Differences between 88-92 Merc Four cylinders….

[ripperace]

Hello all.
I’m not much of a Merc guru by any stretch of the imagination, and this question is for those who are knowledgeable in the area.

I recently picked up a 1990 Merc 100hp cheap that needs rebuilt. The model and serial number indicate it’s a 115, but I am aware both the 100 and 115 were based on the same block. When I build this powerhead, I’d like to get the most out of it that I can. Soooo, what are the differences between the 100, 115, and 125? Are the differences in carburetion, and ignition timing, or is there more to it than that?

Help from Merc guru’s appreciated.
Thanks people!

 

[Texasmark]

Pulling up the parts lists will help to answer your question. Service manual details carburetors for the different models which is another avenue for reference. I have a 2002 115 and its RPM limited to 5250. Possibly the 125 doesn't have a limiter or if so its setup to kick in at 5500 or so....the service manual would detail the RPM for the 125. I am happy with what the 115 does for me and don't want to play around. If I felt that I was HP limited, I might be inclined to disconnect the RPM limiter module and see what she can do on my boat.

I just pulled out my Serv. Manual for '96 to 2004 or 5...what ever year they quit making that engine and the RPM limit is the same for all 100 thru 125 HP engines. The block is the same 112.8 cu in. loop charged. Moving over to section 3B the 115 Carb. numbers are 80-1 thru 4 for the 4 cylinders indicating the size of the jets and the 125 is 81-1 thru 4 with the jet diameters increased by 0.010 on cylinder #1 to 0.020 for the other 3. So it looks like you could just change your jets and get your extra 10 hp.... which isn't all that big of a mechanical change for an engine of that size.

Ask Faztbullet. If anybody knows he does.

 

[flyingscott]

You have the small bore motor, you can make it a 115 with just the carbs I believe. You will have to check the parts differences. They changed the bore size on those motors when the 125 cam out, so they are different. There might be other differences other members can chime in. All the 4 cyl 2+2 motors have an RPM range of 4750-5250.

 

[faztbullet]

The 1988-93 are small bore 105ci engines and the 1994 and up are 113ci motors. Only difference is carbs and jets. You cant put 125 carbs(113ci) on a 100/115HP 105ci block with major re-jetting. If the carbs are marked WME-11 or 20 they are 100hp and if WME-14 or 21 they are 115hp.