Newsletter of the Finger Lakes Chapter, ACBS

Volume 18, Issue 1 Newsletter of the Finger Lakes Chapter, ACBS March 2008

UPDATED March 18th 2008 - page 13


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Outboard Motors Trouble Shooting the Lower Unit By FLC member Dick Newcomb

The two most common problems with the lower unit (not including a damaged prop) are the water pump and the gear case. We'll cover the water pump first.

Water pumps on most outboards built after the 1940s, are housings containing an impeller that is connected to the motor's driveshaft. As the shaft turns, it spins the impeller (looks like a fan) which creates the suction to draw water in and force it up a copper tube to the power head where it passes through interconnected cooling cavities surrounding the motor's cylinders.

As the water circulates, it removes heat from the combustion chambers, and then it is exhausted (underwater in modern motors). As time passes, impellers, which are generally made of a rubber material, get stiff and worn. They gradually lose suction or some of the blades may break off. Now, the motor will overheat.

Because the impeller is enclosed in the top of the gear case, to service it, you need to drop the gear case. That is, you need to unbolt it and slide it down from the housing. In Johnson and Evinrude engines this is accomplished by first disconnecting the gearshift rod inside the housing (tower). There is a small oval plate about half way up the side. Remove the two small screws holding it on.

The shift rod is connected by way of a brass coupler with two bolts. Be very careful loosening and especially retightening these bolts. The coupler is brass and you can easily strip the threads. You really don't want to buy a new one of these; they are expensive. Disconnect the TOP one. You don't need to remove the bottom bolt. Leave the coupler connected to the rod going to the gear case.

Once you've disconnected the shift rod and the bolts holding the gear case on, you can pull the lower unit off. It may take a bit of a tug since the copper tube is usually in a rubber gasket. Note: If you are working on anything bigger than 40 hp, you may need two people; they are heavy and awkward. You'll see the water pump housing on top of the gear case with the driveshaft running through it. Remove the screws and lift off the top (Mercury outboards require a 'special tool' for this procedure. I have these tools if you need help.) Slide the impeller up off the shaft. There is a small pin that holds it on the shaft. Be careful this doesn't fall out and get lost. Note the blades on the impeller: Have they taken a set (bend) leaning backwards in the opposite direction in which the impellor turns? Examine the water pump housing. If water has frozen in the lower unit (a common problem), it may be cracked or warped. In that event you'll need a new one. Let's hope that didn't happen, and that all you need to do is replace the impeller.

Slip the new impellor on the shaft put the notch in it over the pin. While you are doing that, turn the shaft clockwise. This will cause the fins to seat (bend) properly in the housing. Put the top of the water pump back on and remount the gear case. It may take some wiggling to get the copper tube back up into the power head, but be patient. Reconnect the shift coupler, remembering not to over tighten the bolt. There is a notch in the shift rod that should match up with the bolt shaft as it is inserted. Make sure they line up before you tighten the bolt.

Water test your motor to see if it is pumping water well. This all sounds like a difficult job but it really isn't too bad. However, there is some BAD news -- Mercurys are a lot harder to work on, and many motors don't have that troublesome but handy shift coupler. Some require the removal of the power head from the motor to drop the gear case, a major project. In those cases you may want to work with someone who has worked on those before, or pay the Marina. Ouch!

Evinrude warns that running a motor out of the water even for a few seconds may destroy the impeller. This is a little extreme but good advice. Always test motors in a barrel of water, submerging the lower unit to a depth equal to what it would be on the transom of a boat. On more modern outboards, special 'earmuffs' can be purchased to use with a garden hose, but I don't trust them, and I rarely work on motors that new anyway.

Here are some suggestions when working on any part of your outboard: Have a container like a plastic food storage container or small cardboard box handy to put all your screws, etc. in. Be rigorous about saving parts and labeling them, or better yet, take lots of pictures as you go along. Invest in one of the many repair manual guides available. Or the very best idea -- work with someone who has done this before.

Next Time: Servicing the gear case, OR: "This motor is seized up!" Usually, this is NOT what's wrong.