Will the Real Compression Test Please Stand Up!

[Baconaman]

Just wondering if there is a generally accepted way to perform a compression test on a saw. Should it be done cold or after running the saw? Is there s min/max number of pulls? I read where some people write things like the choke was on or the choke was off or this or that is open or closed. I guess if the saw won't start then you have to take a reading when it's cold. So being cold, how does that affect the reading compared to being warm? I guess I would like to know how to do a compression test so that I get the most meaningful reading.

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[Baconaman]

Just wondering if there is a generally accepted way to perform a compression test on a saw. Should it be done cold or after running the saw? Is there s min/max number of pulls? I read where some people write things like the choke was on or the choke was off or this or that is open or closed. I guess if the saw won't start then you have to take a reading when it's cold. So being cold, how does that affect the reading compared to being warm? I guess I would like to know how to do a compression test so that I get the most meaningful reading.

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Try using google you idiot!

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[redunshee]

Not rocket science. Pull till needle stops upward movement. Test cold saw. Think about it. Your testing the amount of compression in the cylinder. Carb, throttle, muffler have nothing to do with it.

 

[Little Al]

I'm not sure but have always understood the accepted practice is saw cold Throttle wide open (Make sure the correct shrader core is fitted to compression tester)& pull until the gauge stops rising usually half dozen or so. Others may do it differently

 

[alexcagle]

Try using google you idiot!

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I always say in mechanics; there is no such thing as a stupid question.
There are stupid answers though......

 

[alexcagle]

Not rocket science. Pull till needle stops upward movement. Test cold saw. Think about it. Your testing the amount of compression in the cylinder. Carb, throttle, muffler have nothing to do with it.

I disagree.
If your filter is plugged it will restrict the amount of air allowed to flow into the combustion chamber during primary compression. Just as a plugged exhaust port or muffler would result in a lower compression reading.
Wide open throttle should be used, and with the choke off, to allow for full airflow as well.

 

[Little Al]

Not rocket science. Pull till needle stops upward movement. Test cold saw. Think about it. Your testing the amount of compression in the cylinder. Carb, throttle, muffler have nothing to do with it.

As I have always understood it the cylinder of whatever you are doing a compression test on requires something to compress mainly air but a 1/14 ratio of fuel mix to the air that is introduced into the cyl if opening the throttle gets to let the cyl fill quicker that is good,maybe removing the muffler would help if it's blocked but that would give a false result/reading as the saw usually runs with it fitted & the purpose is to determine the amount of compression produce to again determine the mechanical condition of the cyl, piston, ring/s etc

 

[HarleyT]

Just pull the damn rope a bunch of times, until the needle stops going up!

But the saw tip must be pointed toward Mecca,

of course!!!!!!!

 

[kevin j]

Two stroke engines draw air backwards through the exhaust so it does not matter.

Crankcase and transfer don’t matter at these low cranking speeds, it only compresses air once the transfers close.

Four stroke engines wityh valves, yes need to have the carb wide open as that is where the air comes from

Having a LIGHT spring schrader valve, not a tire valve, at the very tip of the hose does matter a lot. The cracking pressure of the spring will subtract from gauge reading, and if no valve, the volume of the hose and gauge add to combustion chamber volume and you essentially have a very low compression ratio.

Harley T is mostly correct, but some saws need to be pointed towards Germany or Sweden.

 

[HarleyT]

The carbs pointed toward CHINA!!!!

 

[Baconaman]

Two stroke engines draw air backwards through the exhaust so it does not matter.

So if I am understanding this correctly, you are saying that it is unnecessary to have the throttle wide open when checking compression in a saw. If this is correct, would it affect the reading at all by having the throttle open while cranking?

The compression tester I have came right off the wall at a NAPA auto parts store and the valve that was on it was the right size for my saws. It seemed to mention testing small engines as part of it's designed purpose but I don't remember any mention of it having a "Light" spring on the valve. At what psi reading on a used saw should one become concerned and start thinking about a repair? What psi reading should one expect after rebuilding a top-end? If there is a leak in the crankcase does that affect compression?

I should know these things and think I do on most of them but just trying to verify what I am thinking so it's settled in my brain.

Thanks

 

[wcorey]

The throttle open/closed should have no effect on 2 stroke comp reading.

A new comp tester should come equipped with the light valve, it's not an 'option'. It's possible used ones may have had the wrong type installed at some point.

Though some old reed saws were 100psi, with most saws 150 +/- is more typical for a healthy number.
To some degree judging the health of the piston/rings/bore by comp numbers would depend on what the saw was originally.
There's some that come factory at 200 and some at 130, so it's really relative.
If it's down 10-15 psi, somethings going on, maybe just worn rings but it warrants investigation, at least taking a peak down the exhaust port.

Fresh rings/bore can read a bit low before seating but usually only like 5psi, your mileage may vary...

Leaking cc seals do not effect comp reading, at least not directly, leaking decomp valve, scoring, large ring gap, loose plug will.

 

[dswensen]

Two stroke engines draw air backwards through the exhaust so it does not matter.

Crankcase and transfer don’t matter at these low cranking speeds, it only compresses air once the transfers close.

Four stroke engines wityh valves, yes need to have the carb wide open as that is where the air comes from

Having a LIGHT spring schrader valve, not a tire valve, at the very tip of the hose does matter a lot. The cracking pressure of the spring will subtract from gauge reading, and if no valve, the volume of the hose and gauge add to combustion chamber volume and you essentially have a very low compression ratio.

Harley T is mostly correct, but some saws need to be pointed towards Germany or Sweden.

I don't understand this part. "The cracking pressure of the spring will subtract from gauge reading ". I would agree that, if you have a stiffly sprung Schrader valve, the gauge will read "zero" until the pressure in the cylinder is greater than the pressure it takes to overcome the spring. However, it seems to me that once the spring allows the valve to open, the pressure read will have nothing to do with the Schrader valve any more.

What am I missing?

 

[John Stryker]

I don't understand this part. "The cracking pressure of the spring will subtract from gauge reading ". I would agree that, if you have a stiffly sprung Schrader valve, the gauge will read "zero" until the pressure in the cylinder is greater than the pressure it takes to overcome the spring. However, it seems to me that once the spring allows the valve to open, the pressure read will have nothing to do with the Schrader valve any more.

What am I missing?

It makes sense to me. A valve with say a 10 psi spring would close before that final 10 psi could enter, so it would read 10 psi lower. My experience is that a car tire schrader valve lowered the compression reading by about 30 psi ; it just didn't work.

 

[blsnelling]

1. The gauge MUST have a Schrader type valve at the very tip of the hose where it screws into the cylinder. No adapter can be used.
2. The engine must be cold. Testing hot will result in approximately 10 PSI less.
3. Pull the engine over until the gauge won't go any higher. This will vary depending on displacement of the saw and length of the hose. There is no set number.
4. The throttle does not need to be held open on a piston ported chainsaw engine.
5. Don't waste your time testing an engine that has just been assembled. It should be run, then allowed to cool off completely.

 

[dswensen]

It makes sense to me. A valve with say a 10 psi spring would close before that final 10 psi could enter, so it would read 10 psi lower. My experience is that a car tire schrader valve lowered the compression reading by about 30 psi ; it just didn't work.

Hmmm. Gonna have to go stare at my gauge again. I'm thinking it's just the opposite. In your example, I'm thinking the valve won't OPEN until the pressure reaches 10PSI. It's not the last 10 PSI that is the issue, it's the first 10 PSI. If your cylinder generates 6 PSI, the 10 PSI spring will never allow the valve to open, but once it reaches 10PSI, the valve opens and allows air to enter the gauge.

Not trying to argue - just not seeing it.

 

[kz1000]

All of it depends, are you selling it to me on ebay? If so, add three shots of gear oil to the cylinder, then connect a hose from a 175psi compressor to the carburetor opening and plug the exhaust (one crank is all that is necessary turning slow) take reading and add 30psi for each 100 feet over sea level then put the results in the sales ad.

 

[HarleyT]

synthetic

All of it depends, are you selling it to me on ebay? If so, add three shots of gear oil to the cylinder, then connect a hose from a 175psi compressor to the carburetor opening and plug the exhaust (one crank is all that is necessary turning slow) take reading and add 30psi for each 100 feet over sea level then put the results in the sales ad.

Synthetic I hope!!!

 

[kz1000]

Of course! The readings are artificial!!!

 

[wcorey]

Hmmm. Gonna have to go stare at my gauge again. I'm thinking it's just the opposite. In your example, I'm thinking the valve won't OPEN until the pressure reaches 10PSI. It's not the last 10 PSI that is the issue, it's the first 10 PSI. If your cylinder generates 6 PSI, the 10 PSI spring will never allow the valve to open, but once it reaches 10PSI, the valve opens and allows air to enter the gauge.

Not trying to argue - just not seeing it.

I works on the same principal as a pressure regulator...