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Old 21 September 2008, 14:58   #1
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Yamaha 25/30 HP 2 Strokes Conversions

Hi all

Does anybody knows what to change, alter in order to modify a Yahama 25 to 30 HP. Need to update an engine to use on a larger sib. Any ideas, thoughts, expertise are welcome.

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Old 21 September 2008, 16:52   #2
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Hello
It depends on what type of engine it is! If its a 2 cylinder them it's a 20/25 hp, If its an 3 cylinder than you have a 25/30 hp engine.

Try and find what the engine type is and i wil look in the yamaha manual i have at work, what parts they have in common.

i was thinking of doing the same on my 25hp but it's a 2cyl so im going to have to get some bigger carbs and do a bit o polyshing to get some more horsepower.

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Old 22 September 2008, 08:37   #3
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Hello
It depends on what type of engine it is! If its a 2 cylinder them it's a 20/25 hp, If its an 3 cylinder than you have a 25/30 hp engine.

Try and find what the engine type is and i wil look in the yamaha manual i have at work, what parts they have in common.

i was thinking of doing the same on my 25hp but it's a 2cyl so im going to have to get some bigger carbs and do a bit o polyshing to get some more horsepower.

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Hola Fernando

It's a Yam Enduro Model 25 HP, 2 St, 2 cylinder, 496 CC, 2007 engine. Seing a brochure, there are no 30 Enduros, the next one is a 40 HP. But there are 25/30 in the Sports range, both 2 cylinders, 496 CC too. The only difference between Enduro 25 and same Sport model are better chrome plated parts and 1 Kg more for the Enduro version. So, have a look to see what parts they share and which ones to change. Hope are easy to convert as Tohatsu's 25/30. Down here nobody sells/imports 20 HP engines, you say are same Yam 25/30 engines ?

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Old 22 September 2008, 11:11   #4
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Originally Posted by tsnando View Post
It depends on what type of engine it is! If its a 2 cylinder them it's a 20/25 hp, If its an 3 cylinder than you have a 25/30 hp engine.
Not necessarily true. I used to own a 30hp Yamaha twin, from memory I think it was a '98 model
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Old 22 September 2008, 12:40   #5
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Quote:
Originally Posted by Locozodiac View Post
It's a Yam Enduro Model 25 HP, 2 St, 2 cylinder, 496 CC, 2007 engine. Seing a brochure, there are no 30 Enduros, the next one is a 40 HP. But there are 25/30 in the Sports range, both 2 cylinders, 496 CC too. The only difference between Enduro 25 and same Sport model are better chrome plated parts and 1 Kg more for the Enduro version. So, have a look to see what parts they share and which ones to change. Hope are easy to convert as Tohatsu's 25/30. Down here nobody sells/imports 20 HP engines, you say are same Yam 25/30 engines ?
Loc, Not sure if this will help you or not - I have a 2005 Yam 20D which is (as far as I can determine from the parts manual) is identical to the Yam 25N except for the carbs and exhaust. But this is a 2 cyl, 2str, with 395 cc; weighing 49kg. The next model up in the UK when I bought this would have been the 30D with 3 cyls and 496 cc weighing 59 kg.
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Old 22 September 2008, 12:53   #6
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Loc, Not sure if this will help you or not - I have a 2005 Yam 20D which is (as far as I can determine from the parts manual) is identical to the Yam 25N except for the carbs and exhaust. But this is a 2 cyl, 2str, with 395 cc; weighing 49kg. The next model up in the UK when I bought this would have been the 30D with 3 cyls and 496 cc weighing 59 kg.
Thanks Pol, seems that there are many Yam versions for different countries, as don't use/sell Yam engines don't have the slightest idea betwen engine models. Will ask a local Yam tecnician who is well reputed , if cheap, will do the conversion with him, and definitely will be present to see how's done.

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Old 22 September 2008, 14:39   #7
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Hello
I Have a 25n and as far as i now it is similar to the 20d. At least it shears the same displacement and gearbox.


I will look tomorow at the two service manuals and try to cross reference the engines.
i am going to try and do some porting and exhaust work in the winter when the work slacks a bit, ill keep posting.
Just have to get my Sib back from the repair shop, to go catch some waves!

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Fernando
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Old 23 January 2014, 12:54   #8
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exhaust gas pipe

The difference is exhaust gas pipe.
I have drilled my exhaust gas so its now its like a 25 Hp but has anyone modified the pipe even more then you can get few hp more...?

Quote:
Originally Posted by tsnando View Post
Hello
I Have a 25n and as far as i now it is similar to the 20d. At least it shears the same displacement and gearbox.


I will look tomorow at the two service manuals and try to cross reference the engines.
i am going to try and do some porting and exhaust work in the winter when the work slacks a bit, ill keep posting.
Just have to get my Sib back from the repair shop, to go catch some waves!

Cumps
Fernando
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Old 30 January 2014, 15:02   #9
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i have a 20d which is infact the same as the 25n, different parts are the exhaust tuner tube, 20 has a washer restricting it halfway down

the other part thats different is the carbs,

ive got a spare set of the 25hp carbs with matching part numbers available if anyones interested?

other mods to try are boyesen reeds

vent the leg of the shaft in line with the lower bracket

remove some of the exhaust tuner tube as long as you have the narrowest part there, you can remove anything from about 10mm past it which also allows it to escape poy of the leg hole easier

polish the exhaust ports

advance the ignition 2 degrees @ full throttle
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Old 02 September 2016, 03:26   #10
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Hi supremesoundz

Thanks for the post.
I also have a 20hp Yamaha 2 stroke outboard. Runs ok but I could do with more hp. The serial no 6A-9S-016773 it is mid to late 80's and I am trying to get more hp . It is the single carburettor model and I have put a 25hp carburetor main jet in which is slightly bigger from #135 to #140 , I also advanced the timing but no real difference, How easy is it to remove the washer from the the exhaust do I need to take the engine head off
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