Detune / Retune of Yamaha 6 / 8hp Outboard

OGITD

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I have a Yamaha, long shaft, 6hp outboard.
During my visits to Yampower and Mitchels Outboards I discovered that Yamaha produced two products from the one production line….. an 8hp and a detuned unit of 6hp.

So my question is how was the detune achieved?

The reason I ask is that when on the dinghy and under load it doesn’t appear to rev to its maximum and therefore achieve the plane (it’s good to have fun in a dinghy). :)

A neighbour of mine suggested that it could be something as simple as a throttle-stop, but I still have to investigate this one.
 

VicS

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I have a Yamaha, long shaft, 6hp outboard.
During my visits to Yampower and Mitchels Outboards I discovered that Yamaha produced two products from the one production line….. an 8hp and a detuned unit of 6hp.

So my question is how was the detune achieved?

The reason I ask is that when on the dinghy and under load it doesn’t appear to rev to its maximum and therefore achieve the plane (it’s good to have fun in a dinghy). :)

A neighbour of mine suggested that it could be something as simple as a throttle-stop, but I still have to investigate this one.


Throttle stop, different carb jets maybe.

It might be something more complex.
At first glance it would seem possible to up-rate my 6hp to 8 hp by changing carb jets but a more detailed investigation reveals that it has a different inlet manifold, reed valve assembly and crankcase .


Worth studying the parts lists to identify different parts
 

Seajet

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Old Glow In The Deeps,

if the engine isn't revving fully, maybe it's the prop ?

My Mariner 5 2 stroke wouldn't push the boat at full revs until my Dad ( lifelong engineer ) fettled the prop with a junior hacksaw ( he scribed an equal pattern on the blades first ), reducing the blade width about a centimetre in that case; not difficult, one just has to keep any adjustments even.

If the engine seems 'sticky' and hard to turn over with the starting chord, it may be salt / gunge, I found this despite running an engine in fresh water every chance I got.
 
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theoldsalt

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Being a long shaft model it may have been propped for a displacement boat not a lightweight dinghy. Could that affect it's performance?
 

Lakesailor

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The Tohatsu/Mariner/Mercury 4,5 and 6hp motors are all the same except for the carbs (this is four stroke of course). Unfortunately you could buy another used engine for the price of the bigger carbs!!
If it's an easy fix do it, but be careful you're not chasing up an expensive garden path.
 

landlockedpirate

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The fact that its a long shaft could also be effecting performance, if the dinghy is designed for a short shaft.

I looked at doing this back in the 90's, it is relativly simple to do, but, as LS says the costs were stupid.
 

OGITD

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Cheapest Option

It (the Yam 6) had issues starting and running until I purchased a ‘service kit’ for the Carb. This included the majority of all the internal parts, with the exception of the pump diaphragm and the butterfly valve. Since then it starts on the 2nd compression and revs freely with good throttle response.

If all it requires is a re-jet, prop change and throttle-stop removal / adjustment then it’s going to be doable. There was also discussion that Yamaha (it makes sense to me) used to apply the cheapest option possible which should make it easy to reverse. :)
 

Hobnob

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I have a Yamaha, long shaft, 6hp outboard.
During my visits to Yampower and Mitchels Outboards I discovered that Yamaha produced two products from the one production line….. an 8hp and a detuned unit of 6hp.

So my question is how was the detune achieved?

The reason I ask is that when on the dinghy and under load it doesn’t appear to rev to its maximum and therefore achieve the plane (it’s good to have fun in a dinghy). :)

A neighbour of mine suggested that it could be something as simple as a throttle-stop, but I still have to investigate this one.

I have the engine (8c) and the service & parts manuals. The difference is the carb, and also the exhaust insert, that sits inside the leg.
 

kanguru007

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Bend the throttle stop out of the way and find out for yourself.
You have to push the carburetor lever by hand, the tiler accelerator is not enough (and later you have to find a mechanical solution for that)
Brrrrrr -> BRRRROOOOOOMMM
on a planing RIB it feels even more than a 2Hp change
 

fredrussell

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Bend the throttle stop out of the way and find out for yourself.
You have to push the carburetor lever by hand, the tiler accelerator is not enough (and later you have to find a mechanical solution for that)
Brrrrrr -> BRRRROOOOOOMMM
on a planing RIB it feels even more than a 2Hp change

A useful reply but are you aware you’re eight years too late providing it?
 

kanguru007

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A useful reply but are you aware you’re eight years too late providing it?
It'll be useful to someone in the future. I actually found this forum while I was frantically googling for an answer to this question, and I could not find it anywhere. The F6 motor I bought a few days ago is from 1987, and still looking and feeling like new. I have no idea how many more years it will last.
 

Sandro

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For the same power, for durability and reliability I would like better a detuned bigger motor than a strained smaller one.
Of course weight could be a drawback.
 

kanguru007

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For the same power, for durability and reliability I would like better a detuned bigger motor than a strained smaller one.
Of course weight could be a drawback.
In principle I agree, but I would not say that this is straining it. This Yamaha is a 208cc large bore small stroke motor, originally made to be 8Hp. It is not detuned for the 6HP, it has the same carburetor with the same size jets, the same admission and exhaust valves and opening times. Yamaha just added a simple throttle stop to limit how much the carburetor butterfly can be open. They also do not hide it, there is a table and a drawing in the service manual that shows everything very clearly. I find that very nice of them. I carefully checked the service manual: the F6 and the F8 are exactly the same in every detail and share the same parts, except for the throttle stop, and an ignition cut-out at 6200RPM. And It's not only about high RPM maximum power, even at lower RPMs fully opening the butterfly results in more torque.

I tested the motor further on a Lodestar NSA 260 this morning:
The original F6 will not get the dingy with 2 people onto a plane and speed stays limited to 12Km/h.
After bending away the throttle stop. in 3 or 4 seconds from stopped it gets the dingy with 2 people onto a plane, and it keeps it planing above 20Km/h, with less than 1/2 throttle.
 

Sandro

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I see. So it is exactly the same motor making do for two models. I suppose the F8 is dearer than F6. I am wondering how many will buy an F6 and then bend away the throttle stop (probably illegal).
But I am sure Yamaha know better than me what they are doing .
 

Svenbosma

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I find that very nice of them. I carefully checked the service manual: the F6 and the F8 are exactly the same in every detail and share the same parts, except for the throttle stop, and an ignition cut-out at 6200RPM. And It's not only about high RPM maximum power, even at lower RPMs fully opening the butterfly results in more torque.
I see you are talking about that these motors are the same, I have an 6m8 Yamaha motor, 6HP from 1997. I heard that this one is also the same as the 8HP model except a throttle stop. I want to upgrade or tune it to be an 8HP but I don't know how. Can you explain to me how to remove this throttle stop?
 

kanguru007

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I see you are talking about that these motors are the same, I have an 6m8 Yamaha motor, 6HP from 1997. I heard that this one is also the same as the 8HP model except a throttle stop. I want to upgrade or tune it to be an 8HP but I don't know how. Can you explain to me how to remove this throttle stop?
Remove cover and look at the carburettor while turning the throttle. It will be obvious. If not, ask a mechanically oriented friend for help.
 

crossy

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it has the same carburetor with the same size jets, the same admission and exhaust valves and opening times. Yamaha just added a simple throttle stop to limit how much the carburetor butterfly can be open.

After bending away the throttle stop. in 3 or 4 seconds from stopped it gets the dingy with 2 people onto a plane, and it keeps it planing above 20Km/h, with less than 1/2 throttle.

Apols for resurrecting this older thread and thanks Kanguru007 for this really useful info.

I'm looking at doing exactly the same thing that you have done for the same reasons, my 6 won't quite plane out dinghy with 2 of us in but will with just me so am hoping an extra couple of HP will do the trick.

Looking through the parts catalogue mine correlates with what Hobnob says above, the 6 has a different part number for the Carb Assembly (6H6−14301−01) from the 8 (6G1−14301−01) and the exhaust manifold pipe are also different.

Before i go spending any money on new carbs I just wondered whether you'd mind either popping up a quick picture of the throttle stop which you removed to limit the rpm or maybe a quick screenshot from your service manual?

Thanks in advance,
Andrew
 

kanguru007

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Before i go spending any money on new carbs I just wondered whether you'd mind either popping up a quick picture of the throttle stop which you removed to limit the rpm or maybe a quick screenshot from your service manual?

From "Yamaha F6AMH service manual.pdf"

Screenshot 2020-12-26 at 20.37.38 .png
The only difference between the two carburetors is in the shape accelerator arm. Otherwise they are the same and use the same jets.
Turn the arm by hand: the bit that has to be bent out of the way is the bit that stops it from rotating fully.

Bending the throttle stop away is the easy part, then you need to apply a bit of creative Archimedes physics to the carburetor so that it will rotate to the fully open position driven from the tiler accelerator.

(To gain more power in the upper rpm range we would have to change the exhaust manifold, but I would not even want that, when planning it is fast enough as it is)
 

Rappey

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I would check what size carb the 6 and 8hp have as a popular choice is to downsize a carb to detune an engine.
I had a honda 75 that was a detuned 90, the difference being smaller carbs but also a different inlet manifold to stop you from buying the cheaper 75 and carb set and uprating it.
what it achieved was higher rpm to increase the hp .

Just read above the carbs are same size....
 
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