How to use a compression tester?

jtexas

Fleet Admiral
Joined
Oct 13, 2003
Messages
8,646
Okay, I'm about to reveal some real ignorance here so don't laugh...I have a brand new comp tester, was given to me back in the 80's, never been used.<br /><br />Now I want to decarb my '79 rude 70, and want to get baseline compression numbers.<br /><br />I know I'm supposed to screw the hose end into the spark plug hole.<br /><br />Should the other two spark plugs be in or out? <br />If in, should they be connected or disconnected?<br />When I turn the key, how long do I let it crank? <br />Will the gauge continue to show the reading afterwards or do I have to watch the gauge while somebody else turns the key? <br /><br />I just happen to have a heavy-duty automotive momentary starter switch here, can I use it to start the engine, and if so, how do I connect it?<br /><br />thanks,<br />john
 

dakk1

Chief Petty Officer
Joined
Apr 22, 2003
Messages
418
Re: How to use a compression tester?

Different methods but generally:<br /><br />Motor warm.<br />Throttle wide open.<br />All plugs out but connected to spark plug wires.<br />Plugs grounded or pull the lanyard.<br />4 or 5 revolutions ought to do it.<br /><br />Tester should have a pressure release button so you don't have to watch gauge.<br /><br />First 2 are not absolutely necessary just as long as you do it the same way next time.<br /><br />I have no idea how to connect that switch.<br /><br />Dave..
 

BoatBuoy

Rear Admiral
Joined
May 29, 2004
Messages
4,856
Re: How to use a compression tester?

When I use the switch I made, I connect it between the large post on the battery side of the solenoid and the small post on the ignition switch side.
 

rodbolt

Supreme Mariner
Joined
Sep 1, 2003
Messages
20,066
Re: How to use a compression tester?

the exact method is not overly critical. what is critical is to use the same method for each cylinder. but the above method will work well. like they say either ground the plug leads or pull the stop lanyard. it will blow a fuel/air mist out the plug holes and you really want to eliminate or control any sparks. on the switch hook one lead to the battery positive and the other to the small post on the solinoid that has the yellow/red lead on it.
 

dajohnson53

Lieutenant Commander
Joined
Apr 28, 2004
Messages
1,627
Re: How to use a compression tester?

As for the number of cranks - what I've been told that seems to make the most sense, is to crank until the gauge doesn't rise any more. With the type of gauge with a pressure release button, it will hold on the highest pressure achieved. When I'm doing a small engine where I can crank (e.g. pull start) and observe the gauge at the same time, I just crank and crank until it doesn't rise any more, then stop. Repeat for all cylinders. It is generally 2 - 5 pulls.<br /><br />With a larger engine where I sit at the helm to crank, I crank for around 10 cycles each and call it good.<br /><br />Other than making sure you max out the gauge for each cylinder, as others have all said: cold/warm, etc. etc. it's more important to be consistant.
 

jtexas

Fleet Admiral
Joined
Oct 13, 2003
Messages
8,646
Re: How to use a compression tester?

thanks all, this is exactly what I needed. <br /><br />Oh, and would somebody please speak up if 10ga wire isn't good for a starter switch.<br /><br />thanks!
 
Top