compression test

[chilly1]

Can I run my outboard long enough for a compression check without the lower unit?

 

[Mark42]

Re: compression test

You don't actually run the motor for a compression test. The motor just needs to be cranked over by the starter. So it is OK to spin the motor without the lower unit on.

If the lower unit is connected, you should have a set of water muffs on to keep the impeller lubed up while the compression test is done.

 

[gregmsr]

Re: compression test

I never heard of running the engine for a compression test.

1: Remove all spark plugs and ground plug wires to the block (or simply remove the safety lanyard to kill the ignition)

2: Insert compression tester into plug hole

3: Crank starter for about 4/5 seconds and take the reading

Do this for each cylinder. All readings should be within 10/15 psi of each other and above 100psi.

What type/model outboard do you have?

Should not be a problem doing this without the lower unit installed.

Buy yourself a repair manual for your particular model outboard.

 

[chilly1]

Re: compression test

Oh. I read that a proper test is done on a warm engine. Can I still get a good reading cold?

1971 Chrysler 55 hp...waiting on parts for lower unit, its in pieces right now.

 

[gregmsr]

Re: compression test

Yes, you are correct on the warm engine. But you can get a good enough idea of compression status on a cold one. All cylinders should still be within 10/15 psi of the others. Overall compression is not really as important as all cylinders being close to the same psi.

 

[chilly1]

Re: compression test

Thank you. I got 138/130 cold. I am trying to diagnose why my motor dies. It will run for a few minutes, and I shut it down due to not much water from the tell spout, and I get nervous. I start it again, and it runs for a minute and dies. I start it again, and it runs less before it dies.I do this a couple of times, and then it won't start at all.

 

[Mark42]

Re: compression test

Have you checked the following:

When running, if you keep pumping the fuel bulb, will it keep running? If so replace the fuel filter and try again. If it still dies on its own, replace the fuel pump.

If pumping the fuel bulb will not keep it running, try spraying some fuel/oil mix into the carbs (remove the aluminum cover to make this easier) from a hand pump sprayer. I use an old windex bottle. If it keeps running, you have a bad fuel pump, clogged fuel lines, dirty carborators or clogged fuel filter.

If the above doesn't work, then the problem may be with the ignition. After it fails to start at all, remove the spark plug wires and crank the motor and check for spark. If its not there, the power packs may be bad, or the magnito under the flywheel may be bad.

But don't run it without the lower unit on and a water supply or it will overheat and can seize.

 

[chilly1]

Re: compression test

I have not checked those things. Thank you. I have replaced the carb gaskets, needle and seat, fuel filter, fuel pump diaphram and gaskets, etc. I did clean out the carb b4 replacing those things, but maybe not enough. I also expected to have to adjust the carb, but I am fearful since I am inexperienced in this.