What's Considered Low Compression?

[250R]

When do I know when it's time to do the PC? I mean other than scoring of the sleeve and piston. How low can the saw go w/compression before rebuild? And does the size or CC make a difference?

 

[pioneerguy600]

For me, anything below 120 on a good gauge one can trust.

 

[Woody912]

When do I know when it's time to do the PC? I mean other than scoring of the sleeve and piston. How low can the saw go w/compression before rebuild? And does the size or CC make a difference?

When it will start on choke but not run. Correct compression gauge is a must, probably 100 psi or a little more is too low

 

[Timber MacFallen]

The official stance by Stihl is that anything under 110psi is considered low compression (minimum required) on their saws. However, a quality gauge used in the correct manner is essential.

Some saws run higher compression than others when new, but anything under 110psi is suspect in my experience.

 

[Jasonrkba]

Saws will try to run at 90 but you need 120 minimum. Just been my experience.

 

[pioneerguy600]

Reed valve saws could run lower compression and still be usable but the newer piston ported saws start to suck when they drop below 120psi, the rings and or the piston is badly worn when the comp drops below that.

 

[RED-85-Z51]

It varies saw to saw, but a modern howowner saw may not make 120psi new. An older saw in good shape might make 125. Quality modern saws i like to see 150+. Does that mean it wont run fine at 140, no...but its not ideal

Sent from my LM-G820 using Tapatalk

 

[RED-85-Z51]

Reed valve saws could run lower compression and still be usable but the newer piston ported saws start to suck when they drop below 120psi, the rings and or the piston is badly worn when the comp drops below that.

My 330 is 105psi, old reed motor. Starts fine, idles fine...but you can tell its leaving alot on the table cutting.

Sent from my LM-G820 using Tapatalk

 

[stihl86]

When do I know when it's time to do the PC? I mean other than scoring of the sleeve and piston. How low can the saw go w/compression before rebuild? And does the size or CC make a difference?

Some people swear by compression gauges. Some (me being one) swear at them.
So, look at the posted numbers. Pick one.

 

[Timber MacFallen]

I think one of the reasons people have so much trouble with compression gauges is that it’s one of those tools every assumes they know how to use correctly- but don’t. I saw this in the automotive world when I technician years ago.

If you’re using a decent gauge from a reputable company with some semblance of quality control (Snapon, Mityvac, Motion Pro, OTC, etc) then you should be able to measure relative performance correctly between saws and should be reading within 5psi across gauges. Performance should be repeatable and consistent.

My suggestions:

1) Decide on a warm or cold test as needed (most do cold).
2) Ensure the throttle plate is fixed FULLY open and choke is completely disengaged. None of this “I’m going to try to squeeze the trigger with one hand while I yank with the other” nonsense.
3) Ensure compression release is completely disengaged.
4) Pull aggressively on starter cord in the same manner you would start the saw. Continue pulling on cord until max psi is reached. May take 8-10 pulls.
5) Release pressure and repeat to ensure gauge is reading consistently if it’s an unknown variable. Lack of consistency is a bigger problem than being off in the reading 5psi.

 

[stihl86]

I think one of the reasons people have so much trouble with compression gauges is that it’s one of those tools every assumes they know how to use correctly- but don’t. I saw this in the automotive world when I technician years ago.

If you’re using a decent gauge from a reputable company with some semblance of quality control (Snapon, Mityvac, Motion Pro, OTC, etc) then you should be able to measure relative performance correctly between saws and should be reading within 5psi across gauges. Performance should be repeatable and consistent.

My suggestions:

1) Decide on a warm or cold test as needed (most do cold).
2) Ensure the throttle plate is fixed FULLY open and choke is completely disengaged. None of this “I’m going to try to squeeze the trigger with one hand while I yank with the other” nonsense.
3) Ensure compression release is completely disengaged.
4) Pull aggressively on starter cord in the same manner you would start the saw. Continue pulling on cord until max psi is reached. May take 8-10 pulls.
5) Release pressure and repeat to ensure gauge is reading consistently if it’s an unknown variable. Lack of consistency is a bigger problem than being off in the reading 5psi.

I don't think it's the testing procedure. Its the figure that results.
Coming from automotive, I think you would agree that a compression
test involves all cylinders, not just one. It's for a comparison. Do manufactures supply min/max compression figures? Not many small engine manufactures do anymore.
Leakdown tests have overtaken compression tests.
TBH, I don't know how well they work on 2 cycles.
Believe me, I work on saws every day.
If there was a consistent way to test powerhead integrity quickly, I would.
I've been fooled by numbers near 120 and found scoring.
I pull mufflers on every unit. Especially when recoil ropes are broke!!!LOL
The only way I would use a gauge is with a dry/wet test.
And only very rarely at that.

 

[Kenskip1]

What has not been mentioned is that every time the rope is pulled even with the gage hooked up is that mix is being drawn into the cylinder, thereby raising the compression a bit, unless the compression is so low that it will not draw the mix in the first place. Think about it. Ken

 

[Timber MacFallen]

I don't think it's the testing procedure. Its the figure that results.
Coming from automotive, I think you would agree that a compression
test involves all cylinders, not just one. It's for a comparison. Do manufactures supply min/max compression figures? Not many small engine manufactures do anymore.
Leakdown tests have overtaken compression tests.
TBH, I don't know how well they work on 2 cycles.
Believe me, I work on saws every day.
If there was a consistent way to test powerhead integrity quickly, I would.
I've been fooled by numbers near 120 and found scoring.
I pull mufflers on every unit. Especially when recoil ropes are broke!!!LOL
The only way I would use a gauge is with a dry/wet test.
And only very rarely at that.

Hmm, I think I understand what you're saying, but my understanding of these tests and applications may be different.

Compression tests in automotive identify cylinder variance (what is the difference from the best, worst and average across cylinders) and they identify degradation from your baseline. These tell me two different things. If baseline on a straight six engine is 140 psi and all cylinders are at 120 psi I see an evenly wearing engine. If five of the six cylinders are at 130 psi and one is at 105 psi I have an issue on that cylinder and need to investigate with a leak down test. Specs should be readily available on vehicles.

The leakdown tester will help me identify if I am losing pressure through the exhaust valve, intake valve, head gasket (coolant bubbles) or past the piston. A more advanced issue might be premature valve guide wear which is causing a seating issue on the valve. These tests are extremely valuable on precise multi-valve engines which are sensitive to these problems.

A leakdown test is usually performed after a compression test if there is an identified issue. If you have excellent compression on all cylinders you normally wouldn't be looking for an issue. This is especially true because a compression test is quick and inexpensive to perform while a leakdown test is time intensive and requires getting your crank position and valve timing correct, locking the crank, pressurizing the cylinder properly and then setting it all up again for the next cylinder. It can be time intensive.

In summary, these test measure different things and they can't replace each other. One is much easier to do though.

As for finding scoring and showing a compression of 120. Those things seem fine together. A Stihl or Husky that should be reading 150 psi can easily drop to 120 psi or less with scoring present.

A compression test won't tell you anything except your compression. Scientifically speaking, there is a minimum to create combustion, but beyond that you're measuring your lost potential (if any). I think the value is in trouble shooting and confirming that something isn't right. If spark and fuel seem to check out I go to compression. If compression is strong then I'm wondering if I have a sheared flywheel key and timing is off, etc.

 

[Cantdog]

All depends of the time frame of manufacture and brand. I have some old saws from the early 60's that make, like 70-80 lbs of compression and start and run fine...as good as they ever did. A modern saw ......like from the 70's to present really should make 150 with 140 being the sign you should be buying parts. IMHO.....at 140 you are on the back side of the life of you engine.......likely from excessive end gap on the ring. On the other hand my Jonsereds 80 will pull 238 lbs any time you test it.....and it's pretty well used and it's 45 years old...these old tall domed piston, high compressions saws were certainly an eye opener when they arrived in America in the 70's. Sadly to say they really put the domestic brands to shame.......at least the domestic saws that were below 100cc.....

 

[stihl86]

In response toTimber: Yes, yes and yes. I agree, but look at the responses. How many suggestions are there?
The OP is asking a direct question. He's been given 9 answers.
Which is correct? For each poster's procedures, experience, saw and tool, they all might be. That's the problem with a compression test on a single cylinder.
And, in regards to leakdown. Being involved with Kohler, Briggs, Kawasaki and Honda, all reject use of a compression gauge except for compression release problems.
For the average of 15 minutes, for most saws, removing the muffler and a visual inspection of piston, ring to groove movement and cylinder inspection, both intake with plug removed and exhaust is a far better test.
Also gives you a look at possible carbon buildup.
But, whether using a gauge, a visual or leakdown there's a certain amount of experience involved. Meaning, is a compression test even needed?
The "Hang it by the Rope" trick might be as accurate as any gauge when you have a unicorn for a baseline. (Yes, use another saw, if you have one, but that's a comparison test, isn't it?)

That's what works for me. Just like the 9 different suggestion on compression readings works for those guys.
A difference in opinion.

 

[Cantdog]

In response toTimber: Yes, yes and yes. I agree, but look at the responses. How many suggestions are there?
The OP is asking a direct question. He's been given 9 answers.
Which is correct? For each poster's procedures, experience, saw and tool, they all might be. That's the problem with a compression test on a single cylinder.
And, in regards to leakdown. Being involved with Kohler, Briggs, Kawasaki and Honda, all reject use of a compression gauge except for compression release problems.
For the average of 15 minutes, for most saws, removing the muffler and a visual inspection of piston, ring to groove movement and cylinder inspection, both intake with plug removed and exhaust is a far better test.
Also gives you a look at possible carbon buildup.
But, whether using a gauge, a visual or leakdown there's a certain amount of experience involved. Meaning, is a compression test even needed?
The "Hang it by the Rope" trick might be as accurate as any gauge when you have a unicorn for a baseline. (Yes, use another saw, if you have one, but that's a comparison test, isn't it?)

That's what works for me. Just like the 9 different suggestion on compression readings works for those guys.
A difference in opinion.

Well the OP asked a very general question.......didn't give a particular saw or time frame to address or to ask advice on......there are many answers if there are many options....

But I agree... the redneck drop test is not to be discounted.....it will give you a useable bit of information.....I use it a lot when a saw comes in with problems....

 

[Timber MacFallen]

Stihl86- To be clear, I agree with pulling a muffler and inspecting things. Also, you’re correct that we’re seeing a lot of opinions and most probably work for that individual. However, if someone asked what a typical valve lash setting is on a lawnmower we’d probably get something similar in terms of replies.

We probably should have all told the original poster that it depends and to give us a specific saw. However, I suspected he wanted a range and to understand cutoffs. Most of us would agree that 130-165psi is where you’ll find many 21st century saws that range from entry-level to pro. If we know the model though we should be able to find some specs.

 

[stihl86]

Stihl86- To be clear, I agree with pulling a muffler and inspecting things. Also, you’re correct that we’re seeing a lot of opinions and most probably work for that individual. However, if someone asked what a typical valve lash setting is on a lawnmower we’d probably get something similar in terms of replies.

We probably should have all told the original poster that it depends and to give us a specific saw. However, I suspected he wanted a range and to understand cutoffs. Most of us would agree that 130-165psi is where you’ll find many 21st century saws that range from entry-level to pro. If we know the model though we should be able to find some specs.

Agreed

 

[ChoppyChoppy]

On the other hand my Jonsereds 80 will pull 238 lbs any time you test it.....and it's pretty well used and it's 45 years old...these old tall domed piston, high compressions saws were certainly an eye opener when they arrived in America in the 70's. Sadly to say they really put the domestic brands to shame.......at least the domestic saws that were below 100cc.....

That's around a 16:1 compression ratio.
Usually 10-12:1 is about max on pump gas.

 

[Cantdog]

That's around a 16:1 compression ratio.
Usually 10-12:1 is about max on pump gas.

This has been debated many times about what is and what theory dictates. All I know is that is the actual compression figure on this engine. And if you don't start this saw with authority and it kicks back it WILL pull the handle through your fingers painfully. The Jonsereds 621 will pull around 220 consistently as well....that is what these saws do if they are in decent condition.