Convert 9.9hp - 15-hp 2-stroke failed.

[CaptainA]

My engine struggles to get my little boat up on plane when the whole family is aboard. I had heard the 9.9 and 15 HP versions of the engine use the same block and converting the engine to 15 HP would be pretty simple. Following advice I found in this forum, I got the parts manual (there is only one for both the 9.9 and 15HP versions of the engine and compared parts and there were only a few differences. The main jet and needle in the carburetor and the reed valve block. The difference in the Reed valve block is that the 9.9HP engine only opens 2mm while the 15HP reeds can open to 6mm.

So... I bought a new reed valve block (genuine Yamaha part) for the 15 HP version and verified the stopper on the reed valves was 6mm. And replaced the block in the engine and verified the original block was limited to 2mm.

Not wanting to mess with re-jetting the carburetor, I decided to buy a new one of those also. My engine is not a US model and I was unable to find a way to order a yamaha carburetor, so I ordered an after-market assembly that is supposed to be for the 15HP version of the motor.

I replaced the reed block and the carburetor with the new parts. Engine fired up... no problem there...

But it behaves exactly as it did before. I do not notice any new power. Boat still struggles to get on plane. Max speed with just me and my son in the boat... full throttle, up on plane... is not increased with the new parts.

Is there something else that must be done, or some adjustment that is necessary to gain the benefit of the bigger reed valve openings and new carburetor?

I verified the throttle on the carburetor opens fully when the throttle on the control arm/tiller is turned to 100%, so nothing appears to be artificially limiting speed or power.
Do I need to use/try a bigger prop?
Is there some sort of governor or limit on fuel supply to adjust?

For the record, the engine is a world "Enduro" model 2-stroke purchased new - outside the US - while cruising. This is not a model available in the US so some parts (such as the carburetor) can be hard to source.

Model: E9.9DMH
Serial # 6B3K S 1012370 C
Model year... 2012, I think.

Thanks for any help

Andy

[99yam40]

what RPM does the motor turn at WOT?

maybe the motor does not have enough HP to push the load you have with the prop you have

[boscoe99]

What country did the motor come from?

[boscoe99]

Is the carburetor that you purchased the same size as the one that it replaced? Throttle bore that is?

If you have changed all of the parts that are different, except for the Yamaha carburetor, then that could very well be the difference in power.

As 99yam40 notes, the motor must be propped so that it can turn at or near 6000 RPM when the boat is loaded and the throttle is wide open.

[boscoe99]

https://www.amazon.com/6B4-14301-00-.../dp/B07XP2TDMJ

[CaptainA]

The motor came from Panama.

The carburetor recommended by boscoe99, above, is the very one I purchased and installed. It is an aftermarket product made in china. To my untrained eye it is nearly indistinguishable from the original that came on the engine.

I am not a "mechanic" (though I maintain the outboard on my tender and the diesel in my sailboat). I am an engineer and am mechanically inclined, but I don't have a strobe or other useful tool for measuring the engine speed of rotation. If such a tool is key to solving this mystery I can probably get my hands on one.

[CaptainA]

Should also note, as stated above, with both the original reed block and carb and the new reed block and carb - and in both cases, the same prop. I am getting what seems to be the same power output. I can measure the boat speed. I can't measure the rotation of the engine, but I can measure the boat speed. With me and my 7yo son in the boat, it planes easily and goes 18.5 +/- 0.2 kts. There was no change in this top speed when I changed the pair of parts. That would suggest to me the engine speed is nearly the same in both configurations. I would expect a 50% boost in power (9.9-15Hp) I would see a measurable increase in top speed if I change nothing else.

Could it be the engine is speed limited (somehow) and I am not requiring the full power of the engine to turn it at max RPM's? ie... do I need a larger prop so that the engine could put out more power for lower RPM?

Note, the prop I have on it is whatever came on the engine when I bought it at the Yamaha dealer in David, Panama.

Thanks for puzzling through this with me...

[99yam40]

if you do not know what RPM the old unit turned and you do not know what RPM it turns now it is real hard to say what is going on.
if they both turned the same RPM they turn with the same prop, I would think the speed would be the same.

now if the HP increases, you would think a increase of RPM would happen unless the motor was over loaded too much to begin with.
no one but you knows what size boat and load you are putting on this motor and we sure have no way of knowing what prop it has on it.
you will have to provide more info

[boscoe99]

Should also note, as stated above, with both the original reed block and carb and the new reed block and carb - and in both cases, the same prop. I am getting what seems to be the same power output. I can measure the boat speed. I can't measure the rotation of the engine, but I can measure the boat speed. With me and my 7yo son in the boat, it planes easily and goes 18.5 +/- 0.2 kts. There was no change in this top speed when I changed the pair of parts. That would suggest to me the engine speed is nearly the same in both configurations. I would expect a 50% boost in power (9.9-15Hp) I would see a measurable increase in top speed if I change nothing else.

Could it be the engine is speed limited (somehow) and I am not requiring the full power of the engine to turn it at max RPM's? ie... do I need a larger prop so that the engine could put out more power for lower RPM?

Note, the prop I have on it is whatever came on the engine when I bought it at the Yamaha dealer in David, Panama.

Thanks for puzzling through this with me...

If not an OEM Yamaha carburetor all bets are off as to what the outcome will be.

They make clip on tachometers that can be used to determine the WOT RPM.

The engine is not speed limited except for over revolution protection. 6150 RPM or so.

It is counter intuitive but a larger propeller can result in less power being produced. Less power = less speed. The motor has to be able to turn X RPM to produce rated power. Generally 5000 - 5500 for your model.

[boscoe99]

I posted a reply but it has been flagged for approval. Getting harder and harder to offer folks help here.

[boscoe99]

Here is what I posted.

[99yam40]

I posted a reply but it has been flagged for approval. Getting harder and harder to offer folks help here.

I have a feeling the links are giving them problems.
not enough mods looking to see if they are OK

well I was wrong, no links I saw

[CaptainA]

More details

The prop is: 9 1/4 x 8 - J

The boat is a 9' "ultra light" Aluminum RIB that weighs ~85lbs.
Total passenger weight in test above (me and my son) is just over 200lbs
Total weight (Engine + passengers + boat + fuel) ~ 400lbs
Boat planes easily.

If I add my wife, daughter and dog that adds another 300 lbs,
Boat will get on plane but struggles. Once on a plane all is good

No difference in behavior with new parts as noted above.

[panasonic]

I would go back to the parts diagrams and make sure the exhaust tuner is the same part number for both....

[99yam40]

I would go back to the parts diagrams and make sure the exhaust tuner is the same part number for both....

Pan, what are your thoughts and the prop he has?
I guess you deal with the smaller motors a lot from what you have posted in the past.

I had a 15 on a 12 foot may years ago, but I do not remember what the prop was