Uprating Mariner (& Mercury) 25 to 30 - simple as a gasket?

[Max...]

I did not want to bump the topic below as it's so old but can any of the engine gurus on here comment?

Currently looking for a 2000's 25 2 smoke either Merc or Mariner - were they the same in this era as per the 15?

Reference below seems to be the 'Marathon' version - is the uprate applicable to all types?

http://www.rib.net/forum/f36/upratin...0hp-25518.html

[Bartol]

All I replaced on my 1999 Merc 20 was carb from 2004 Merc 25 (WMC101a). Inlet throat is noticeably larger, everything else is the same. Same gaskets, same ingition timing, just more power.

[Max...]

Bumping this - where are all the gurus?

[samt]

The Tohatsu built 25hp can be uprated to 30hp in theory by just swapping parts, but to get 30hp from the Mercury built 25hp would involve reboreing and proper tuning. See photos for reference, both were avaliable new at the same time and even the 'Marathon' or 'Seapro' doesn't necessarily indentify either one. But i believe the Tohatsu built engine was known as the international model.

[Max...]

Thanks Sam, I thought it was as simple as just changing a gasket for a de-restricted one? All carb parts, jets etc remain the same? Just cannot seem to get a definitive answer on this!

[uncle al]

I would suggest thatyou go on any free site with parts lists, and compare the 25hp tohtso , with a