Newsletter of the Finger Lakes Chapter, ACBS

Volume 17, Issue 1.................................................................. March 2007

UPDATED: March 13, 2007


















Troubleshooting an Outboard Motor By FLC member Dick Newco

This is the first in a series of restoration articles that Dick Newcomb has agreed to prepare for Brightwork. -Ed.

Compression tester (left), and spark checker


Suppose you've acquired a classic outboard motor, or you are looking for one to buy. What should you check to find out if you can restore it for your boat or collection?

The first step should be the overall condition. Are all the parts there? If not, for example, buying shrouds, cowels or trim parts for the motor may wind up costing more than the motor, if you can find them. Look for broken cavitation plates, cracks in the gear foot, swapped pieces that don't match. These are sure signs of trouble. It is best to pass on motors like that unless they are very rare.

Let's say the motor passes the 'looks' test, what next? Most people will try to pull the starter rope to see if it turns over. That's a good idea but not necessarily the sign of a "frozen" powerhead. In my experience 90% plus of these "set up" motors really weren't hopelessly rusted tight, especially Evinrudes and Johnsons. The ones I've worked on typically seize up in the gear foot where the driveshaft goes through a bronze bushing to the pinion gear. A tiny amount of rust on that driveshaft is all it takes to lock up the motor and make it seem frozen, as if the pistons were melted to the cylinder walls. This is very easy to fix. Mercurys have a complex shifting connection that often gets out of sync, that, too, can make the motor seem frozen.

Once you get your prize home, it all boils down to spark, compression and gas. There are two essential tools I'd recommend you invest in if you are going to do any messing around with engines, be they outboards or inboards: a compression tester, and a spark tester (see picture). My compression tester came from Harbor Freight Tools and cost about $15. My spark checker was ordered through Sienna Marine and was $45, well worth it.

The MOST important test is spark! If you don't have spark, no amount of gas or great compression will matter; your motor will not run, period. They will run on bad compression, and fairly well on dirty carburetors.

To test spark, remove the spark plugs -- all of them so you don't have to fight the compression. Attach the plug wires to your spark checker and find a good ground to clip it to. Position it so you can easily see it as you pull the rope or crank, if it is an electric start. You should see a nice blue spark jumping between the points on the spark checker. Let's say you are only getting one to fire (out of two on a two cylinder motor, for example). There are several possibilities, but the most likely causes are fouled or dirty contact points or a bad coil. In future articles I'll describe how to check, clean and/or replace these.

If you've got fire (spark), move on to compression.

Since you already have the spark plugs out you can check compression easily. Squirt a little WD40 or oil into each cylinder first. This lubricates the walls and helps the rings seal. Carefully screw your compression checker into the spark plug hole until it is snug. You don't need to tighten it too much, in fact, just snug it up. Now, pull the starter rope three or four times or crank the electric starter for about 3-4 seconds. Read your gauge. Great compression on a typical outboard is 125+ lbs.; good is 100+; trouble is lower than 90. New rings and possibly pistons are a pretty major overhaul. Sometimes it's easier to get a good replacement powerhead.

Last is gas. You can get a fair idea of weather your motor will run (if it has passed the spark test and the compression is good) by spraying a small amount of starter fluid into the carburetor Turn the throttle all the way open before you spray and do not put the choke on. Then pull the rope (or crank). It should fire for a short time -- even a brief pop will tell you that it should run.

I'll go into cleaning and rebuilding carburetors in future issues.