coverting Yamaha 4hp 4stroke to 6hp 4 stroke

ranuplh

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Joined
Dec 21, 2012
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2
If you cant afford to buy another motor like me....try this

yep the motor for the 4, 5 and 6hp 4stroke is the same weight, same bore stroke, same cc of 139.
based on 1hp=25cc then 139cc =5.56hp but most engines are tuned down to a lower HP for better engine life.


Ok what to do

......change the carb from 4hp to 5 or 6 hp equivalent (carb venturi/throat size is the difference between these motors) .
change the exhaust outlet size to the 5 or 6hp one
reset the idle stop adjustment, and verify/adjust full throw of the throttle cable (from idle to WOT)

and thats it.

Rick
 

boobie

Supreme Mariner
Joined
Nov 5, 2009
Messages
20,826
Re: coverting Yamaha 4hp 4stroke to 6hp 4 stroke

What difference is two hp going to do for the performance of the boat for the cost of doing it ??
 

Capt Ken

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Joined
Jul 30, 2002
Messages
2,269
Re: coverting Yamaha 4hp 4stroke to 6hp 4 stroke

Faster trolling speed. Real sleeper outboard. Just think of all those other boats with a 4hsp you can outrun.
 

ranuplh

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Joined
Dec 21, 2012
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2
Converting Yamaha 4hp 4stroke to 6hp 4 stroke

Converting Yamaha 4hp 4stroke to 6hp 4 stroke

Faster trolling speed. Real sleeper outboard. Just think of all those other boats with a 4hsp you can outrun.

Well, I'm coming from an engineering approach, rather than a simplistic cost/benefits approach or making fun of small engine four stroke users, when the planet is facing an energy crisis.

To answer your question, it is possible to buy second hand and through understanding the problem solve it.

The 4hp is the underpowered 6hp for the same stroke. The inital torque of both motors is the same, but the output of the 4 to 6hp motor have been limited.

I'd welcome other members who have engine development performance in these small motors.
 

Capt Ken

Commander
Joined
Jul 30, 2002
Messages
2,269
Re: Converting Yamaha 4hp 4stroke to 6hp 4 stroke

Re: Converting Yamaha 4hp 4stroke to 6hp 4 stroke

You have not discovered anything new. Engine builders have been restricting HP on certain engines for decades. It's due to boat builders restricting their boats to certain horsepower. Or they need to compete with a competitors engine. So the question is, is gaining 2 hsp worth the cost of changing the fuel and exhaust system?
 
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