Okay here's the deal. I'm one of them guys who learns best when I write something down. If I write it down I'll remember it. Always been that way, big note taker.
So, I am going to write this down in a fairly selfish endevour. See, I'm having this warranty go round and what happens is, I end letting myself get confused, flustered, side-tracked, or whatever. I know how it works and what is going on, but the truth is I have limited hands on experience. So when I am engaged in discussion and don't have/take the time to think it through and remember to keep it simple, I forget stuff and what not.
That happened to me today and I forgot to ask some questions I should have and to key into things I should have. So, knowing what works for me, I'm writing it down.
Maybe as a side benefit of me writing it from a fairly mechanically adept yet limited experienced person in those type terms will help others of the same status.
I am NO EXPERT!!!! And it is not my intent to write this as a tech manual type thing or to say that I will get it all correct. Just writing it in the way I understand things, in simple terms, and certainly on a most basic level. Obviously there is a lot more than I'll be touching on here. And I have no doubt (and encourage) those of higher knowledge to step in and correct any mistakes I make.
So...
First I suppose we need a basic understanding of how a four stroke combustion engine works.
Very down and dirty...It is exactly what the term implies. An engine that functions off of four strokes and an explosion to create power.
I found this most basic cut away of a generic four stroke cylinder:
So the four strokes are
-Intake
-Compression
-Combustion (ignition)
-Exhaust
Basically, during the Intake stroke, the piston moves down inside the cylinder. Simultaneously, the intake valve opens allowing mixed air and gasoline to be drawn into the cylinder.
The fuel/air mixture needs to be vaporized to properly explode. So during the next stroke, the Compression stroke, the piston moves back up the cylinder, basically crushing the air and fuel. Both valves are closed during this stroke to essentially create a sealed chamber.
Boom time. At this point, the Combustion stroke, the spark plug fires igniting the fuel and the piston moves (slams) back down the cylinder.
Now we have to purge the chamber of any unburnt gases. So, during the fourth stroke (Exhaust), the exhaust valve opens and the piston moves back up through the cylinder to push these gases out and ready the chamber to start all over again.
As I said, that is EXTREMELY basic. Valve timing, ignition timing, etc, etc. all play a much bigger role and I didn't even touch on any of that because that isn't the intent here.
So...
Leak down and compression tests are fancy names for a deceptively simple process. And each are performed in pretty much the same basic way.
I believe the piston, is brought to Top Dead Center (TDC) which really just means all the way up to the top of the cylinder. But either way, both the intake and exhaust valves must be closed to create the sealed chamber that is required for compression during operation. If one of the valves is slightly open obviously the chamber isn't sealed. So they need to be closed and everything sealed up.
Then the spark plug is pulled and, in the case of a Compression Test, an air guage is screwed into the hole. Then the starter is engaged and the guage reads the cylinder's ability to hold air in the combustion chamber. If it shows a significant discrepency (based on pre-determined and known numbers) then you (they) must start the process of finding out why the chamber is not sealing.
This is where a Leak Down Test comes into play.
Same basic set-up, valves closed, plug pulled, etc. except this time, instead of a guage, what is essentially a threaded nozzle, is screwed into the plug hole. Air is pumped into the combustion chamber (cylinder). If everything is sealed up properly, only a small amount of air should escape down the cylinder, past the piston, and into the crank case (the bottom part of the engine).
If air is coming out the exhaust, there is a problem of some kind with the exhaust valve.
If air is coming out the carb, there is a problem of some kind with the intake valve.
If "substantial" air is going into the crank case, then there is an issue with cylinder/piston relationship and/or integrity. The piston actually has three rings that slip into a groove that is cut around the radius of the piston. The piston itself is ever so slightly smaller than the inside of the cylinder. These rings are what seal against the cylinder wall. This allows oil to come up into the cylinder to keep things lubricated, but prevents it from just splashing all the way up and into the valves, plug, etc.
Since we're talking tight tolerances and need a complete seal, any scratches, loose fits, warps, nicks, etc, on the valve faces, valve seats, pistons, cylinder walls, etc. will not permit that tight seal. So during the leak down, you're trying to pinpoint where the air is getting by. Then you dive in to determine why and what the exact cause is.
Woosh. Okay I feel better.
Feel free to full on ignore this if ya want but if it helps it to make sense to some folks then cool.
Four Strokes and Leak Down/Compression Tests - How They Work
1994 XLH [Black of Course]
" A fear of weapons is a sign of retarded sexual and emotional maturity " -Sigmund Freud 
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