Checking compression without elec harness attatched....bad idea?

[woodsyfeller]

I want to check the compression on 80-81 mercury 140 straight 6 that has the thuderbolt ignition system. I was wondering is there a chance I would damage any of the elec/igniton system if I spin the flywheel to check for compression without the wiring harness attached to the motor.Should I hook up the wiring harness and battery? The engine is not currently on a boat but on a stand.If I should hook up the harness and battery would it be a big deal to use the starter to turn over the engine without it being in water or will that damage the impeller even though the motor isnt going to actually be started just turned over a few times for compression readlings.

Should I spray some fogging oil or some other product into the pistons when doing this to help lube the piston walls ect?

 

[CharlieB]

Re: Checking compression without elec harness attatched....bad idea?

Distributor ignitions are OK, it is the non-distributor ignitions that will fire without the wiring harness as they are essentially a magneto-style, their switchbox have a black wire with a thin yellow trace that must be grounded to kill spark, or all the plug wires must be grounded.

Pull all plugs, ground all the plug wires. This will protect the ignition from damage and ease the strain on the starter and battery.

Crank speeds just for testing will not hurt an impeller. They are easily damaged from friction of running speeds without the lube and cooling of the water.

 

[cyber]

Re: Checking compression without elec harness attatched....bad idea?

I'm interested in doing this as well.
How do you 'ground' all the plug wires.

 

[Chris1956]

Re: Checking compression without elec harness attatched....bad idea?

That 140 should be a distributor ign motor. If so, remove spark plugs and just use some jumper cables from a battery to the starter and ground. On ADI models, remove spark plugs, reinstall them into their wires and tape the bases together. Now ground the bases to the block.