Leak down test, sort of

[LoHo]

I have hooked my compressor to each cylinder with the cams off, so the valves are closed. When I apply air, I hear the piston go down, and pressure builds, but I also hear air escape into the engine, like around the rings. That is normal, is it not? I know #3 has an intake valve issue, as air escapes through the snorkel quite readily. All of the other three leak down slowly and pop if I pull the air nozzle out.

[natemoen]

You will get some going past the rings (there is a gap at the end of each ring). You really need a guage on there so you can measure the rate at which the air escapes.

[Rasputin]

The piston must be at absolute TDC to do this test. If the piston goes down then the test is null and void. That is what is said about doing the test with the engine parts ready to run.
As you have the cams removed so all the valves are closed it should not matter so much, other than a possibility of the cylinders being worn so they will leak past at other than ignition point.
A side note is to anyone trying to do this test with a leak down tester is that all components should be assembled so you do not drive the piston down with air pressure if you are not at TDC, thus sending another piston flying up into your unprotected valves. IE do not do this test with CCT removed or other such possibilities.

[skids]

I hope you had it on the centerstand! :-) They way you did it would be with the valves all closed. The problem with an air compressor is that any leakage is constantly being replaced, but if it helps you find where it is leaking it helps! If you have a leaky valve, now is the time to fix that. Is the leakage into the crankcase severe? You would get a lot of air out of the breather tube if that is the case (I think) as long as the CCT is installed. You could sort of compare that with a known good cylinder, and if the troublesome cylinder is leaking more into the crankcase with a leaky valve as well, I would say that the rings MIGHT be trashed. I am just guessing...

[motoman]

You will get some going past the rings (there is a gap at the end of each ring). You really need a guage on there so you can measure the rate at which the air escapes.

.........'leakdown test'...........

[Rasputin]

The problem with an air compressor is that any leakage is constantly being replaced, but if it helps you find where it is leaking it helps!

That is the whole point of a leak down test... to hear if it is going out the carbs or the exhaust or the crankcase. The gauge is to simply define how badly it leaks.

[motoman]

I have hooked my compressor to each cylinder with the cams off, so the valves are closed. When I apply air, I hear the piston go down, and pressure builds, but I also hear air escape into the engine, like around the rings. That is normal, is it not? I know #3 has an intake valve issue, as air escapes through the snorkel quite readily. All of the other three leak down slowly and pop if I pull the air nozzle out.

Even done incorrectly, that pretty much tells you what you were wanting to know pertaining to that suspect valve. Larry, for that test to be lagitimate, it has to be together like it is gonna be run as Rasputin stated.

[Rasputin]

Even done incorrectly, that pretty much tells you what you were wanting to know pertaining to that suspect valve. Larry, for that test to be lagitimate, it has to be together like it is gonna be run as Rasputin stated.

+1 it will still work done the way he has done it due to all the valves being closed. Just do not want anyone to think they can plug 120psi compressor into a plug hole and see how it goes without understanding how this test works.

[LoHo]

I was going to call it the "What-the-hell's-wrong-with-this-bike?!" test, but "leak down" sounded kinkier. I know it isn't the full monty, but it worked.

All I wanted was to see where my compression was going; I suspected a valve in #3, and this experiment has confirmed that at least. I have a salvage head coming; I'll scavenge a valve, clean it up and lap it in, and then be certain of my timing before I try her again.

[petejw]

The piston must be at absolute TDC to do this test. If the piston goes down then the test is null and void.

hey ras,
it isnt necessary to have it at tdc, the best thing about a
leak down test is that u can move the piston up or down and
get several readings, an example could be a score in the cylinder bore,
at tdc the leak down test wouldnt show this til the piston was in
a postion of where the scoring is.



also if possible its best done at operating temp, that way there
should be no gap in the rings.

[TADracer]

+1 on that Pete said. For your edification, here is what I do.

Most leakdown testers say to use 90 PSI pressure but I use 50 psi instead and rotate the motor slowly through the power stroke using a strap wrench on the alternator stator (mainly so I do not round off the square lug on the timing plate). I have the oil fill plug removed and carbs open and air filters removed so I can hear the crankcase and intakes. Hearing the exhaust is a bit harder unless you remove the pipes but you can still hear all that if you are away from noise like street traffic.

If you read the two gauges then anytime the valves are closed and you are at TDC you can hear the air escape through the oil fill plug but not through the carb or exhaust. If you hear any air at carb or exhaust then it is a valve. If the reading on your two gauges is more than 10% apart then you are having some wear issues in the rings that should be addressed. Then I move the piston down slowly through the power stroke and the the gauges should remain constant. If you hit a bad spot in the cylinder then the lower gauge will lose some pressure and you know that some cylinder work might be needed.

[Rasputin]

Point I was trying to make is that if the engine is not together with all CCT's, chains etc in place, you can destroy parts by not being at TDC as the pressure will cause that piston to go down. That will in turn cause all of the other related parts to move as well. If the CCT is out of the bike for instance and you applied the pressure it would push the piston down and turn the rest of the motor with it. Now with the CCT being out and no tension on the valve train from the cam chain things may move and get bent.
Yes you can use it other than at TDC, just be sure that if the pressure turns the motor you are prepared for it. Not all the guys who will try this know all they should before trying it so it is just a warning.
Air pressure is the same as combustion pressure when it comes to turning a motor.
The other reason to do a leak down test at TDC is that you are now testing the valves and rings and head gasket only. Once that test is established you can now do it with the piston part or all the way down (while maintaining the engine rotation) and see the difference. Now when you have X number of pounds loss thru a intake valve you can subtract it from Y value of loss while passing the score in the cylinder wall and determine which is causing how much loss so you can analyse it better.
There are many ways to use any tool and opinions vary. It is just the way I was taught to do it.

[petejw]

The piston must be at absolute TDC to do this test. If the piston goes down then the test is null and void.

lol thats not what u said in ur first post
but all good.

[LoHo]

Skids asked : Is the leakage into the crankcase severe?

Nope, just a slight hiss like a nail in a tire before you pull the nail out. My heart sank when I heard the sound coming from the cam chain galley, but then I realized that there must be some blowback past the rings.

Anyway, I think that once I replace the valve and CORRECTLY set the timing, I'll be gold. It's funny, I've had the head off twice, setting the timing right the first time and screwing up the second. Okay, maybe funny is not the word I'm looking for...

[DGXSER]

I feel for you, really I do. I have not fubarred any of my valves just yet, but I screwed up a frame stud once.

Tried to use the wrong thread nut on shock, snapped the threaded stud off that is welded to the frame, I do not weld. It was one of the final steps in putting the SH back together.

Anyway, I know the feeling of thinking I was putting the ribbon on the package only to figure out I just stuffed up the package completely.