Tune up 4 cylinder mercruiser engine

can you provide me with the orderly steps to replace plugs, wires, distributor cap, rotor and condenser on a 4 cylinder boat engine. working space is good.

You need a instructor standing next to you when you do these things mainly to inspect your work. Would you want to be out on the lake and you engine not start? (I guess thats why you must carry oars).

Really, first timer on a boat motor, not good.

Really, the steps are no different than a car with points just the consequences of a mistake are greater.

First and foremost: get a service manual designed for that motor. It will have all the torque specs and other necessary info.

A friendly word of advice: Buy and use OEM only. I tried using aftermarket on my 150hp Yamaha outboard and had nothing but trouble.

When replacing plugs beware of two things: they must be gapped correctly and be of the correct type.

Replace plug wires ONE at a TIME to avoid mixing them up. They are different lengths so compare before installing.

Note: Plug wire replacement should be done using a wire boot puller to avoid damaging the wire. I recommend replacing the wires every two years.
A lot of people think this is overkill, but that’s just me.

I’ve done this for each boat/motor (3) I’ve had and (aside from that one time using aftermarket) have never broken down.

Distributor cap, rotor and condenser will also be shown in the manual. The cap must be identical to the one coming off and replaced accordingly.
The rotor has to be replaced and adjusted for proper spark gap. The book explains this.

This is also a good time to check out the engine for oil and water leaks, throttle linkage, drive belts, etc…
Don’t forget that battery and the cable connections that’s hidden away. Good, clean and wrench tight always.

I got into a habit of keeping some spare parts as well as tools (to which I tie a (retrieving) string to). Yeah, I’ve lost my share of tools over the side too.

Keep a magnet handy to retrieve dropped tools and bolts/nuts from the bilge.

It’s much wiser to do this work on dry land for the obvious reasons.

Hope that helps and have a SAFE and a fun summer boating.

This is all relatively easy stuff. I would snap off the distibutor and change the rotor and condenser first. Then snap on the new cap and put on the old plug wires in their proper places on the new cap.

Fire up the engine and see if it is running as well or better than before you replaced these items. If all is OK move on.

Replace the plugs one at a time and place the old wire back on each plug one at a time. When all 4 plugs are done fire up the motor to see how it runs. If all is OK then go onto the wires.

The new wires maybe different lengths. Sort them out longest to shortest. The coil wire should be obvious. Take off the longest old wire and compare the length to the new wires and put on the appropriate new wire, then the next, until all 4 wires are on and the new coil wire is on as well. Fire up the motor and it should be running just fine. You are done. This method will take longer but you will never have more than 1 wire off at anytime so you’ll not get confused.

Before you tackle this locate the firing order stamped on a manifold somewhere in case you need it. Marine motors sometimes run backward so you need to determine the rotation direction of the distributor if you have to redo the wires according to the firing order.

You didn’t mention points, so I don’t know if they are part of the tune up. If you change the points you’ll need a dwell meter and timing light to properly set the ignition timing and you’ll need the spec on how many degrees before TDC is the correct setting.

Make sure you buy marine grade parts for this job. Auto spec parts do not tolerate moisture well enough to be dependable in a boat motor application.

Thanks for your feedback shoreview

If you have the time could you explain rotor adjustment, I have never been called upon to do this, Thanks.

Is this electronic ignition??? Many boats still use points.

If points…it’s a little bit more tricky.

Whoops! My bad.

I should have said ‘points’.

If this engine uses points they are fairly easy to set.

After replacing the points, don’t tighten the set screw so tight that you cannot move them (just snug).

With the screwdriver, and a spark gap gauge handy, (years ago I used the front cover from a book of matches) turn the engine over (by hand) until the plastic tab rests against the high step of the distributor cam.

With the gauge between the points, move the point set in or out until you feel a little resistance when you try to remove the gauge.

Tighten the set screw.

If you don’t have the proper point gap spec it will be a crap shoot to get it set correctly. Maybe use the matchbook for a start.