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21 September 2008, 20:58
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#1
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Member
Country: Other
Town: Lima-Peru
Boat name: Nautile
Make: Sea Rider 450 Rib
Length: 4m +
Engine: Tohatsu 5/18/30 HP
Join Date: Sep 2007
Posts: 1,998
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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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21 September 2008, 22:52
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#2
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Member
Country: Other
Town: Labutes City
Make: bombard
Length: 3m +
Engine: Yamaha 25
Join Date: Aug 2008
Posts: 15
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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.
Cumps
Fernando
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22 September 2008, 14:37
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#3
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Member
Country: Other
Town: Lima-Peru
Boat name: Nautile
Make: Sea Rider 450 Rib
Length: 4m +
Engine: Tohatsu 5/18/30 HP
Join Date: Sep 2007
Posts: 1,998
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Quote:
Originally Posted by tsnando
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.
Cumps
Fernando
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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 ?
Happy Boating
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22 September 2008, 17:11
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#4
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Member
Country: UK - England
Town: Bristol
Boat name: n/a
Make: Honwave T35AE
Length: 3m +
Engine: Tohatsu 18hp 2stroke
Join Date: Feb 2006
Posts: 379
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Quote:
Originally Posted by tsnando
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.
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Not necessarily true. I used to own a 30hp Yamaha twin, from memory I think it was a '98 model
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22 September 2008, 18:40
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#5
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RIBnet admin team
Country: UK - Scotland
Boat name: imposter
Make: FunYak
Length: 3m +
Engine: Tohatsu 30HP
MMSI: 235089819
Join Date: Sep 2005
Posts: 11,632
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Quote:
Originally Posted by Locozodiac
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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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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22 September 2008, 18:53
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#6
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Member
Country: Other
Town: Lima-Peru
Boat name: Nautile
Make: Sea Rider 450 Rib
Length: 4m +
Engine: Tohatsu 5/18/30 HP
Join Date: Sep 2007
Posts: 1,998
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Quote:
Originally Posted by Polwart
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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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.
Happy Boating
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22 September 2008, 20:39
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#7
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Member
Country: Other
Town: Labutes City
Make: bombard
Length: 3m +
Engine: Yamaha 25
Join Date: Aug 2008
Posts: 15
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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!
Cumps
Fernando
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23 January 2014, 18:54
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#8
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Member
Country: Finland
Length: 4m +
Join Date: Jan 2014
Posts: 1
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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
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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30 January 2014, 21:02
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#9
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Member
Country: UK - England
Town: kingsbridge
Boat name: felix
Make: cesar
Length: 3m +
Engine: yam 20hp
Join Date: Aug 2010
Posts: 31
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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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02 September 2016, 09:26
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#10
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Member
Country: UK - England
Town: UK
Length: 3m +
Engine: Yamaha 20 hp (2 STR)
Join Date: Sep 2016
Posts: 6
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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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02 September 2016, 09:46
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#11
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RIBnet admin team
Country: UK - England
Town: Cambridgeshire
Boat name: Nimrod II
Make: Aerotec 380
Length: 3m +
Engine: Yam 15 Tohatsu 9.8
Join Date: Nov 2007
Posts: 8,994
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Shane you might get lucky but supremesoundz hasn't posted on her for 2.5yrs.
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02 September 2016, 14:26
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#12
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Member
Country: UK - England
Town: UK
Length: 3m +
Engine: Yamaha 20 hp (2 STR)
Join Date: Sep 2016
Posts: 6
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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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02 September 2016, 14:54
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#13
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Member
Country: UK - England
Town: UK
Length: 3m +
Engine: Yamaha 20 hp (2 STR)
Join Date: Sep 2016
Posts: 6
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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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03 September 2016, 07:56
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#14
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RIBnet admin team
Country: UK - Scotland
Boat name: imposter
Make: FunYak
Length: 3m +
Engine: Tohatsu 30HP
MMSI: 235089819
Join Date: Sep 2005
Posts: 11,632
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Quote:
Originally Posted by Shane Hethering
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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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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03 September 2016, 14:35
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#15
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RIBnet admin team
Country: UK - England
Town: Cambridgeshire
Boat name: Nimrod II
Make: Aerotec 380
Length: 3m +
Engine: Yam 15 Tohatsu 9.8
Join Date: Nov 2007
Posts: 8,994
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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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05 September 2016, 10:20
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#16
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Member
Country: UK - England
Town: UK
Length: 3m +
Engine: Yamaha 20 hp (2 STR)
Join Date: Sep 2016
Posts: 6
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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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05 September 2016, 12:52
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#17
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RIBnet admin team
Country: UK - Scotland
Boat name: imposter
Make: FunYak
Length: 3m +
Engine: Tohatsu 30HP
MMSI: 235089819
Join Date: Sep 2005
Posts: 11,632
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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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