Does the carburetor have to be wide open and all spark plugs removed

 

Have to? No. But best testing methodology is Yes to both.

Removing the spark plugs allows the engine to spin faster without having to deal with 7 other chambers having compression strokes — easier on the starter.

Carb fully open allows maximum air flow to enter the chamber quickly.

 

Thanks Barking rats

 

Plus, having throttle locked open, you avoid drawing fuel into the cylinders while cranking.

 

Also, when and If you ever participate in competitive motorsports, there are often classes whose track rules regulate Maximum amount of compression permitted.
This is checked & measured via a calibrated gage; usually by the track's 'tech man' ... often soon after a win is protested by an 'aggrieved' competitor
That procedure spelled out in track rules and it typically MANDATES butterflies / throttle plate Must be wide-open while testing. Wide open will pump highest.

 

Whenever I do a Compression test or Leak Down test I start with a Fully Charged battery. Then during the test I keep the battery charger charging the battery to assure the voltage stays high and the cranking speed doesn't change.

Then I remove all the Spark Plugs and then I secure the throttle blade in the wide-open position. At this time ground the spark while testing the engine. If the car is fuel injected then remove the fuse that powers the fuel pump, you do not want fuel in the mix while cranking the car.

Finally while testing the cylinder I run the starter for approximately five seconds or so to get the best reading. The battery charger should make the test a bit more useful as the cranking speed can change the out going numbers. I record the reading's numbers and then before moving to the next cylinder I add a tablespoon of engine oil and repeat the test. After both tests are complete and recorded, I move on to the next cylinder.

With both a dry reading as well as the wet reading you will have a better idea of what the health of the engine is. Personally I tend to prefer the Leak Down Test because it gives you better and more precise information. The compression test will help narrow in on the problem if it is done correctly. The Leak Down test can identify a specific part that is causing the problem. Both have their uses and both are very helpful when working with older engines.

Best Regards,
Chris

 

Thanks ctm

 

Chris, how do you do the leak down test? I know in piston engine aircraft you had two gauges with a metering hole between them and you recorded both values.

JT