The service bay at Norfolk Marine is headed up by Yamaha Master Tech Chris Breeden, and he shows us exactly what goes into the 100 hour service of a two-stroke outboard engine.


Lenny: The 100 hour service on a two-stroke outboard is probably not something you're going to try at home. But you want to know what's going on at the dealership when they do that work, right? We're here at Norfolk Marine today, and Yamaha Master Tech Chris Breeden is going to show us step number one, of the 100 hour maintenance interval.

Chris: The first thing you're going to do is drain the lower unit oil, by taking out the lower drain screw, and then you release the upper drain screw. Then you would tilt the engine down.

(See our How to Change the Lower Unit Oil on a Boat video).

Okay, the second step is going to be removing the spark plug boots. Then we're going to break the spark plugs loose and remove them for the compression check. When these plugs are out, now's the time to check them for how they're burning. You want the plug to be as white as possible on the center electrode. This one's in pretty good condition, this one here is starting to foul. One is pretty typical of a 100 hour service, and the other is running a little rich.

Now we take the compression gauge, put it in the number-one cylinder. Make sure the safety lanyard is disconnected, so you don't make any spark as you do the compression check. Now we're ready to crank the engine over.

We have 125 on that cylinder, and we have about 130 on this one.

Lenny: So Chris, one was 125 and one was 130, and I know you're looking for those numbers to be close together, right?

Chris: That's correct, as long as they're within 15 percent, that's acceptable.

OK, now we install the new spark plugs. And, let's re-fill the lower unit. Lenny, could i get you to trim this down level so we can get the correct amount of oil in? Thank you. Now, you just pump it up until oil comes out of the top hole. You also want to clear the tip of the magnet of any metal debris before putting the screw back in.

And now, we're going to grease the zerk fittings. Then we're going to remove the prop, inspect it for any fishing line, and grease it.

Lenny: If anyone has any questions about how to do this, we do have a How to Change a Boat Propeller video in the video library. Now, why would you be concerned if you saw fishing line wrapped around there?

Chris: Fishing line can cut into the lower unit seal and cause the seal to fail, letting water into the lower unit.

Lenny: And that could cause catastrophic damage.

Chris: Definitely. We also have a zinc in bad shape to replace, and we need to change the fuel-water separator in the boat.