15hp mariner 4 stroke upgrade to 20hp?

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AaronR
Posts: 11
Joined: 08 Aug 2010, 22:06

15hp mariner 4 stroke upgrade to 20hp?

Post by AaronR » 02 Aug 2016, 10:07

I have a 1998 15hp mariner 4 stroke made by Tohatsu on a 14' Lowe semi-V.I'm only getting around 16-18mph out of it. From google I can see that people have done research to see the engine is the same between the 15hp and the Tohatsu 20hp but the main jet, slow jet, main jet plug, and the main jet nozzle are the only differences but they didn't think to add the needed part numbers.

I first called a Mercury dealer who told me they only sourced a 15hp and 25hp from Tohatsu and that I would need to call a Tohatsu/Nissan dealer to get the jets I need.

Called the Tohatsu dealer who said to call a different mercury dealer because he couldn't help.

Called another mercury dealer who said the carbs did look the same but there were multiple jet choices and he didn't know which ones I needed but told me the Mariner 15hp 4stroke was supposed to be shipped to Belgium and I may have difficulty locating parts and I'm wasting my time anyway because going from 15hp to 20hp would give me no performance gains. He said I would need to go to a 30hp just to gain 1-2mph. Seems to me that a 30% increase in power would be noticeable.


Does anyone have a good online resource for parts? I've tried iboats and marineengine with no luck.

Thanks for any help!

Xtremeboats
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wmk0002
Posts: 596
Joined: 17 Apr 2014, 15:39
Location: Cullman, Alabama

15hp mariner 4 stroke upgrade to 20hp?

Post by wmk0002 » 02 Aug 2016, 10:15

What are your rpms and what pitch is your prop? I can't help you on the conversion you are talking about but 18 mph isn't that bad for a 15 HP on a 14 ft boat depending on the load.
2016 Alumacraft 1648 NCS - 1992 Johnson 9.9 HP Tiller
http://www.tinboats.net/forum/viewtopic ... 21&t=40764

AaronR
Posts: 11
Joined: 08 Aug 2010, 22:06

15hp mariner 4 stroke upgrade to 20hp?

Post by AaronR » 02 Aug 2016, 22:18

Rpms are about 5600-5800.The motor was actually brand new from 1998 still on a crate when i bought it last year and it has the factory prop, it says 48-9 so im not sure if its 9 pitch or 9 diameter.

I had a '76 Johnson 20 on the boat before and it ran about 24-25 on the same load, that's my main motivation for the conversion.

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wmk0002
Posts: 596
Joined: 17 Apr 2014, 15:39
Location: Cullman, Alabama

15hp mariner 4 stroke upgrade to 20hp?

Post by wmk0002 » 03 Aug 2016, 10:43

Oh, I see. Sorry I can't help. I'm gonna follow your thread out of curiosity to see if it can be done though.
2016 Alumacraft 1648 NCS - 1992 Johnson 9.9 HP Tiller
http://www.tinboats.net/forum/viewtopic ... 21&t=40764

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