onthewater102
Member
Registered: 04/14/05
Posts: 1649
Loc: Kent, CT
Guys - you can simply swap in the carb off either a similar 25 HP XD series engine or one of the newer marathon series motors - either one will work. The powerhead parts and lower units are all the same for the 18/20/25 HP engines, I've checked plenty of serial #'s on boats.net at this point to verify there are no differences in any of the parts. The intake gasket and intake plates are all made for the 25hp size carb but used on all 3 motor sizes without an issue.
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. when I ordered the rebuild kit every gasket and part in it fit fine except for the idle mixture screw, which fortunately I still had the original one.
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 - slightly easier as you don't need to re-bore or replace the intake (if you want more than 25hp the intake needs to be replaced on the Johnsons.)
I know these engines well as I have two of them, but both have Tillotson carbs and the Tillotson carbs are junk. I have a rebuilt Walbro that I am going to mount soon so that I can get going again.
I know that the timing is different between the HP's for engines with Tillotson's as my first 25hp started out as an 18HP. That motor is a Frankenmotor.
I know that the timing is different between the HP's for engines with Tillotson's as my first 25hp started out as an 18HP. That motor is a Frankenmotor.
please elaborate. Ken and i tested that motor out with the bigger carb and found no difference in speed, and it didnt sound like it was reving any higher either. something isnt right, or we'he is missing something.
onthewater102
Member
Registered: 04/14/05
Posts: 1649
Loc: Kent, CT
Not entirely true, it ran 18mph before the change, 20mph after with 2 people - then after changing the trim angle I went out by myself and hit 22mph, though I'm not sure if that was the trim angle or the extra person less weight.
I didn't think about needing a timing adjustment. I don't see a difference in the steps in the FSM - I can share the google drive link if needed - but perhaps it will need to be set differently within the same bounds.
It is definitely adjusted to idle smooth and slow - you can troll on the damn thing if you wanted to...granted, that doesn't necessarily mean WOT is perfect.
Sorry for the late response. I was referring to Tillotson carb'ed motors. The ones in the pictures are Walbro's (WMC's).
If I remember correctly, the 25hp Tillotson's have designations like BC1A, BC1B and BC1C. The 18hp has BC2A, etc. My (POS) Clymer's manual for this engine lists different timing at idle. At WOT (wide open throttle), I think they are all the same. Regardless of the HP and the carb maker.
Also, on the Tillotson's, there are two lines on the throttle cam. One is for the 18hp and the other is for the 25hp. Remember that the Walbro's are different.
My Tillotson carb(s) has a chainsaw float needle installed and it has the material for the tip that withstands the scourge of ethanol in our gas. The original tip swelled up so much from the ethanol that no fuel could get into the carb even though the bowl (plastic and not ethanol resistant either) was bone dry.
My Frankenmotor has a 25hp carb on when used to have the 18hp one. Same the throttle cam/linkage was adjusted for the 25hp setting. The idle timing was set something like 5-BTDC whereas the 18 was something like 2-ATDC. My boat (Abalone, a 14-ft Duranautic), is now wicked fast and I like to race dual-motor Regulators on the CT river. Scary and I always lose, but it is a lot of fun.
Hi I know this is an old thread but i need some help with my Merc 18HP early 80's
The carb is a tillotson #bc2b carburetor.
Are you saying I can simply mount another carb on my motor?
Im having trouble with my current carb, seems to need rebuilding every year, ive stored properly. I cant find another one of these carbs as a replacment
I know these motors well. Well enough that I can tell you that the Tillotson's do NOT like gasohol. The o-ring on the main jet is not compatible and neither is the tip of the float needle. YOu can get a Viton o-ring for the jet if you don't destroy it while removing same. The float needle can be replaced with that from a chainsaw, can't remember which (the internet is your friend). Lastly, the plastic fuel bowls tend to warp and the cork gasket for same becomes hard a rock from the gasohol. It is time for you to move on from the Tillotson, in my opinion.
What I did to keep my motor alive is to get a WMC 25hp carburetor. It is not a completely bolt-in operation but it can be done. Think I had to change the reeds because the holes in same don't exactly line up with the WMC carb. Also, you will have to grind away some metal on the forward cowl cover "hook" near the fuel inlet. If you go down this route, you want an earlier WMC carb, something like a WMC-19 if memory serves me correctly. Timing (at idle) is a little different as well.
I can definitely tell you that after I did this, I felt more confident in the motor and it runs like new. The boat flies with the max pitch propeller made for this motor. If you upgrade to a 25hp carb from an 18hp one, your fuel economy will be the inverse of your horsepower gain, but for me was desirable and well worth it. It pushes a 1967 Duranautic 14, "Abalone."
Regardless, good luck to you and let the community know what your plans are for your motor.
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