Re: Do I have the right pump?

[johnyrude200]

Wish I saw this thread sooner. Let me offer some help since I work on around 50 of these units a year (18/20/25hp pre 1977).

Your gearcase is not a newer style gearcase. The prop shaft doesn’t have splines on it. In 1977, it switches to a splined prop shaft (correct me if I’m wrong). As mentioned, check your exhaust housing to see if there is an orifice to accomodate the driveshaft ‘spacer.’ These GREATLY improve water pumping action because they help shield the impeller housing from exhaust gases getting in.

These motors pump a ton of water, even prior to this update around 1970. You can even run one of those motors that are updated to take the driveshaft spacer without one and they still pump a lot of water, but I wouldn’t recommend it. If the impeller gets weaker, there is a much smaller margin of error. FrankR may be able to clarify, but I believe around 1973, a water restrictor was added under the powerhead on top of the exhaust housing to slow down water supply. They can run cold (and are not as good idlers) the closer to 1976 that you get.

As mentioned, you may have an odd combination impeller housing to the exhaust housing. You just have to look and see. If you have a motor with an exhaust housing indicating it should have a driveshaft ‘spacer,’ than get the correct impeller housing.

In terms of the 4 holes in the gearcase, all of these gearcases are drilled for 4 impeller housing screws, but may only accept 3. You’ll see the SS impeller plate only accomodates 3 holes too. The size of the screws changed from really skinny, fine thread to thicker standard thread at some point. I don’t remember when. I all I know is the older skinny, fine thread screws SUCK. They almost always get stuck, break, or the heads pop off. Trying to drill them out and retap, forget it. Just go 1 size bigger and update to better screws.

Now identifying what motor you have isn’t going to be easy if folks have been combining odds/ends motors. Keep in mind the difference between the 18/20 and a 25 is really the cylinder head (clearly different if you pull them), and the crank, manifold, rods, and internal porting of the block. Other than the cylinder head, these others things aren’t something you can casually identify. I guess if you pull the flywheel to service the magneto, you might get lucky and still have a P/N stamped on the taper. But that is rare. Your motor has a newer style carburetor, which I suspect is a 25hp carburetor. The older 18/20 carbs were used (I believe) up to 1970.

Here is a pic of the cylinder head differences on my website: http://runneroutboards.com/motor101/powerhead/powerhead.html#18-20_Differences

You can go by block color to a certain extent; keep in mind in the early 70’s johnson kept the 20hp line, evinrude the 18hp line. But both carried 25hp motors? Disregard identifying based on anti-ventilation plate or shift levers. If you don’t have the ID tag, or have a core plug, it’s likely best guess. You’ll know once you go full throttle because there is a marked difference between an 18/20 and a 25 at the top end.

For what it’s worth I have 3 or 4 of these frankenmotors waiting to be fixed this winter where I’m going to have to figure out what they are using the same detective work. And the only difference between a 18 and 20 is the leaf plates. It really gets fun when some knucklehead decides to try and convert an 18/20 to a 25 (Can’t be done!) and starts strapping components of a dead 25 to an 18/20. When you see your compression gauge read 185-200PSI, that means they put the wrong cylinder head on a 18/20! I wouldn’t want to be around that motor when they run it!