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1985 20hp to 25hp carb difference

N

newyota

Regular Contributor
Can anyone tell me what the carb. differences are between the 20 and 25hp .I just bought a intake and carb.per my marina mechanic used of course for my 20hp to upgrade and thought the venturi would be bigger on the 25hp carb, but it is the same as what my 20hp is.Is it just the jetting that is different?I have the restricted intake so I have the 25hp unrestricted intake now that came off motor the guy got carb.Part numbers on top of carb and bottom of bowl are same as what I got on my 20hp carb.thanks
 
Yes it should be a 47D for the 20hp carb. You may need to grind a wider screwdriver off just enough to fit inside the threads for the plug they are usually in there pretty tight.
 
kimcrwbr1 said:
Yes it should be a 47D for the 20hp carb. You may need to grind a wider screwdriver off just enough to fit inside the threads for the plug they are usually in there pretty tight.
Click to expand...
ok-I was just looking at parts list for both 20 AND 25 and they both call for a 47D.Now what.Someone else did get back to me and say the venturi should be bigger on the 25.So either I already have the 25 or I was sold another 20?Part tag on the carbs has been no help.
 
I just got back from marina and showed him carb and told hime the venturi measure the same and main jet in 20-25 call for same jet.He said some of these do use same carb, but just restrict intake like mine is.Got the right intake now.Hopefully I can eturn the carb per my marina mechanic.
 
newyota said:
Can anyone tell me what the carb. differences are between the 20 and 25hp .I just bought a intake and carb.per my marina mechanic used of course for my 20hp to upgrade and thought the venturi would be bigger on the 25hp carb, but it is the same as what my 20hp is.Is it just the jetting that is different?I have the restricted intake so I have the 25hp unrestricted intake now that came off motor the guy got carb.Part numbers on top of carb and bottom of bowl are same as what I got on my 20hp carb.thanks
Click to expand...

Since I had some time to spare, I took the time to go through my OMC 1985 20hp/25hp parts book to compare exactly what is different between the 20hp model and the 25hp model.

The cylinder head, piston, rings, connecting rod, crankshaft, and the crankcase (block) itself are identical.
The flywheel and ignition system are identical.
The long exhaust housing and tubing tube are identical.
The lower unit and its internal components are identical.
The carburetor assembly, INCLUDING the #49D (part #324020) high speed jet (NOT a #47) are identical.

Etc, so forth, and so on...... are identical....... EXCEPTING the "Intake Manifold"

Intake manifold #395691 = 20hp
Intake manifold #395698 = 25hp

Meaning.... to upgrade the 1985 20hp to a 25hp model... simply change the intake manifold. There would be no need to change anything else UNLESS a propeller change would be needed to obtain the proper WOT recommended rpm.

This suggests that the venturi cast opening would be smaller on the 20hp intake manifold to create a fuel restrictive fuel flow from the wider venturi of the dual purpose carburetor (actually rated at 25hp) to drop the hp rating to 20hp. This is similar to a practice used by taxi companies (to name just one), that use a restriction plate between the carburetor and intake manifold to limit the speed range.

Bottom line..... the carburetor purchase was a waste of money.... and check the purchased intake manifold to make sure the venturi hole is larger than your old 20hp one.
 
joereeves said:
Since I had some time to spare, I took the time to go through my OMC 1985 20hp/25hp parts book to compare exactly what is different between the 20hp model and the 25hp model.

The cylinder head, piston, rings, connecting rod, crankshaft, and the crankcase (block) itself are identical.
The flywheel and ignition system are identical.
The long exhaust housing and tubing tube are identical.
The lower unit and its internal components are identical.
The carburetor assembly, INCLUDING the #49D (part #324020) high speed jet (NOT a #47) are identical.

Etc, so forth, and so on...... are identical....... EXCEPTING the "Intake Manifold"

Intake manifold #395691 = 20hp
Intake manifold #395698 = 25hp

Meaning.... to upgrade the 1985 20hp to a 25hp model... simply change the intake manifold. There would be no need to change anything else UNLESS a propeller change would be needed to obtain the proper WOT recommended rpm.

This suggests that the venturi cast opening would be smaller on the 20hp intake manifold to create a fuel restrictive fuel flow from the wider venturi of the dual purpose carburetor (actually rated at 25hp) to drop the hp rating to 20hp. This is similar to a practice used by taxi companies (to name just one), that use a restriction plate between the carburetor and intake manifold to limit the speed range.

Bottom line..... the carburetor purchase was a waste of money.... and check the purchased intake manifold to make sure the venturi hole is larger than your old 20hp one.
Click to expand...
Thanks-I did try getting info like you just gave me before purchase and had not any come back like yours.I knew the intake I got was needed so that is ok.Am writing to seller of carb to to return it. thanks again.
 
(NOTE: I'm aware that later in years, the carburetor part number along with the intake manifold(s) were superseded but avoided including that initially as it would drive a person nuts. Listing it here as an after thought just as a info thing. The bottom line listed above still holds true.)
********************
Carburetor Assembly: 395279 = 20hp 433358.... 25hp 433357 (Both N/A in 1997).
Intake Manifold: 395691 20hp & 395698 25hp = Both supersede to 396788.
********************
 
racer should know better but I did a search also and for the 1985 20 and 25hp had the same size jet but the only difference I could find was the manifold part number. Are you just trying to get a few more horses out of that motor or were you having fuel issues to begin with? Did you pull the jet out of your new carb and see what size it is? You should do a good rebuild before you install the carb anyways.
 
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