Checking compression - some questions before I go for it!

[Splaker]

Hey folks,

Those of you following my post regarding the poor running 70 HP evinrude will know that I am trying to diagnose a problem and a compression test has been recommended.

So far this is what I learnt I must do:

1. warm up engine, then shut down
2. disable the ignition system either flick the lanyard or ground each plug wire or unplug power pack (don't even know where that is!)
3. remove all plugs
4. throttle wide open
5. install the gauge and crank the engine so gauge gets 5 to 7 hits

In the manual that came with the compressor tester, it mentions "opening the throttle carburetor plate" - how do I locate that on my motor? What am I looking for? or, is this even necessary?

thanks!

 

[wilde1j]

Re: Checking compression - some questions before I go for it!

I never bother with #1 & #4 ... #4 is pertinent only for 4 stroke motors anyway and #1 may result in a slightly higher reading. The important thing is that readings be with 10% of each other.

 

[Silvertip]

Re: Checking compression - some questions before I go for it!

The throttle plates being referenced are the butterflies in the throat of each carb. That's what controls engine speed. The farther they are open the more air and fuel enters the engine. On a two stroke having the throttle open won't hurt anything but is does not good either so it doesn't matter.

 

[milmat1]

Re: Checking compression - some questions before I go for it!

Your outline is correct and I Always warm the engine first.(Especially with modern engines that have a lot of Aluminum!) I want to replicate the running temp as much as possible.
The throttle should be wide open as well, It cannot compress what it cannot pull in.

The Compression test is actually a test of a cylinders pumping ability, More than anything elsel So Watch the gauge as it goes thru 4-5 compression strokes. It should "Pump" up to its final reading. And note the Pumping action, is it the same on all cyl's etc...

You want to as closely as possible duplicate the circumstances for each cylinder. Because what you are really doing is comparing cyl's and looking for Changes.

A Cylinder leakdown test is the only real way to determine the cdlinders health....

JMHO !

 

[F_R]

Re: Checking compression - some questions before I go for it!

If you are testing compression to evaluate whether or not it is viable to get running, how ya going to warm it up?

Open the throttle if you want to, I don't care. But it doesn't make any difference. A two stroke must have a charge of compressed air going in the intake ports and flushing the exhaust out the other side in order to run. At that moment in time, before the ports close, the pressure in the cylinder is zero (on a non-running engine). So, you always start the stroke at zero psi. That scavenging happens at any throttle setting. It also is why the throttle plates have holes in them.