[lucky1]

Do you have to remove all the spark plugs prior to conducting a compression test

To get reliable reading?

Thank you.

[b-man]

Remove them all.

Otherwise the closed off cylinders with varying amounts of compression will alter the results of the cylinders you’re trying to test.

You want the engine to spin at a consistent speed to get valid results.

[25stevem]

I bet you want the plug on the drivers side under the power brake booster to stay in if it could, but don’t do that during a compression test.

I find the easiest way to get that out is with a spark plug socket with a hex on top and then I use a closed off set box wrench on the hex to bust it free.

[67Fbird]

also note...you will want to make sure the battery is fully charged......1 less variable associated with cranking rpm

[lucky1]

Thank you all!

[Gach]

Yes and make sure carb is wide open.

[lucky1]

It’s tri power, how and why ?

[chuckies76ta]

Originally Posted by lucky1

It’s tri power, how and why ?

Just open your throttle and secure it open. No fuel in case you have FI.

The engine needs air in the cylinders to do a compression test. It basically uses the air into the cylinder to show compression on the gauge.

Another good Idea is to disable your coil, remove the positive wire off the coil.

I turned mine over about 4 cycles. Your hoping for all cylinders to achieve close to the same pressure. like 170 to 180 psi for example.

[Schurkey]

Originally Posted by chuckies76ta

Another good Idea is to disable your coil, remove the positive wire off the coil.

...or GROUND the heavy "coil wire" at the distributor end. In other words, pull it out of the distributor and secure it to a decent ground.

But removing the little positive wire, while slightly more difficult, is a better solution.

[lucky1]

Originally Posted by Schurkey

...or GROUND the heavy "coil wire" at the distributor end. In other words, pull it out of the distributor and secure it to a decent ground.

But removing the little positive wire, while slightly more difficult, is a better solution.

What’s the purpose of that?
To reduce drain on battery ?

[Schurkey]

You must kill the spark on a carbureted vehicle when doing a compression test. The carb is still metering fuel just as it would when cranking the engine to start it. That fuel is blowing out the seven open plug holes while you're testing compression pressure on the eighth..

Add a spark, and you maybe need clean underwear.

The point to grounding the coil wire is to prevent the coil from building sky-high voltage that destroys insulation. With the coil wire grounded, you build almost zero voltage although there'd be some current flow.

Disconnecting the coil + wire also prevents the coil from building voltage; the coil would then be totally non-powered.

[TammyTA]

Originally Posted by 25stevem

I bet you want the plug on the drivers side under the power brake booster to stay in if it could, but don’t do that during a compression test.

I find the easiest way to get that out is with a spark plug socket with a hex on top and then I use a closed off set box wrench on the hex to bust it free.

Just stumbled on to that trick last summer. It needs to be a sticky!

[JLMounce]

Didn't see this mentioned yet, but unless otherwise unable to, the compression test should also be done on an engine that is at operating temperature. This takes any piston to cylinder wall clearances out of the equation.