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30 July 2024, 06:39
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#1
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Member
Country: UK - England
Town: Colchester
Length: 5m +
Join Date: Sep 2016
Posts: 3,124
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Yamaha 15 hp 4 stroke convert to 20 hp
This would be a carb model 15 hp. Has anyone done this is it simply a matter of swapping over to a 20 hp carb
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30 July 2024, 08:00
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#2
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Member
Country: UK - Scotland
Town: wormit
Boat name: lots of them
Make: various
Length: no boat
Engine: all types
Join Date: Dec 2004
Posts: 632
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You need a brain.
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30 July 2024, 09:38
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#3
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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: 8,996
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>>You need a brain.
Is that an ECU or CDI on those models... and in addition to a carb or jet swap I assume? How about the exhaust, is that changed on the 20hp?
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30 July 2024, 11:24
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#5
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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: 8,996
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If that is around 2010-2014 or so I reckon it won't have an ECU but a standard CDI ign. If so I've looked at the parts lists (always the place to turn to) and the CDI units are the same part number, as is the exhaust stub, so the only part that could be replaced is the carb which has just one character different in its part number. It could just be that the jet set is different but be aware sometimes the carb bore is loads smaller as well as jet differences. I'll let you check that out.
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30 July 2024, 12:07
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#6
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RIBnet admin team
Country: Ireland
Length: 4m +
Join Date: Feb 2008
Posts: 14,912
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30 July 2024, 12:21
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#7
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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: 8,996
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That's a good call Willk and seems to confirm the parts fiche research that's it's just the carb required to gain 5hp.
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30 July 2024, 12:26
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#8
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RIBnet admin team
Country: Ireland
Length: 4m +
Join Date: Feb 2008
Posts: 14,912
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No probs - I was wasting 90% of my mental capacity at work today anyway...
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30 July 2024, 13:00
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#9
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Member
Country: UK - England
Boat name: Redneck
Make: Excel
Length: 3m +
Engine: 20 efi & 9.8 2s
Join Date: Jul 2020
Posts: 3,496
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Thought you wasn't keen on heavy 4 strokes Dennis, that's got to be around 50kg. Or are you going to leave it permanently fitted?
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30 July 2024, 15:33
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#10
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Member
Country: UK - England
Town: Colchester
Length: 5m +
Join Date: Sep 2016
Posts: 3,124
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Quote:
Originally Posted by Steve509926
Thought you wasn't keen on heavy 4 strokes Dennis, that's got to be around 50kg. Or are you going to leave it permanently fitted?
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Hello Steve. I do indeed prefer lighter 2 strokes but this is for my new Honwave T40 that is already a big heavy beast and stays on a Trailer. I did its maiden voyage with a Mercury 20hp 4 Stroke which I was very pleased with. I just fancied seeing what the Yam would be like but only if it is 20hp. The facebook link was one at what I thought was a reasonable price but a 15hp
So yes this is one engine that will be left on the boat on a trailer if I get it if not I will stick to the Mercury. It is the gear selection that I like better on the Yam
Video soon of the maiden voyage of the T40 
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30 July 2024, 15:39
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#11
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Member
Country: UK - England
Town: Colchester
Length: 5m +
Join Date: Sep 2016
Posts: 3,124
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Quote:
Originally Posted by Fenlander
That's a good call Willk and seems to confirm the parts fiche research that's it's just the carb required to gain 5hp.
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Very useful information. Thanks
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30 July 2024, 16:44
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#12
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Member
Country: UK - England
Boat name: Redneck
Make: Excel
Length: 3m +
Engine: 20 efi & 9.8 2s
Join Date: Jul 2020
Posts: 3,496
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Quote:
Originally Posted by smallribber
So yes this is one engine that will be left on the boat on a trailer if I get it if not I will stick to the Mercury. It is the gear selection that I like better on the Yam
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Just shows everyone is different, if it's the Merc with the gear selection on the tiller I much prefer it like that.
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30 July 2024, 17:11
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#13
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Member
Country: UK - England
Town: Colchester
Length: 5m +
Join Date: Sep 2016
Posts: 3,124
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30 July 2024, 17:13
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#14
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Member
Country: UK - England
Town: Colchester
Length: 5m +
Join Date: Sep 2016
Posts: 3,124
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Quote:
Originally Posted by Steve509926
Just shows everyone is different, if it's the Merc with the gear selection on the tiller I much prefer it like that.
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Mine is very very stiff to go in gear. I am going to have another look at it as when I took off the carb about 6 months ago I am wondering if I put all the linkages back together properly. In that area so much is connected to so much else
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30 July 2024, 17:48
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#15
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RIBnet admin team
Country: Ireland
Length: 4m +
Join Date: Feb 2008
Posts: 14,912
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Quote:
Originally Posted by Steve509926
Just shows everyone is different
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I'm not! 
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30 July 2024, 20:39
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#16
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Member
Country: UK - England
Boat name: Redneck
Make: Excel
Length: 3m +
Engine: 20 efi & 9.8 2s
Join Date: Jul 2020
Posts: 3,496
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Quote:
Originally Posted by smallribber
Mine is very very stiff to go in gear. I am going to have another look at it as when I took off the carb about 6 months ago I am wondering if I put all the linkages back together properly. In that area so much is connected to so much else
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Just looked at the older Merc 20hp carburettor model and it is a different gear selector on the EFI model.
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30 July 2024, 20:51
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#17
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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: 8,996
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Is yours gears on tiller twist throttle Steve?
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30 July 2024, 21:06
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#18
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Member
Country: UK - England
Boat name: Redneck
Make: Excel
Length: 3m +
Engine: 20 efi & 9.8 2s
Join Date: Jul 2020
Posts: 3,496
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It is, I find it great to use but I know some don't like it.
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30 July 2024, 21:10
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#19
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Member
Country: UK - England
Town: Colchester
Length: 5m +
Join Date: Sep 2016
Posts: 3,124
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Mine is operated by twisting the throttle. It is the push forwards or back that I prefer like the Yamahas
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31 July 2024, 19:30
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#20
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Member
Country: UK - Scotland
Town: wormit
Boat name: lots of them
Make: various
Length: no boat
Engine: all types
Join Date: Dec 2004
Posts: 632
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Quote:
Originally Posted by Fenlander
If that is around 2010-2014 or so I reckon it won't have an ECU but a standard CDI ign. If so I've looked at the parts lists (always the place to turn to) and the CDI units are the same part number, as is the exhaust stub, so the only part that could be replaced is the carb which has just one character different in its part number. It could just be that the jet set is different but be aware sometimes the carb bore is loads smaller as well as jet differences. I'll let you check that out.
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Part number for the f15c 6ah-85540-00
Part number for the f20b 6ah-85540-10
Can only assume the timing curve/ max advance is different.
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