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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.

Happy Boating
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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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Fernando
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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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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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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, 13:54   #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, 16:02   #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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Old 02 September 2016, 03:46   #11
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Shane you might get lucky but supremesoundz hasn't posted on her for 2.5yrs.
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Old 02 September 2016, 08:26   #12
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Thanks for spotting this - do you or anyone else have any knowledge on the matter then.
Please advise thanks, Shane
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Old 02 September 2016, 08:54   #13
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I understand not everyone is always in front of thier PC so so if anyone else can help with the above it would be really appreciated,

Many thanks Shane
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Old 03 September 2016, 01:56   #14
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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
I had a fairly recent discussion with a local, but very helpful and knowledgable outboard mechanic (one with real experience doing this sort of stuff rather than just servicing engines). My engine is twin carb and more modern but I suspect the same applies:

1. The carb body needs replaced not just the jets.
2. The exhaust manifold needs replaced (it may be possible to widen it out, but they are two different parts not just one part with a washer fitted loose in it. After many years of use in a hot salty environment, I am not sure I'd expect to attack it with a drill and not damage it.)
3. The timing needs adjusted.
4. Its a "relatively easy job" assuming everything unbolts easily. If any bolts shear then its just become a major effort. On a 30 yr old engine that is a very real risk for you.

Off the top of my head the parts (at Yam prices) were c. 600 GBP [can't remember if there was VAT on top]. Labour would be more on top, and for a motor of my age wasn't going to be worth it. For a motor of your age it definitely won't, unless you can find secondhand parts and DIY the work.

These engines hold their value well, but there is not much difference is price between 20 and 25, and you'll get no extra resale on a converted 20-25 (unless you happened to have a test rig to prove it was giving 25!)
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Old 03 September 2016, 08:35   #15
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>>>not much difference is price between 20 and 25, and you'll get no extra resale on a converted 20-25 (unless you happened to have a test rig to prove it was giving 25!)

Yes my thought was sell yours and buy one that's a proper 25hp. If you are careful with both buy and sell it will cost nothing.

Further I'd personally downvalue a DIY uprated engine because you have no idea if it was done to spec and how well.
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Old 05 September 2016, 04:20   #16
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Hi Poly,

Thanks for your very helpful post.
Yes my model is slightly different to yours, in that the Yamaha 1984-1986 20hp and 25hp outboard engine model shares the same single carburetor, which the local Yamaha dealer confirmed when I bought the 25hp Carb jet a few months ago. The only difference is the Exhaust tube and the Timing.

I went out yesterday and I adjusted the manual trim/tilt pin yesterday to lift up the bow more and it made a big difference on the boat speed, I was very surprised.

Poly, I am just wondering
1) What pitch propeller you are using? on your 20hp
2) Have you been able to find any alternative reed valves for the engine like the Boyseen Reeds, I know that they give much better performance from when I installed some on another engine (a 9.9 Yamaha) .. if so do you have a part number.

Many thanks Poly and look forward to your reply
Shane
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Old 05 September 2016, 06:52   #17
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Shane, I've not tried any fancy reeds. I can certainly make a couple of knots difference getting the trim right, and i recon I've lost a knot or more just with all the crap I carry (2 anchors, lots of rope, fire extinguisher, spare oil, spare fuel etc).

I've got a selection of props (all standard yam issue) in slightly different sizes. I can't remember exactly what they are and I've never been bothered to test them all on the same day in the same conditions. If i remember I swap between the biggest and smallest for running alone and with the whole family - but I mostly use the middle size and forget about it.
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