How to check cylinder compression

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PNW Sawduster

I cut it off twice, and it's still too short!
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Ok, so this will no doubt sound like super remedial question, but what do I need to check the compression on my saws? I've got a compression test kit from Harbor Fright, and it works fine on my outboard and other larger engines, but when I hook it up to either of my Jonny Red 49sp's all it shows is 30 psi, tho both saws will hold that compression without noticeable leak-down. Cylinders/rings in both look good, looking thru the exhaust port and thru the plug hole. I don't believe for one second that they only have 30 psi compression, so what am I doing wrong? Do I need a special small-engine compression tester?
 
I use a Strandix 1023. It has a 1" hose and a valve core to help get accurate readings on small engines

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Ok, so this will no doubt sound like super remedial question, but what do I need to check the compression on my saws? I've got a compression test kit from Harbor Fright, and it works fine on my outboard and other larger engines, but when I hook it up to either of my Jonny Red 49sp's all it shows is 30 psi, tho both saws will hold that compression without noticeable leak-down. Cylinders/rings in both look good, looking thru the exhaust port and thru the plug hole. I don't believe for one second that they only have 30 psi compression, so what am I doing wrong? Do I need a special small-engine compression tester?
How many times did you pull the starter rope? It takes 5-6 pulls to reach full compression reading
 
How many times did you pull the starter rope? It takes 5-6 pulls to reach full compression reading
I gave 'er a good 10 pulls at least. If the saws really had low compression I'd expect that the gauge wouldn't hold steady afterward, correct? Maybe it's just because it's a cheapie gauge - ?
 
The procedure for testing compression is as follows:

1) Secure throttle in wide open position.
2) Make sure choke is completely disengaged
3) Make sure compression release is disengaged (if relevant)
4) Make sure all gauge connections are seated properly.
5) Pull cord aggressively until gauge maxes out (usually 7-10 pulls)

Quality gauges may vary 5 psi. On that engine I believe you're looking for 150+psi.

Can you confirm you're getting a correct reading on a different engine? Things can suddenly fail.
 
The procedure for testing compression is as follows:

1) Secure throttle in wide open position.
2) Make sure choke is completely disengaged
3) Make sure compression release is disengaged (if relevant)
4) Make sure all gauge connections are seated properly.
5) Pull cord aggressively until gauge maxes out (usually 7-10 pulls)

Quality gauges may vary 5 psi. On that engine I believe you're looking for 150+psi.

Can you confirm you're getting a correct reading on a different engine? Things can suddenly fail.
I have heard this sequence before, but I have never understood why the throttle has to be wide open.

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If you wind up needing a new compression gauge I like the offerings from Motion Pro, MityVac, Echo 91147 and OTC. I believe any quality gauge with a schrader type valve end should be able to handle your needs. You need to be able to build up pressure in your line and can't rely solely on a chainsaw engine given their small displacement.
 
I have heard this sequence before, but I have never understood why the throttle has to be wide open.

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The goal is to maximize airflow so you achieve the highest possible reading. When a piston lowers and draws air- any restrictions are going to limit the air volume in the chamber. Depending on the engine type and other variables you might see a 10+ psi difference.
 
The goal is to maximize airflow so you achieve the highest possible reading. When a piston lowers and draws air- any restrictions are going to limit the air volume in the chamber. Depending on the engine type and other variables you might see a 10+ psi difference.
Thanks, never heard it explained before.

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All good information however,,, every time you pull the rope on a saw with running or better compression numbers, you are drawing mix into the cylinder. Mix has oil and oil will not compress and this could raise the compression but not buy a significant amount. In a perfect situation I would run the saw out of mix then do the test. However this does not always happen as described. Ken
 
All good information however,,, every time you pull the rope on a saw with running or better compression numbers, you are drawing mix into the cylinder. Mix has oil and oil will not compress and this could raise the compression but not buy a significant amount. In a perfect situation I would run the saw out of mix then do the test. However this does not always happen as described. Ken

The amount of fuel in the incoming fuel/air mixture is minuscule and the difference will not be noticeable on a typical gauge.
Multiple pulls exchange the mix for new, don’t add more.
Not worth even considering.


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