Mariner/Tohatsu 15hp 4 stroke to 18/20 HP?

[00V8]

I have a 1999 mariner(made by tohatsu) 4 stroke 15hpoutboard. I was doing some research and if what I've seen is correct only the 2 jets are the only thing different between the 15/18hp engines.

What about the newer 20hp? It has some changes to the carb part numbers but inspecting one in person it looks identical to my engine.

It's the conversion to the 20 hp from 15hp going to provide much gain?

It seems like it would be substantial to me as it is a 25% increase in power but the dealer I called said it wouldn't change the power of the engine at all.

Thanks for the help!

 

[Sea Rider]

Both carbed 4 strokes 15 & 20 have same 350CC, both run with same timing advance, 20 carb should have wider fuel passages, that's where the extra HP power comes from. Swapping 20 jets to a 15 carb is a no go, waisted time and money.

Will need to check if both models share same prop pitch, if not will need to swap 20 HP prop, tach it as usually loaded on a wot spin to check if OB is revving middle to max wot rpm range.

Happy Boating

 

[pvanv]

I have a 1999 mariner(made by tohatsu) 4 stroke 15hpoutboard. I was doing some research and if what I've seen is correct only the 2 jets are the only thing different between the 15/18hp engines.
What about the newer 20hp? It has some changes to the carb part numbers but inspecting one in person it looks identical to my engine.
It's the conversion to the 20 hp from 15hp going to provide much gain?
It seems like it would be substantial to me as it is a 25% increase in power but the dealer I called said it wouldn't change the power of the engine at all.
Thanks for the help!

Incorrect. Changing jets would cause incorrect mixtures,
The newer 15/20 is a completely different block.
You could change complete carbs to go to 18 hp.
If you install a carb for the 18, you would gain 3 hp, which is about 20%

 

[Sea Rider]

Can't find a 4 strokes 18 HP listed on Tohatsu Jap web site, is it only for US market ?

Happy Boating

 

[pvanv]

It is now obsolete.
The 9.9 was replaced by the 9.8.
The new 15/20 replaced the 9.9/15/18.
(four strokes, that is.)