Compression Test: Damage Coils on Small 2-cylinder Johnson/Evinrudes?

[cprodave]

When testing with compression gauge on small (15HP or less) 2 cylinder older OMC's with Magneto Ignition do(es) the Spark Plug Wire(s) need to be grounded to avoid damaging the Coil/other? If I have only 1 Compression Gauge, what is the best method for Compression Testing (other Spark Plug installed but with Spark Plug Wire disconnected vs.disconnected from Spark Plug but grounded, etc)?

Also, what is best Compression Testing method on the newer Electronic Ignition (around 1977 etc) outboards?

 

[Crosbyman]

In order to protect the coil's secondary high voltage inner windings it is best to leave the plugs in their boots and ground them both to the engine block . I use very large clips on the plugs both connected to a comnon wire terminated with another large alligator clip . this single clip is grounded to a bolt head or any engine block part it will clip on.

this will direct the HV spikes to ground otherwise they may internally puncture the thin layer of varnish used as an insulator on the secondary thin wire and then jump to the coil laminates.

HV will always seek the easiest path to ground so just give it one with the fat grounding clips on the plugs

this will leave both cyl to open air and you can alternately measure the compression on each cyl. knowing the HV spikes will safely jump the plug gaps to frame ground

 

[cprodave]

Thanks Crosbyman, I will conduct my tests using your method going forward.

 

[F_R]

I agree with what crosby just said, with one exception. I prefer to ground the wires to the block instead of using the plugs. A set of wires with alligator clips on both ends will do it.

Why?? Well because sparking in the vicinity of the open plug holes can start a fire. How do I know? Because I almost caught myself on fire. And/or the shop. I did what crosby suggested and hit the starter. Turns out the motor was flooded from previous starting attempts and gas shot out of the plug holes and the sparking ignited it. Flames shot out about ten feet. Thank God, I was standing to one side of the inferno.

 

[geoffwga1]

I know it's not funny and could have been serious but I would love to have seen that.

 

[thatone123]

Wow, Fifty years of outboards and I have never bothered to do anything for a compression test except take the plugs out and let the wire and rubber boot just hang there. Never had a problem but you never know I guess. Soon many of us will be required to wear a life preserver in the shower I guess. But the above may be a good idea, except I'm always in a hurry.