Mercury outboard carb swap for increase HP

[nelsonag]

Hello all,
I have an 85 merc 18xd that came on a 16 foot tin boat I purchased. It does well but with 3 guys and duck gear, I might need more juice. Saw some videos online of a guy swapping out the carb and exhaust tuner on this model to upgrade to a 25hp. From research online, the engine block and all other components are the same across the 18, 20, and 25 with only the carbs and tuners differing. Has anyone done this before with any result? I am curious if I just buy a carb if it will be negated by the smaller exhaust tuner? Or should I just sell it and put the extra $200 I was going to use for the carb into a 40hp engine. thoughts?

Also, if anyone has any advice on turning this from a tiller into a remote steer, Im all ears. I attached some pics

 

[lckstckn2smknbrls]

Go 40hp

 

[CedarRiverScooter]

Go 40hp

Good advice.

Carb swaps come with a lot of rejetting & fiddling. May not have good idle ever again.

 

[jethro]

I agree, if you want a 40 and the remote control, sell it and go to a 40. You'll be happier in the long run.

 

[RaisedByWolves]

Yeah, with the OMC engines there is a tested recipe that will get you more HP out of your engine.

The Mercs always made more speed stock per HP but were more high strung out of the box and didn’t respond as well to modifications.

 

[lckstckn2smknbrls]

Even if the mods go perfect 25hp is not 40hp.

 

[onthewater102]

It's a straight forward mod, nothing is different other than the carb, I've done it, you get more power and it idles just fine because it doesn't know it wasn't a 25hp to start with, but you go from 18 to 25...not 40. If your boat can take it go for the 40.

 

[GYPSY400]

Sell the 18 and buy a 40. Even if you went with the 25 mod plus the remote steer option it's not worth the trouble.. and later on if you modded it for remote steer and try to sell it, no one will want it.

 

[Crazyboat]

Going for 18-25 is not bad on a light boat, but with a couple of buddies you'll be lucky to notice a 2-3 mph gain IMO. If you can trade her for the 40 everyone else talks about, do so. If not change both parts, to realize the increase, doing just one won't do what you want. Picture a 1/2" garden hose, you open one end up to 1" but leave the other @ 1/2", you don't get any more flow out of her, you need to open both ends.

 

[thill]

Get the 40. You will be very pleased at the difference.

When I upgraded, the biggest surprise is that the larger engine burns LESS fuel to cruise at 25. The little engine would have to run full blast to reach 22, where the bigger one was just humming along, not straining at all. Mine now tops out at 30-31, if I need to ever go fast.