Compression/spark testing 2stroke

[Fab1]

Hi guys,I was teaching my nephew how to do a compression and spark test using my outboard to teach him and thought if document it for future reference for anyone in the market for buying a boat.

You need a compression tester,spark tester and a means of removing the plugs as I have here.On some engines you can use something as simple as a shifter in a pinch to remove the plugs.

We'll start with the compression test first.

You want to isolate all spark/fuel to the engine for this test.

Here I'm going to be isolating the ignition system to prevent firing at the coils that can damage them by removing the kills witch and leaving the key in the off positioln.In my case I have a carbureted engine and I want the throttle plate open wide open and have the throttle wide open(make sure someone spins the prop as you do this)Now all those precautions are taken it's time to remove the plugs to install the compression gauge.If you have gunk around the spark plug holes now is a good time to wipe it away to prevent debris from falling into cylinder.

Here I've removed the spark plug boots by firmly grasping the boot while twisting and pulling at the same time..Here I've removed the spark plugs and an screwing the compression gauge into the cylinder by hand using the correctly sized adapter for my plugs until the o ring is seated snug into the spark plugs hole being careful not to cross thread or overtighten.

Adapters showing the o rings to prevent leakage.

Here the gauge is all connected and zeroed to take a reading of the cylinders compression.

Now what you want to do is pull on the starter rope a few times or hit the key for a few seconds until the reading on the gauge no longer moves up which usually takes 4-5 compression strokes.

This is the reading on my top cylinder of around 140psi.

This is the reading on my lower cylinder of around 140psi.

I don't like to see a variation of more than 7-10% between cylinders in any engine.

Engines are all about balance the closer you can get those compression numbers between cylinders the better balanced,smoother an engine will be.

Most engines I've seen will run with around 90-160psi of compression(high compression engines or ones with large amounts of carbon deposits will have more)but what is more important is that the variation between cylinders is as close as possible to each other.

In other words it's no good having 60psi in one cylinder and 150psi in another.

Now how to do a quick check for spark.

Again isolate the fuel but this time you want your kill switch,fuses etc connected.

All you need for this test is to disconnect cylinder #1's spark plug boot and connect the alligator clip on the tester to a ground(in this case a bolt on the cylinder head)and the tip of the tester into the terminal inside the boot to complete the circuit,then i have a second jumper wire running from cylinder #2's boot terminal and gronded to the same cylinder head bolt i used on cylinder #1 to prevent possible damage to the coil packs when cranking as I've done here.

Now all you need to do is pull the starter rope or hit the key and every time the coils fire you'll see the red light in this case flash indicating you have spark as in the photo.

And that's just one way of quickly determining if an engine has compression and spark.

This method can be used on everything from outboards,cars,small engines such as mowers etc with variations in how you isolate the systems prior to testing.

Hopefully this will help someone somewhere down the road looking at maybe purchasing a used outboard or diagnosing there own one when having issues.

Cheers.

Edited September 27, 2015 by Fab1