'86 Mercury XD 20hp to 25hp conversion

[onthewater102]

Anyone here done this one? As best I can tell from comparing parts on the schematics the lowers are all the same and the only differences are in the carburetor. The carb mounting plate and reed block are the same part # for both serial# groups.

I see the guys with the Johnson's from this era knocking their 20's up to 35's, which I know the Merc won't do, but to go from 20 to 25 wouldn't be bad if it could be done. It just seems that the merc/mariner XD's weren't as popular for some reason, I'm guessing the range of their power-class probably had something to do with it.

 

[muddywaders]

Was told by a merc guy that the porting on the 20 was different and would not benefit from a 25 carb.not sure.see if the part #s of the blocks are the same.Let me know what youve found.mw

 

[onthewater102]

They're all the same part#'s from what I can tell...at least they seem to have been on the old XD series motors. Perhaps he was referring to newer motors?

25hp XD:
https://www.boats.net/parts/search/Merc2/Mercury/25XD/6443973%20THRU%200B238463/CYLINDER%20BLOCK%20AND%20COVERS%20%28SERIAL%20GROUP%20%231%29/parts.html

20hp XD:
https://www.boats.net/parts/search/Merc2/Mercury/20XD%20%282%20CYL.%29/6443973%20THRU%200B238463/CYLINDER%20BLOCK%20AND%20COVERS%20%28SERIAL%20GROUP%20%231%29/parts.html

 

[T 300]

I swapped my 20 hp with a 25 carb. Much better, carb on the 20 is tiny.
3/4 throttle now is the same as wot before.

 

[onthewater102]

Sweet...that's kinda what I was hoping...now to find one.

 

[onthewater102]

Well, finding one for the XD series is damn near impossible. I'm thinking I can use one off the newer marathon 25's provided I use the enrichment assembly off my 20hp.

This is the carb on the newer XD series motor (https://www.boats.net/parts/search/...0/CARBURETOR (MERC 25)(MARINER 25)/parts.html) that shares many parts from the marathon series (https://www.boats.net/parts/search/MercuryMarine/0C221500/367/100/Carburetor(25)/parts.html). The parts i noticed being the same that lean me toward thinking they'll mate up just fine are the side plate (#13), the fuel pump body (#11), the throttle shaft (#3 - indicating the carb body is the same width) the top plate cover (#17) & the fuel bowl (#4) indicating both carb castings have the same upper and lower components...and the idle wire (#25). That is an assortment of parts from every end of the carb that define the outer points of the carb assembly and they're all the exact same part numbers, so I'm thinking the main casting is interchangeable. I was especially interested in the idle wire as it has to be the same distance from where it attaches to the trigger, which puts the mounting point for the idle wire to the enrichment assembly at the same point.

The throttle valves are the same, but they are a different part # from the valve on the 20hp which is why I think i need to change the entire carb and not just the jet as the bore is probably larger.

I'm really thinking the only difference between the 85XD carb and the 88Marathon is the enrichment components as they relate to the lower section of the cowl housing and a different supplier for the casting so that it comes with a different casting #. If I recall correctly, the XD motors were made domestically whereas the Marathons are from Belgium.

 

[onthewater102]

Was told by a merc guy that the porting on the 20 was different and would not benefit from a 25 carb.not sure.see if the part #s of the blocks are the same.Let me know what youve found.mw

The mounting plate for the carb is the exact same part # for the 20hp as the 25hp for both the marathon motors (part #34 in each diagram, merc part #41420A 1):
25hp intake plate https://www.boats.net/parts/search/.../367/120/Cylinder Block and Covers/parts.html
20hp intake plate https://www.boats.net/parts/search/...007 & Up/Cylinder Block and Covers/parts.html

and the XD series motors (part #34 in each diagram, merc part #41420A 1 - same as the marathon motors):

25hp intake plate Cylinder Block Group 1 https://www.boats.net/parts/search/...BLOCK AND COVERS (SERIAL GROUP #1)/parts.html
25hp intake plate Cylinder Block Group 2 https://www.boats.net/parts/search/...BLOCK AND COVERS (SERIAL GROUP #2)/parts.html

20hp intake plate Cylinder Block Group 1 https://www.boats.net/parts/search/...BLOCK AND COVERS (SERIAL GROUP #1)/parts.html
20hp intake plate Cylinder Block Group 2 https://www.boats.net/parts/search/...BLOCK AND COVERS (SERIAL GROUP #2)/parts.html

After comparing all those I'm quite certain that everything will be interchangeable.

 

[JMichael]

I'm not sure but on a 2 cycle, isn't there some porting on the cylinder/block that is separate from what the carb mounting plate has. Have you checked to see if the blocks between the 2 motors are the same?

 

[onthewater102]

yup - they are

 

[onthewater102]

Part arrived today - going to get it on the boat when I get home & take her for a spin. I'll post the webpage for the late model parts supplier I found when I get home & test it - the carb looks like it was pulled from a brand new motor.

 

[onthewater102]

First off, a big thank you to Robert Jones over at https://www.northernoutboard.com for the prompt delivery of the beautiful 25hp carb. I did pull off the fuel bowl and inspect the innards which were nice and clean, but the float had the same pin-hole issue taking on gasoline as my old one and needed to be replaced. No big deal - just something these old merc carbs must have issues with I guess.


Well, anyone with any concerns can set them aside - the differences in the airflow are entirely due to differences in the carb casting. The gasket from the 20hp is cut out for the hole on the back of the 25hp, as is the intake plate, which is why the part #'s are all the same. The gasket cutout is oversized on the 20hp, as seen in my pics below. Bottom line - you can definitely take the 25hp carb from the newer Marathon series of Mercury engines on the older XD series engines with no engine modifications.

Side by side, can you tell which is which???

Due to the difference being so drastic, and the fact that the casting # from my carburetor (WMC-2) is not listed on the 18/20/25 XD series parts schematics I'm not sure exactly what part I had originally as I'm not the first owner of this engine.

Showing the measurement of the main inlet opening to be the same 1 5/16", though the casting internally is totally different.
Original Carb:

25HP Carb:

Engine side of the carbs, 25hp first with measurement of 1 1/4" the outlet from the carb to the engine:

And the original carb with it's 1" engine side outlet

Here's the original carb with the original gasket on it with the 1 1/4" measurement of the new carb's oultet for reference:

The original gasket is definitely larger than the original carburetor outlet...however - when you put that gasket on the intake plate you see that the intake plate is bored to the size needed for the 25hp carburetor

You will notice the fuel pump body on the 25hp carb is significantly wider than the original. This did not create clearance issues with the original throttle plate. All the powerhead part numbers and lower unit parts are the same for the various motors, which makes sense, as everything from the intake gasket on is designed for the dimensions of the 25hp. While not as drastic a HP increase as you'll find available to similarly sized Johnson motors (that can go all the way to 35hp with the right intake and carb combination) it is just as simple an upgrade.

 

[onthewater102]

Boat went from topping out at 18mph to 22mph...not bad, not as good as I'd hoped, but not bad. A member of my local CT fishing forum suggested I might need to adjust the timing for the new carb - I'm going to look into that & see if there is anything to it.

 

[lckstckn2smknbrls]

You might need to reprop the motor.
What rpm's is it turning with the new carb?

 

[onthewater102]

The 25hp motors came with the same 10.125x13P that I had on the motor already. I had it cupped earlier trying to fix a slipping issue, but now I think that's holding the RPM's back. As far as exactly where I'm at with RPM I'm not sure yet - I've only had it out once and I forgot the tach at home. I'm not getting 5,500 RPM for sure, as I know what that sounds like and I'm certainly not there, but where exactly I'm at I'm not sure.

My tow vehicle is a Chevy, so naturally because I used it last weekend it needs work this week, so I'll be lucky if I get out there Saturday afternoon to test everything.

 

[nelsonag]

Hey there, do you happen to have the part numbers for the available 25hp carbs that are compatable? I have the 18xd and am looking to do something similar. I currently have the WMC 9 carb.

Unrelated to the carb, do you happen to know how to set this up for remote throttle? The pully system is wonky, but Ive seen there are people that have it done and theres a cutout on the cowl for the throttle so im sure theres an adapter somewhere

A

 

[onthewater102]

Mine was originally a tiller setup - there are a number of components to be replaced to convert it, not just the plate on the side of the cowl. The cams on the side of the motor and the rod linkages were all different. I found someone parting a motor out on Ebay and picked the parts up there, some of them were out of production if I recall.

 

[onthewater102]

As far as the parts for the carb, look to boats.net at any of the 25hp motors. Look for the cam style engagement of the throttle plate - any of those carbs will fit.

 

[nelsonag]

Thanks. Could you post a picture of your throttle linkage setup so I have something to compare to?