<VV> Compression test and TDC

[Charles Lee]

Thanks...

I'm not really doing a full compression test here, just one #1 to see
which TDC is the compression stroke.

Looks like I have good compression because it goes to 60 on the 1st TDC,
then 120 where I stopped, because I just needed to see SOME compression
to be convinced I was on the right cycle.

With the "remote starter" cranking the engine, I'd probably be able to
feel the compression with my finger, but not when I was cranking
manually though.

Tomorrow we'll see if she fires up - it's been almost 10 years actually,
now that I count the time she's been decorating my driveway !   I heard
some life before the battery died, so it looks good so far.

Later,
Charlie



-----Original Message-----
From: virtualvairs-bounces at corvair.org
[mailto:virtualvairs-bounces at corvair.org] On Behalf Of Sethracer at aol.com
Sent: Wednesday, September 24, 2008 10:14 PM
To: chaz at ProperProPer.com; virtualvairs at corvair.org
Subject: <VV> Compression test and TDC


 
In a message dated 9/24/2008 4:48:30 PM Pacific Daylight Time,  
chaz at ProperProPer.com writes:

On  closer examination, I get 0 on the alternating TDCs, so that must be
the  exhaust.

I do get 60, then 120, so maybe it's 60 on the 1st compression  TDC,
then 120 on the 2nd time compression TDC ? Seems like good  compression
though.


 
Charles - The distributor makes one revolution for every two revolutions
of  
the crank. Each time the rotor passes the wire for number one it is at
the top 
 of a compression stroke. To perform a correct compression test, all
plugs 
should  be removed, and the throttle should be held open during
cranking. There 
will be  a build-up as the piston goes up on each of the 5 or 6
compression  
strokes - that is providing the check valve is still in place in the 
compression  tester AND there is some compression in the cylinder!
Cranking it a half 
dozen  times will usually cause the cylinder to reach it's maximum
reading on 
the gage.  If you end up with a cylinder that is low, say 30 lbs less
than the 
others,  chances are you have a problem. If a cylinder is down a bit,
the most 
common  next test is to squirt a little (I mean just a little) oil into
the 
cylinder and  see if the compression comes up a bit with another test.
If it 
does, it is  probably ring wear. If it doesn't, there might be a valve
problem - 
burnt   or bent? The better next step is a cylinder leak-down test. This

involves  putting a known air feed into the cylinder (At TDC) and seeing
what 
percentage  of "leakage" takes place. There is a special device for
this. If 20 or  
30% is leaking, not only do you have a problem, but you can probably
find the 
 culprit. The air will exit the motor and you can usually hear it. If it
is  
coming out the carb inlet, a burned intake valve, out the muffler,
burned  
exhaust, if out of the oil filler cap, perhaps a burned or broken piston
-   or 
worse. 
 
Maybe I just have a sensitive index finger, but I have always been able
to  
tell when the piston is approaching the top under compression and
blowing the  
tip out. Hey, that's why I call it my Index finger!  -  Seth



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