, but as I understand it, the engine design didn't really change for about 20 years from mid nineties til a couple of years ago.

There are two aspects to doing this work:
1. Adjusting the reed valves
2. Removing the throttle limiter.

First off was adjusting the reed valves.

The reed valves sit on a plate between the carb and the block, so to get at it you need to remove the carb. These 2 10mm bolts go through the plastic airbox (?) and the carb.


There were then various linkages between the throttle and choke mechanism. Most uncipped, but I found that I needed to undo this screw, which attaches the choke mechanism onto the side of the plastic airbox.



I also had to loosen this screw to slide this linkage out. I marked its position with a marker pen.


Once the linkage was loosened, I could rotate the carb to the side, without removing the fuel line. This revealed the reed valve plate


There are 3 10mm bolts holding on the reed valve plate.

This is the reed valve. Notice one side I have opened out and one is in the stock condition:


I had read of people opening the reed valve out with pliers, but I was worried of twisting it, or damaging the rest of the assembly, so I removed the plate and rigged up a bending fixture in my vice, using a bit of brass I found.



I needed something flat to bend out the clearance, and settled on a bolster chisel.


My feeler gauges don't go up to 6mm, so I measured various things in my toolbox til I found something that was right. (I've since heard of using a 6mm drill bit, which would have been easier!)

[Blankton]

The finished article


The second stage was the "rev limiter", this was a little tab, which stopped the throttle fully rotating. I was thinking about bending the tab, but didnt want to strain the mechanism. I also thought about remving it totally, but was worried that when I remved the bolt it would spring apart. Not sure what to do, I decided to losen the bolt slightly to see if I could rotate the tab. After a couple of turns, the tab just fell off - it turns out that its slotted. So this bit was very easy:




And that was that. I ran out of time, so I haven't fired her back up. I also received a tachometer in the post today, so I'll have that installed for when I fire the engine back up.

[Fenlander]

Great work and nicely documented. Look forward to hearing how it is on the water.

[Blankton]

Originally Posted by Fenlander

Great work and nicely documented. Look forward to hearing how it is on the water.

Me too. I now have the same power as my 4 stroke, but with 14kg less weight hanging off the back of the transom. I am hoping for an improvement in planing performance.

[909]

Bump ^^^

[Blankton]

The result was 21 kts by myself , which is pretty much the same top end I had on my 4 stroke 15hp, so I think we can be sure that the modifications have had their desired effect.

The real difference was in ease of getting onto the plane, it stepped up onto the plane every time it was asked with a quick squirt of the throttle. Even though it is only 14kg less in weight than my 4 stroke, the position of those 14kgs seems to have made a real difference.

[Fenlander]

That’s a great result…. and for no cost.

[A1o]

Hi mate,

What app are you using to measure speed and distance please??

[Blankton]

Originally Posted by A1o

Hi mate,

What app are you using to measure speed and distance please??

It is just a screen within navionics. Whenever I go fishing I start a "track" which my phone saves. I can then review the tracks and it summarises the vital statistics from each one.

I suspect you could use other stuff like strava or a fitness tracker watch to achieve the same thing.

[A1o]

Thanks mate, all new to me!

[909]

Excellent post. By far the most detailed and well written post about this conversion on the internet . Period.

Originally Posted by Blankton

I understand it, the engine design didn't really change for about 20 years from mid nineties til a couple of years ago.

Do you think this would work on a 1985 Yamaha 9.9SK . Do you think the process is the same ? What was the initial spacing before you bent the stoppers to 6mm.

Originally Posted by Blankton

After a couple of turns, the tab just fell off - it turns out that its slotted. So this bit was very easy:

It's a simple as just removing the tab. What effect would removing the rev limiter stop have on the engine without doing the reed valve mod ?

Keep up the good work

[Blankton]

Originally Posted by 909

Do you think this would work on a 1985 Yamaha 9.9SK . Do you think the process is the same ? What was the initial spacing before you bent the stoppers to 6mm.

It's a simple as just removing the tab. What effect would removing the rev limiter stop have on the engine without doing the reed valve mod ?

Keep up the good work

I don't know about that engine. The best advice I have seen is to find a parts listing for the 9.9 and 15 and check through them back to back and identify what is different. If you Google it, there are online parts catalogues.

The Reed valves were 1.5mm previously. I'm not sure of the affect of doing one without the other. The Reed valves are acting to choke the flow of fuel and air into the motor and so is the rev limiter...... In fact I'm not sure why you need both, when I'd have thought either /or would have the desired affect of reducing the power.....

[TmMorris]

The Reed valve element is essential as this is what ultimately regulates the fuel flow into the chamber.

Probably the best first step is to get the manuals for your 9.9 and the matching 15. Then check the volumes are the same which seems very likely and then go through the fuelling section cross checking parts numbers.

With a 2 stroke where the engine is throttled down it does seem most likely that the core differences will be the carb needle to lower fuel draw, a choking plate to lower the air flow and then a different Reed valve to sort all that out.

The clues to me would be to first check the cc of the pair to see if they match as that is the relatively binary answer as to whether a power upgrade is on the cards and then check if the two engines run the same timing as that's a clue to me that making the change could be as simple restrictor and Reed valve.

The other thing worth double checking at the back end of a conversion is whether you need different spark plugs rated for a higher temp.

[TmMorris]

Originally Posted by Blankton

The Reed valves were 1.5mm previously. I'm not sure of the affect of doing one without the other. The Reed valves are acting to choke the flow of fuel and air into the motor and so is the rev limiter...... In fact I'm not sure why you need both, when I'd have thought either /or would have the desired affect of reducing the power.....

I think the crude answer lies in the fact that you didn't have to change the carb jetting? If that's the case then the same air and fuel is flying through the carb regardless of whether a 9.9 or 15 and the restrictor is mostly about blocking some air and by nature some of the fuel while the Reed valve just ensures the fuel element is right that.

Someone will have the specifics but I've generally considered the restrictor plate to be like a large gate that holds back the mob but still allows it through in an unruly fashion and the Reed valve is like a turn style that puts it all in order.

[Dawsman]

So after completing this did you find that you had gained much for actual top speed? Or is it more so just more”torque” per say

[Blankton]

Originally Posted by Dawsman

So after completing this did you find that you had gained much for actual top speed? Or is it more so just more”torque” per say

I never used it as a 9.9, but as I've said above it gave the same performance on my boat as a known brand new 15hp, so I am pretty confident that it was producing 15hp (or thereabouts).