Chainsaw Compression Question

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If this turns into some kind of war of opinions I am on your side. Besides it is the primary compression that is more important than the secondary compression . You need both for the engine to run properly . You can be a little weak on compression on the top end of the stroke but you can't be weak on the bottom end of the stroke.
How does that happen? Gauge will only maintain and read highest compression pumped into it, or does it measure something else?
 
Duce, I see you got ahold of my post after I edited it to save some controversial arguments. Primary compression is the condition of the seals . When you do a pressure /vac test on the bottom end you are measuring in a way your primary compression to see the integrity of the seals . Secondary compression is at the top end of the stroke which is measured with a regular compression gauge.

Have you ever seen any published compression numbers from Stihl??? You won't because they consider primary compression more important. I know it is confusing but it is what it is. I have been down this road before and kind of started a fire but I thought I would put it out before it spread.
 
Duce, I see you got ahold of my post after I edited it to save some controversial arguments. Primary compression is the condition of the seals . When you do a pressure /vac test on the bottom end you are measuring in a way your primary compression to see the integrity of the seals . Secondary compression is at the top end of the stroke which is measured with a regular compression gauge.

Have you ever seen any published compression numbers from Stihl??? You won't because they consider primary compression more important. I know it is confusing but it is what it is. I have been down this road before and kind of started a fire but I thought I would put it out before it spread.
That's no fun!!!
 
What about the location of the Schrader valve?
You still only get a highest reading on any gauge. Did not say it would be a good reading with a large volume adapter either, or location of valves. :crazy2:
 
I just looked at you tube. One guy, I gather he has a repair business, he does chainsaw compression tests both ways.

I guess no matter which way we stir the pot everyone is correct. Some stir the stew counter clockwise, some stir clockwise it tastes the same.
 
I'll jump in on the Schrader valve question. Two things you have to have to get an accurate compression reading on a small cylinder (such as a chainsaw).
(1) The right valve - one with a low opening pressure, about 1 psi, not one from a tire, and (2) the right location - at the spark plug attachment, not at the gauge end of the hose. Basic physics: PV=nRT, the ideal gas law. When the valve is at the end of the hose, you have added the volume of the hose to the compression volume. No matter how many times you pull, compression will level off at a lower value than if the valve is at the plug. Why do gauges generally have the valve at the end of the hose? (1) Convenience, the pressure release valve, (2) the gauges are usually used on cars - how much difference will the added hose volume change the compression reading on a 427 cu.in. V8? Not much; on a chainsaw, enough to throw the reading way off.
 
Nope, how do you stop air from coming in through exhaust port?

Seriously? You simply leave the test blind on!

I once did a compression test right after I vac & pressure tested an 056 Magnum that I just rebuilt.

Scratched my head for a few minutes, as I was way short of the expected 150 psig.

If you’re totally blinded like that, it’s makes a huge difference on a 2 stroke engine, because it’s starved for atmospheric air.

The very first pull is as high as you’ll get on the gauge.
 
Seriously? You simply leave the test blind on!

I once did a compression test right after I vac & pressure tested an 056 Magnum that I just rebuilt.

Scratched my head for a few minutes, as I was way short of the expected 150 psig.

If you’re totally blinded like that, it’s makes a huge difference on a 2 stroke engine, because it’s starved for atmospheric air.

The very first pull is as high as you’ll get on the gauge.
Have not done that yet, but give me time. Now, I understand how it could be done.
 
Thanks :) just realised the piston passes the air intake and thus that’s where the compression is created... rookie question!

Got this from Supercheap Auto, which here in Australia is basically a car parts shop. It doesn’t work.. reads like 25psi on my brand new saw and the exact reading on my old one that has half the compression ! It’s going back tomorrow and have ordered one front “All mower spares” it’s twice the price, but hopefully works!!

Tom, when your brand new saw only gave a 25 psi reading- was the decomp valve depressed?
Cheaper gauges can be used- may not give a true reading, but if you set them up on the powerhead correctly and pull a few times until the needle will not go any higher on a known good saw like your new one- usually you can get a benchmark for that gauge. Equally if you know you have a saw not making compression, you can get a no go benchmark pressure for the same gauge.
But 25 psi on a new saw, I suspect something was not sealing well- or a goofed from the factory gauge.
 
Tom, when your brand new saw only gave a 25 psi reading- was the decomp valve depressed?
Cheaper gauges can be used- may not give a true reading, but if you set them up on the powerhead correctly and pull a few times until the needle will not go any higher on a known good saw like your new one- usually you can get a benchmark for that gauge. Equally if you know you have a saw not making compression, you can get a no go benchmark pressure for the same gauge.
But 25 psi on a new saw, I suspect something was not sealing well- or a goofed from the factory gauge.
Hey Bob, na not compressed. Took the thug apart to check for leaks but everything seems fine. Looked at reviews online seems like more duds than successes for that model. I have gotten a new one from All Mower Spares. Hopefully I have better luck!! :)
 
Hey Bob, na not compressed. Took the thug apart to check for leaks but everything seems fine. Looked at reviews online seems like more duds than successes for that model. I have gotten a new one from All Mower Spares. Hopefully I have better luck!! :)

Supercheap Auto mate....... super cheap is not always super good.
Be interesting to see what kind of reading the new gauge gives you on the new saw.
 
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