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Sportster Motorcycle Motor - Top End Discuss Sportster Motorcycle Top End issues. Rockerboxes, Valves, Cylinders, Pistons, Rings, Lift Rods, etc...

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Old 14th April 2006
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Default Four Strokes and Leak Down/Compression Tests - How They Work

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.


Last edited by GOTWA; 14th April 2006 at 10:50..
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Old 14th April 2006
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Oh yeah Erik, don't forget, valve seals play no part in combustion chamber integrity. As Stevo said, they play no mechanical function in the process...

They are up above and out of the entire gig for this.

Hey laugh if you want, it works for me.
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Old 14th April 2006
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Nice job Eric. Most leak down gages pressurize the cyl thru an orifice of known size, with a pressure gage on both sides of the orifice. If there is no or low leakage in the cylinder, the gages will read the same. As the cylinder leakage increases, there will be a reading difference, the gage on the cylinder side will read lower. This will give a good idea of amount of leakage and is most times equated to a percentage. Your explanation makes the tests understandable. Thanks.
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Old 14th April 2006
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You got the basic idea, I only think a few terms a little off. Not really wrong, just not right if you want to be super technical about it.

Like, fuel doesn't get vaprized, it gets atomized, and the carb/fuel injectors really do this and not the compressions stroke (though it may play some role, but I'm not certain).

Also (and this is just me being a stick in the mud about this) gasoline doesn't explode - it burns, and there is a difference. The burning is what allows power to be driven down on the piston for the full stroke, and is also what allows spark advance in higher RPMs (in addition to cam timing and a slew of other things)

And the gasses that are pushed out during the exhuast stroke are burnt, not unburnt. Anything that's unburnt would be raw fuel.

Now that that's out of the way...

As far as the compression/leak down test goes, you're pretty mcuh spot on, only there are more places that aire can leak through. You are correct that valve seals play no role as they merely keep oil from dripping down into the combustion chamber, but other things can leak, like head gaskets. Actually, that's the only other thing I can think of...

Good read...I always find myself intently reading anything to do with this stuff even though I already have it down pat...Thanks for the post!
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Old 14th April 2006
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one thing to remember in leakdown test is as your pumping air into cylinders is that the pistons will want to go down wich can be dangerous. if the bike is in gear and the piston goes down the bike will walk on you. strap the bike down to something or lock the crank in position. if your holding crank with a rachet and socket, hold on tight as it may swing really fast.
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Old 14th April 2006
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Oh sure, get all technical on me, damn gearheads...

No, I know all of what you said, but like I said, was just keeping it simple and straight forward for my way of thinking and maybe for others. But you are correct for sure on all points.
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Quote:
Originally Posted by xl1200r
Like, fuel doesn't get vaprized, it gets atomized, and the carb/fuel injectors really do this and not the compressions stroke (though it may play some role, but I'm not certain).
Now that is good info that I knew but until you just pointed it out I hadnt really keyed into the fact I knew. I should have worded that better and thought it through more. See there, this has already been a good exercise.
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Quote:
Originally Posted by GOTWA
Now that is good info that I knew but until you just pointed it out I hadnt really keyed into the fact I knew. I should have worded that better and thought it through more. See there, this has already been a good exercise.

like you said, sometimes you just have to write (type) it down.
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Old 14th April 2006
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Quote:
Originally Posted by GOTWA
Now that is good info that I knew but until you just pointed it out I hadnt really keyed into the fact I knew. I should have worded that better and thought it through more. See there, this has already been a good exercise.
No biggie...I'm the same as you as I know this stuff cold, when I'm trying to talk with a service guy or looking for advice, I just forget the science beheind everything for some reason.
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Old 14th April 2006
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Well, I certainly wouldn't say I have it down cold by any means. But I can relate to what knowledge I do have getting garbled when I'm in discussion. It's like some bizarre phenomona.
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