Correct Way To Check Compression ?

[Aahhyes68]

How do you perform an accurate compression test on a saw ? I've searched and found all kinds of stuff but nothing clear to me. Warm, not warm, run first and allow to cool, room temp (WTF is that lol..), throttle wide open, throttle doesn't matter, pull 4-6 times, pull until you get the highest reading, etc... I dunno. I want to check my 024 and learn how to do it in case I run across a used saw I'm interested in.. I would like to be an educated consumer. Used saw is another thing..pull the muffler and look for shiney walls, but that doesn't mean ...etc.. Just trying to learn something..

Thanks, Steve

 

[o8f150]

cold and keep pulling until the needle stop climbing

 

[NewToStihl]

How do you perform an accurate compression test on a saw ? I've searched and found all kinds of stuff but nothing clear to me. Warm, not warm, run first and allow to cool, room temp (WTF is that lol..), throttle wide open, throttle doesn't matter, pull 4-6 times, pull until you get the highest reading, etc... I dunno. I want to check my 024 and learn how to do it in case I run across a used saw I'm interested in.. I would like to be an educated consumer. Used saw is another thing..pull the muffler and look for shiney walls, but that doesn't mean ...etc.. Just trying to learn something..

Thanks, Steve

Not warm; test it prior to running, or give it sufficient time to cool down completely after running. Throttle makes no difference; compression is generated when the piston rings are above the intake. o8f150 is right; pull until the needle stops climbing - I'm OCD so I always pull the rope 12 times.

 

[Andyshine77]

A freshly assembled saw needs to warmed up and ran a few seconds at WOT to clear out the assembly oil, if you don't you will get a false high reading. Other than that always make sure the engine is cool, keep pulling until the needle stops climbing.

 

[w8ye]

I've done it all different ways.
But agree that the chainsaw should have been recently run (maybe the day before?) if possible, and not hot.
Out of habit from four strokes, I put the carburetor on fast idle.
12 pulls, anyway.

 

[50blues]

I think that putting it on fast idle will make it climb faster, but I have been called crazy before. Remember to close the decomp. plug before you try. Not to much of a problem on an 024 but there was a few.

 

[w8ye]

Yes on the decompression plugs. You can often see soap bubbles coming from the decompression (if you put soapy stuff on them) when doing a compression test.

Some people even remove the decompression and put a plug in the hole.

A leaky decompression valve will often be oily and messy all around it?

 

[workshop]

I've run tests on this before. Room temp equals cold engine. Throttle position makes no difference so leave it closed. I pull about 6 times or until needle stops climbing. You want oil in it like the engine would normally get but too much can give a false reading as will not enough.

 

[nmurph]

If it hasn't been run in the last couple of weeks and you want to get the most accurate reading, run it and let it cool for an hour or so.

Pull until it stops climbing.

Make sure any assembly oil is burned off.

Cold compression will drop 5-10lbs when a saw is hot.

 

[ttyR2]

Two strokes will pull air in through the exhaust at slow revs (starting speed), so opening the carb doesn't really do much. I never noticed any difference.

Hands down the most important thing is to have a *reliable accurate* compression gauge. The KD Tools one I used for a while turned out to be very inaccurate. It's worth the money to buy quality in these types of tools.

 

[Guido Salvage]

You need a compression tester with a Shrader valve to get an accurate reading. I have a Snap-On I found in a pawn shop for $35.

 

[ttyR2]

I bought the $60 kit from Sears. Uses Shrader valves and has various adapters. Works well and has a good precise gauge on it. Highly recommended.

 

[XSKIER]

I've gotten several years out of an Actron gauge from Autozone. It is completely accurate for short reach 14mm plugs, but the adapters add volume and lower the readings for all other threads. I do notice that the needle climbs quicker with an open throttle. But I don't work on saws much. Mainly .5L-3.5L two stroke liquid cooled.

 

[ladrhog]

I've gotten several years out of an Actron gauge from Autozone. It is completely accurate for short reach 14mm plugs, but the adapters add volume and lower the readings for all other threads. I do notice that the needle climbs quicker with an open throttle. But I don't work on saws much. Mainly .5L-3.5L two stroke liquid cooled.

Could you explain that to me I thought pressure was pressure and it didn't matter the volume

 

[XSKIER]

Adding an adapter puts the Actron Schrader valve further out from the combustion chamber. This adds volume to the combustion chamber before the Schrader can see the pressure generated. Same piston, same compression ratio, larger combustion volume, then there is less observed pressure on the gauge. The nice compression gauge sets have the Schrader valve at the end of each adapter, ensuring a more accurate reading.

The compression gauge is meant to compare cylinder integrity in a single multi cylinder engine at the same moment. A change in over 10% will show where a misfire could be. Taking a reading from your 20 year old saw, and comparing it to someone else on the net is not what a compression gauge is for. The readings they generate vary too much by altitude, relative air density, barometric pressure, humidity, operator influence, and gauge condition.

Just read about the poor guy beating his brains out over a MS 362 that "has enough comp for a work saw". The compression gauge didn't do him any favors...

 

[Edge & Engine]

According to Echo and Husqvarna, the correct procedure is with the engine cold, throttle held at WOT and pull until the needle stops rising. If you continue to pull, you should see the needle "bounce" both ways but always come back to the same position. To get the most accurate reading you need to use a compression tester designed for two cycle engines, (We use an Echo one, Husqvarna has one also). They have very short stiff hoses that are swappable and come in various sizes, usually 14mm and 10mm. Use of an adapter is not advised, (unless it has a schrader valve built in).

 

[Aahhyes68]

http://www.baileysonline.com/Chains...ression-Tester/Compression-Tester.axd#reviews


Anyone use the one from Bailey's ??


I have a cheap set from China Freight but would like something better...

 

[Aahhyes68]

Ran the saw for a few minutes yesterday, WOT, pulled 12 times, Harbor Freight compression tester and retested it three times,.... Looks good for an old 024 I guess.

 

[nmurph]

I've had an Actron from AutoZone and the one pictured above from O'Reilly's. The Actron lasted maybe a year but seemed to be accurate. The one from Autozone only lasted a couple/three months. I bought a Snap-On a couple of years ago and couldn't be happier. I have had to replace the shrader valve one time, but that is just a wear item.

 

[surfincr]

i have the actron from autozone, mine lasted a good 3 years then started reading everything 120lbs than 90lbs which didnt occur to me until after i tested the 3rd or 4th saw that came in my shop... i couldnt figure out why the heck the equipment was running so good with such low compression until i verified with my air compressor