Schrader valve?

[Ghillie]

I noticed in a few posts that there was some confusion to where the schrader valve is on a compression tester.

Here are a few pictures.

This is the schrader valve in the end of the hose. It acts as a one-way valve so the pressure can build up after a couple of pulls on the starter cord.

This is the valve unscrewed to show you how it is made.

This is the relief valve so you can bleed off pressure without taking the hose out of the spark plug hole.

I hope this helps out some confusion. I have never seen a tester with a schrader valve in the gauge. I am NOT saying they don't make them, just haven't seen it.

Fred

 

[serial killer]

I've seen some posts reminding people to make sure that the Schrader valve is on the bottom of the hose. My compression tester, which I believe is the same model as yours, came with an extension/adapter that is about two inches long and screws onto the end of the hose. This places the Schrader valve up inside and a little bit away from the engine's cylinder. I took the warnings to mean that I should not use that extension because it reduces the accuracy of my readings.

 

[Ghillie]

You shouldn't use the extension if you don't need it. The closer the valve is to the cylinder the more accurate it will be.

Some plug holes are too deep and narrow but on a saw you shouldn't have that problem.

 

[gafirefighter]

Thanks for the informative post.

Adam

 

[kevin j]

The gauge is still 'accurate' in the strictest sense, it will just read lower. so 'not accurate' in the real world sense.
The extra volume on the cylinder side (hose, tube, etc) effectively lowers the compression ratio, so it will always read lower psi. It will be consistently low and repeatable.

volume after the schrader is not so bad. '10 feet of hose' after the schrader check valve would take forever to charge up, each rope pull would rise the gauge higher and higher, but it will eventually get to the correct final pressure, because the combustion chamber volume is correct if the valve is close to the end of the adaptor.

Some of the testors must have special schraders with light springs. I replaced one in an auto tester with a standard tire valve core and it read low. It took about 20 psi to crack the spring in the valve core, so the gauge was always 20 psi low if I recall. Got the right part and gauge was good again.

k

 

[Crofter]

[Q

Some of the testors must have special schraders with light springs. I replaced one in an auto tester with a standard tire valve core and it read low. It took about 20 psi to crack the spring in the valve core, so the gauge was always 20 psi low if I recall. Got the right part and gauge was good again.

k[/QUOTE]

You are definitely correct about a common tire Schrader valve core having a much heavier return spring and not giving the correct reading if used in a compression guage.

 

[16:1mix]

The gauge is still 'accurate' in the strictest sense, it will just read lower. so 'not accurate' in the real world sense.
The extra volume on the cylinder side (hose, tube, etc) effectively lowers the compression ratio, so it will always read lower psi. It will be consistently low and repeatable.


k

+1
Well said.

 

[WesternSaw]

Shrader ValveAgain

As a novice to the chainsaw thing it would be really great if this was cleared up for me once and for all,so I can get to it.The main focus where I seem to be stuck is. Is there always a shrader valve in the end of the spark plug adapter that you thread into the plug hole? If not is having one there better?Should it be a short distance from plug adapter to gauge? Should there be a pressure release next to the gauge so you can release the pressure and repeat the process without removing the plug adapter from the hole to release pressure.I am sure a person would want to do a couple of checks for consistency sake ie. accuracy.Who do you guys think make the good gauges?
When The Student Is Ready The Master Will Appear! I'm the student here.
Thank's Lawrence

 

[blsnelling]

There must be a Schrader valve immediately at the end of the hose where it screws into the cylinder, plain and simple. Anything differently and you reading will be low.

 

[LazyJ]

The model in Ghillie's pics looks like the Actron brand I have, $15 bucks on Amazon. Actually, for $25 bucks I got the comp gauge and the fuel pressure/manifold vacuum gauge.

 

[gafirefighter]

That makes sense. I do wonder one thing though. Why do the large majority of the testers you see on the market not contain one? Is it just because they are cheaper testers?

Adam

 

[blsnelling]

All of the ones I've looked at do have them. I bought mine at Auto Zone. My Dad's ancient tester has one.

 

[Ghillie]

That makes sense. I do wonder one thing though. Why do the large majority of the testers you see on the market not contain one? Is it just because they are cheaper testers?

Adam

I do not see how it would work correctly without a schrader valve. The pressure in the gauge would not build up.

It would release everytime the exhaust side opened up and it would be worthless. It has to have some sort of one-way valve to work correctly.