Compression Test without a battery?

[djmeaney]

I have read the FAQ on compression testing. I would like to know if, or how, you can compression test a larger motor 75 - 125 HP without a battery.

I am considering a couple of boats and neither has battery for the motor.

Can I use jumper cable from my car to crank an outboard without damage?

Do I need to buy a marine battery?

Does it matter if the motor is cold to get a good idea of compression?

Don't think the motor has been run for a while and its going on a boat that it has never been mounted so controls, etc. are not attached -- would this keep me from being able to test it?

Thanks,

Dan

 

[Haffiman]

Re: Compression Test without a battery?

You may use any batter (12V) for cranking as long as it holds the needed cranking Amp's.
Be careful with a 'dry' motor that has been sitting for some time. Bring along a spray can of fogging oil (or two), open throttle plates on carbs, fill up with fogging oil, turn flywheel by hand and notice if engine turns freely.
Then you may perform a comp test.
If no water for impeller available, be prepared for that to be ruined!!

 

[F_R]

Re: Compression Test without a battery?

Depends on the motor. Some can be seriously damaged by cranking without a proper and secure electrical hookup.

 

[sschefer]

Re: Compression Test without a battery?

Depends on the motor. Some can be seriously damaged by cranking without a proper and secure electrical hookup.

Not sure of the setup on the boat you're testing but if it's just lacking a battery, why not buy a good one, charge it and take it with your for the test. Just hook it up and test away, you're going to need a good battery anyway.

The engine should be tested warm and will give the best test when warm but you're probably going to read anywhere from 5 to 10 % higher after fogging.

Besides the impeller issue, if this motor does not have controls and your start it you won't be able to shut it off so make sure you know where the ignition ground is and put a toggle switch in so that it acts like the key (open = run, closed = stop). If you have a set of controls you can just hook them up temprarilly with the harness connector and a normal key switch.

 

[djmeaney]

Re: Compression Test without a battery?

Not sure of the setup on the boat you're testing but if it's just lacking a battery, why not buy a good one, charge it and take it with your for the test. Just hook it up and test away, you're going to need a good battery anyway.

The engine should be tested warm and will give the best test when warm but you're probably going to read anywhere from 5 to 10 % higher after fogging.

Besides the impeller issue, if this motor does not have controls and your start it you won't be able to shut it off so make sure you know where the ignition ground is and put a toggle switch in so that it acts like the key (open = run, closed = stop). If you have a set of controls you can just hook them up temprarilly with the harness connector and a normal key switch.

The motor is currently off the boat, but it does have the controls. I guess it's going to have to back on the boat if I buy it -- so it should be a big deal to temporarily hook everything and place it in a barrel. I need to buy a deep cycle marine battery for a battery back up sump pump anyway so it won't go to waste if I don't buy the motor.

Thanks.