<VV> Compression test and TDC

[Sethracer at aol.com]

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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