STEP

ACTION

TIP

1

Place outboard shift lever on the power head in neutral.
 

* After removing the lower assembly, DO NOT operate the shift lever on the power head or the rod on the lower assembly.  This keeps the shift mechanism in sync with the gears when reinstalling the lower assembly.  See step 11.
 

2

Remove the nuts that hold the lower assembly (gear case & propeller) to the power head.
 

The lower assembly of a small outboard will usually separate now.

3

NOTE - For an outboard that has been converted to a long shaft, you may have to separate the lower assembly ~1/2" then jiggle the shift lever to forward to expose the shift lever screw.  This connects the upper portion of the shift lever to the lower portion.
 

NOTE - Having removed the screw through the gap you should be able to pull the lower assembly away from the power head. 
* If you have an outboard that was converted to long shaft, look for a screw above the extension that was inserted in the leg. 

4

Remove the nuts that hold the top half of the water pump housing.  Remove the cap and note the SS disk against the top of the impeller that seals the top of the pump.  Remove the disk and note the number & condition of the impeller vanes and in which direction they point. 
 

Place the impeller on the work bench with the vanes pointing in the same direction when it was in the pump housing.  No point getting confused!
* Replace the impeller if the vanes are hard, cracked or missing.  You MUST install the new impeller with the vanes pointed in the same direction.
 

5

With the impeller removed, inspect the inside of the pump housing; walls, top and bottom sections.
 

If the inside surfaces are smooth then the pump is OK to return to service.  If they are rough, you should replace the pump.  A rough surface can quickly wear out an impeller.  Roughness is an indication of having sucked up sand in shallow water. 
* Outboard water pumps are displacement type that depend on the seal of the vane tips and the sides to move water.  If you run this pump dry for only one turn you will likely tear off the fine side seals, rendering it useless.
 

6

If even one vane is missing from the rubber impeller count on it being inside the block.  It must be removed so water flow is not restricted.
 

Direct full pressure tap water into the engine flushing port, letting water flow at full volume.  Use a fine tipped nozzle on the end of a garden hose to seal it to the block.  This back flushes the cooling channels and should remove any clumps of rubber.  On my Merc 75 it spit rubber particles out on the driveway almost immediately.  If you can't spot them and notice that the water flows more freely, consider it fixed. 
 

7

Install the key pin that locks the impeller to the drive shaft.  Ensure the key pin is squarely seated on the drive shaft so it can slide into the slot of the impeller.
 

I use a dab of grease to hold the key pin in place.

 

8

Install the new impeller by rotating it into the water pump housing in the same direction it turns when the outboard runs.  Don't tear or cut the rubber on the sharp corners of the pump housing during installation.
 

* If you have difficulty compressing the vanes, tighten a hose clamp around them.  Hold the vane over the pump base and nudge the impeller in.  This is similar to compressing piston rings during an engine rebuild.
* If you install the vanes pointing backwards they are likely to break when the outboard starts.  Dang!
 

9

Place the SS disk on top of the impeller, being careful to align it the key hole that prevents rotation in the housing.  Lower the pump housing cap over the base and tighten the nuts. 
 

With some outboards is it important to torque these nuts to specs to ensure the pump doesn't leak.  A leaking pump can't deliver all the water to the block.
 

10

Install the lower leg assembly to the power head.  Tighten the nuts you removed in step 1. 
 

It is fairly easy to align the propeller shaft and shifter rod to the power head.
* For some obscure reason I have to invert my Merc 75 outboard to align the propeller shaft, shift rod and water delivery tube.  Upside down works most every time.

On my Merc, and I presume others, it is difficult to align the 1/4" water delivery tube into the pump outlet after the propeller shaft and shift rod is engaged.
* To solve this I inserted a 2" long vinyl tube to the top of the pump to guide the water delivery tube in while I can still touch it with the bottom section slightly out.  (Pretend the pencil is my water delivery tube).
Confirm all three are installed correctly.  Don't guess.
 

11

Verify that the propeller turns freely with the gear shift lever in neutral.  Likewise, it should rotate in the correct direction when in forward or reverse.
- Now is also a good time to check the propeller nut.  That can be expensive as it sinks to the bottom.
 

* If the propeller doesn't turn in neutral, the gear shift lever and the lower unit are out of sync with respect to each other.  Remove the lower unit, rotate the shift rod till the propeller turns freely (neutral), then reassemble with the gear shift lever in neutral.
 

12

Start the outboard with the leg in the water at least as deep as the cavitation plate.  After the outboard starts you should immediately see a strong stream of water leaving the outlet nozzle.  Verify that the propeller turns freely in neutral and has thrust in forward and reverse.
 

* It is absolutely essential that the lower leg be immersed in water for impeller lubrication and cooling but cooking oil or white grease can protect the vanes for the first few turns of the outboard. 
* If there is no stream of water, I hate to say this but you "screwed up."  Not nice to say but either the water delivery tube is not inside the water pump (step 10) or the key pin is not seated correctly (step 7).  I learned this lesson the first time I replaced an impeller.