Compression testing partially assembled engine

[HotTommy]

I am putting my 1993 Force 150 outboard back together after replacing the pistons. It is off the boat. I'd like to check the compression by hot wiring the starter to a battery to turn it over. I understand that I need to ground the spark plug wires to do this test on the boat using the key because it engages the ignition system. If I hot wire the starter only, do I still need to ground the plug wires? I'm uncertain because the spinning flywheel will generate current with the rotor and stator. .. Anyone know the for sure answer?

 

[pnwboat]

Depends on what type of ignition system you have.

If you have the Prestolite ignition system, the stator is used to generate the ignition system voltage. You can engage the ignition Kill circuit by grounding the white wites on the CD modules. You can also temporarily disconnect the stator leads and tape them up.

If you have the OEM Mercury style Thunderbolt ignition system that uses a Switch Box, the stator is only used to generate voltage to charge the battery. You can tell because the stator only has two wires the go to the Rectifier. The ignition system voltage is provided by the battery voltage to the Converter box, not the stator. As long as you don't provide battery voltage to the Converter box, you won't have any spark.

If by chance you have the after market CDI stator that has the stator the provides the ignition voltage and eliminates the Converter box, ( you can tell because the stator has 4 leads. Two go to the rectifier, and two go to the Switch Box) then again, engage the ignition Kill circuit by grounding the Black/Yellow wire on the Switch Box or temporarily disconnect the stator leads that go to the Switch Box and tape them up.

 

[HotTommy]

Wonderful answer. I have the third type. Thanks.

 

[pnwboat]

Then you're good to go. Good luck with your project!

 

[Jiggz]

Don't forget to spray fuel oil mix to lube the rings and cylinders especially for new rings.

 

[tommarvin]

You could do a leak down test.
HF has the kit for around 40 bucks,you need compressed air only.that's all you need. You never turn the key, it's very fast, do three and average them.
For a compression test do you have to have water flow to the muffs, a wide open throttle, all spark plug boots grounded to block, all plugs out, is there anything else you have to do?

 

[jerryjerry05]

No you don't need to ground the plugs if your only energizing the starter.
​
You don't need, water hooked up, or
the throttle open all the way.(try both ways)

The only thing you need to do is remove the plugs.

If your not using the key, your not energizing the ignition system.

You more than likely have the prestolite system.
They started with Mercury stuff in the later models of 94