Compression test on an older meccury 9.8 hp 110 outboard

[tacx]

I am going to do a cylinder compression test on my 1970 110 merc outboard. Is there any concern regarding the ignition system when doing the test. I read somewhere that the plug wires need to be grounded when cranking ( by pull cord ) to prevent issues with the ignition system?

Or is it OK to just pull both plugs, leave the plug wires hanging, install the compression tester and just pull away. I certainly do not want to screw up the stator or coils.

Thanks
Tom

 

[ThomW]

Does it have a kill switch or kill lanyard? If so, pull the lanyard or kill switch and test away. With the kill switch or lanyard pulled there will be no spark, and no damage to the ignition parts. If no kill switch, pull spark plugs, leave them in boots and run a wire from each spark plug to ground them to the block.

 

[merc850]

Any spark plug should be grounded, when it's connected to a coil, for testing or checking for spark; the voltage must make a complete circuit or it builds up and finds another way to ground, usually thru the coil insulation.

 

[flyingscott]

Do not use the kill switch on that motor. Make sure the plugs are grounded have you done a spark test yet.

 

[tacx]

No spark test yet. Headed up to northern Michigan where the motor is. Plan on doing a spark test and then a compression test. The motor has a kill button and I understand from these forums that it is normally grounded and breaks the circuit when pushed, unlike most kill buttons. As of now it does not stap the engine. I have been using the choke. Will also check out the kill button when I get up there.

 

[tacx]

UPDATE,

​​​​​​1. Checked kill switch. Its bad. Internal contacts sometimes break sometimes dont. Eliminated kill switch and grounded stator wire.

2. Checked compression. Both cylinders around 113 psi. So thats good.

The engine had NGK BUHW-2 plugs in it when I got it. ( these are the recommended plugs)
So I replaced them with new NGK BUHW-2 plugs. Engine was still hard starting and if it did, it ran terrible. Could not keep it going in low speed.

I gave up and took it to a old timer who works on boat engines. He noticed that the coils were replacement coils and not original merc coils. He yanked the NGK BUHW-2 plugs, which by the way are "surface gap" plugs, and he install a set of Champion QL77JC4 plugs ( which are standard electrode plugs

pulled the start cord and Walla! The engine ticked like fine swiss watch. Low speed, high, speed and has great throttle response. I'M A HAPPY CAMPER

I believe the issue had to do with the replacement coils not liking the surface gap plugs.

Tom

 

[tacx]

Update