How to properly do a compression test

[donmakesallthefuelbegone]

I am planning to buy a second hand chainsaw and like everyone knows: if you’re not careful you’ll buy someone else’s problems. Thus to avoid that I bought a compression tester but I’ve heard so much different stuff like: run the chainsaw, others say to not do that then some say do it with full open throttle and then there are people that say don’t do that.

Needles to say as a beginner it gets overwhelming and I’m confused. I am planning to either buy a 460 or a 462 (if I get lucky, either way it’ll definitely be a Stihl) I don’t know if make and model changes any of this but I’d love to get some good advice that hopefully isn’t an entire mess of people saying this and that

 

[heimannm]

The most important thing is the tester must have a Schrader valve in the tip where is connects to the spark plug hole. the displacement of a chainsaw (or other small engine) is so small that the additional volume of the hose or connectors will throw off your reading. The valve should be a low pressure one intended for a compression tester, otherwise the gauge will read lower by whatever the "pop off" pressure of the valve is.





Second most important is to pull until the gauge stops rising. This can take 8-10 pulls at times.

Warm compression will generally be a bit less than cold, and wet (with fuel or oil) will be higher than dry.

If in doubt, put a small amount of mixed fuel either through the carburetor or down the plug hole and pull it over 8-10 times before testing to make sure the crankcase and cylinder are like normal cold start conditions.

Throttle open or closed doesn't make much difference if you keep going until the gauge stops.

Mark

 

[donmakesallthefuelbegone]

The most important thing is the tester must have a Schrader valve in the tip where is connects to the spark plug hole. the displacement of a chainsaw (or other small engine) is so small that the additional volume of the hose or connectors will throw off your reading. The valve should be a low pressure one intended for a compression tester, otherwise the gauge will read lower by whatever the "pop off" pressure of the valve is.

View attachment 1181814

View attachment 1181815

Second most important is to pull until the gauge stops rising. This can take 8-10 pulls at times.

Warm compression will generally be a bit less than cold, and wet (with fuel or oil) will be higher than dry.

If in doubt, put a small amount of mixed fuel either through the carburetor or down the plug hole and pull it over 8-10 times before testing to make sure the crankcase and cylinder are like normal cold start conditions.

Throttle open or closed doesn't make much difference if you keep going until the gauge stops.

Mark

Thank you so much, this is very useful information, I’ve been screwing around with it and already noticed that throttle doesn’t make a lot of difference (what is logical if you think about it but you never know so I just tried)

I’m grateful for you taking the time to explain this, thanks

 

[PV Hiker]

A compression tester is good to help with diagnose. Consider a vacuum/pressure tester to go along with your diagnosing tools. You could have a leak that reduces your compression where as a vac/press tester can help you locate leak, fixing it therefore raising your compression (like a pressure DECOMP relief valve).

**EDIT to change pressure to decomp. Typed to fast and awkward on phone. Sorry for the confusion.

 

[buzz sawyer]

A compression tester is good to help with diagnose. Consider a vacuum/pressure tester to go along with your diagnosing tools. You could have a leak that reduces your compression where as a vac/press tester can help you locate leak, fixing it therefore raising your compression (like a pressure relief valve).

Except for a malfunctioning decomp valve or damage to the spark plug hole, how would an air leak affect compression?

 

[PV Hiker]

That is why I mentioned the valve.... good for you to add spark plug threads.

 

[Okie]

Yes little confusing checking out a used chainsaw when buying or getting ready to work on one.
Buying a used chainsaw is kinda like buying a pig in a poke if you cannot examine the saw before buying and even then it can be iffy.

I've seen chainsaws with very bad piston or cylinders have good compression.

I prefer pulling the muffler and looking at the piston/cylinder 1st instead of a compression test.
I've seen some very sad/bad looking pistons and the saw still have good compression and run ok.
Before I even start to work on a new to me chainsaw I remove the muffler and look at the piston/cylinder and if they are ok I then go by pull rope feel for compression and might not ever drag out the compression gauge. If the piston/cylinder is scored especially the piston skirt the engine is going to at least need lots of work and if running lean it can easily destroy a new piston fast.

If you do not have the correct type small engine compression gauge that uses the low pressure schrader valve most gauges will lie to you and check a small engine a bad with very low compression.

 

[donmakesallthefuelbegone]

Yes little confusing checking out a used chainsaw when buying or getting ready to work on one.
Buying a used chainsaw is kinda like buying a pig in a poke if you cannot examine the saw before buying and even then it can be iffy.

I've seen chainsaws with very bad piston or cylinders have good compression.

I prefer pulling the muffler and looking at the piston/cylinder 1st instead of a compression test.
I've seen some very sad/bad looking pistons and the saw still have good compression and run ok.
Before I even start to work on a new to me chainsaw I remove the muffler and look at the piston/cylinder and if they are ok I then go by pull rope feel for compression and might not ever drag out the compression gauge. If the piston/cylinder is scored especially the piston skirt the engine is going to at least need lots of work and if running lean it can easily destroy a new piston fast.

If you do not have the correct type small engine compression gauge that uses the low pressure schrader valve most gauges will lie to you and check a small engine a bad with very low compression.

I knew about pulling the muffler, that is something I would do with everything that is 2 stroke and second hand, thanks for the advice though, I didn’t know that a bad looking cilinder could still have good compression

 

[Okie]

here is some info I've saved and links on this site about compression testers/testing/making your own, etc:

Actron CP7828A Professional Compression Tester Kit with 10mm, 12mm, 14mm, 14mm Long Reach, and 18mm Adapters $45 or so



Amazon link for reviewing:

https://www.amazon.com/gp/product/B00020BM28/ref=as_li_ss_tl?ie=UTF8&linkCode=sl1&tag=howtomotorepa-20&linkId=cdbd6f22890e891c57a50a4c630c7756



A good compression tester for small engines:



Actron CP7828A Professional Compression Tester Kit with 10mm, 12mm, 14mm, 14mm Long Reach, and 18mm Adapters $45 or so







Amazon link for reviewing:



https://www.amazon.com/gp/product/B00020BM28/ref=as_li_ss_tl?ie=UTF8&linkCode=sl1&tag=howtomotorepa-20&linkId=cdbd6f22890e891c57a50a4c630c7756



Link to video about compression testing chainsaws: 6/23

https://www.arboristsite.com/community/threads/compression-tester.336165/

 

[Okie]

and more about compression testing chainsaws and small engines.



Link to a video about small engine compression testing6/23

https://www.arboristsite.com/thread...-04&utm_content=daily_newsletter#post-8020777



Here is a link about modding Schrader valves and homemade testers, actron tester, etc

Compression tester issues:

http://www.arboristsite.com/community/threads/compression-tester-issues.310133/



I got 3 of the Napa 700-9556 compression gauge Schrader valves today after the local NAPA ordered from their warehouse. The come 5 in a box and you can buy them each @ approx. $2.20 each. They worked A-ok. One was defective out of it's sealed package, was not spring loaded on back check due to a broken spring, noticed immediately before attempting install for test..





3 /2018
Napa #700 9556 is obsolete.

Here is a special low pressure schrader valve for compression gauges Milton and Lisle part number's and I seen them on flea bay as still available.

Milton 1251-2 @ about $5 each

Lisle 20100 @ about $7



Update info 12/20

Possible good compression gauge for chainsaws at $40, STANDIX 1023

Mentioned at this link 12/20

https://www.arboristsite.com/commun...mpression-gauge-and-tested-a-few-saws.348043/

 

[stihltech]

If it wont stay up holding by the starter handle, you must check for decomp open, loose sparkplug, AND pull the muffler. That will tell you more than any compression reading.

 

[Vintage Engine Repairs]