Poor man's compression test?

handyrandyrc · Dec 9, 2009

Just talked to a two-stroke guru here at work. Now, he mostly works on snowmobiles and dirt bikes, but he said the principle is the same. At this altitude, he says he likes to have 120 psi for 'good' compression on a 2-stroke.

He told me the easiest way for him to do a quick test is to pull the spark plug and turn the crank over. If he can hold his thumb over the plug hole, he knows compression is dead. If air is pushed out, he knows compression is good. Now he said he's used to 200-400 cc 'lungs' so there is more volume of air. However, our thinking was 'pressure is pressure', right?

120 psi is 120 psi whether it's 50cc air volume or 400cc volume, right? Or am I missing something else here?

mtngun · Dec 9, 2009

Pressure is pressure.

But ..... what's so hard about using a compression gage ?

bcorradi · Dec 9, 2009

120# of compression may be good on a snowmobile, but not on a saw.

StihlyinEly · Dec 9, 2009

mtngun said:
But ..... what's so hard about using a compression gage ?

Nothing. If you have one. Given the title of the thread, my guess is he doesn't have access to one and, even if he does, it's useful information for those of us who don't.

handyrandyrc · Dec 9, 2009

Well, that answers my question on the compression on this Poulan 4200 I am working on.

I can hold my thumb over the hole and rotate the flywheel by hand. I feel pressure build under my thumb, but I can definitely hold it over the plug hole without leaking any.

I'm gonna have to pull the head off and see if piston/rings can be done.

93Dakman! · Dec 9, 2009

And that depends on how fast you turn the crank too. I know my 6401BB has more compression than a Diesel volvo. Its still hard to pull even with the decomp pushed in. But I can turn it slowly with my thumb over the hole and no air pushes past.

mowoodchopper · Dec 9, 2009

Compression tester World !

It should have its own forum and I already named it! LOL

I know alot of guys on here love their compression testers and their minimum
compression reading for a saw to be a runner.
IMO its pretty much pointless , I you cant tell from pulling over a saw and the way it runs if it has good compression, you should get an electric saw and then you dont have to worry. Also all of these saws that are torn down or junked because they blow under 150 what a waste . And the myth a saw wont run with under 120 is just that a myth. Its very easy to start a saw and see if it is strong or not.

Next I'm gonna invent a chain sharpness gauge, it will be computer operated and it will scan your chain cutters and if it says no good you just throw it away. It will save all that silly stuff like feeling your chain or trying it in wood that's all a thing of the past. If you dont have a sharpness tester before long you will be a rookie!

Slamm · Dec 9, 2009

mowoodchopper said:
Compression tester World !

It should have its own forum and I already named it! LOL

I know alot of guys on here love their compression testers and their minimum
compression reading for a saw to be a runner.
IMO its pretty much pointless , I you cant tell from pulling over a saw and the way it runs if it has goof compression, you should get an electric saw and then you dont have to worry. Also all of these saws that are torn down or junked because they blow under 150 what a waste . And the myth a saw wont run with under 120 is just that a myth. Its very easy to start a saw and see if it is strong or not.

Next I'm gonna invent a chain sharpness gauge, it will be computer operated and it will scan your chain cutters and if it says no good you just throw it away. It will save all that silly stuff like feeling your chain or trying it in wood that's all a thing of the past. If you dont have a sharpness tester before long you will be a rookie!

Yeah !!!!! What he said ........... Who needs compression anyways?? I think its over rated at best.

Instead of making pop-up pistons and increasing compression, I'm going to dish out my pistons to lower the compression. I'm sure they will run a lot strong after that.

A lot of times when I compression test a saw and its running over 150 psi. I'll dishout the piston to bring the static compression down to something more mangeable like 120 psi. It makes for an easier starting saw, while simultaneously increasing the amount of time I get to spend running my beloved chainsaws, because it takes longer to get anything cut. Perfect concept.

Thats what I do,

Sam

mowoodchopper · Dec 9, 2009

Slamm said:
Yeah !!!!! What he said ........... Who needs compression anyways?? I think its over rated at best.

Instead of making pop-up pistons and increasing compression, I'm going to dish out my pistons to lower the compression. I'm sure they will run a lot strong after that.

A lot of times when I compression test a saw and its running over 150 psi. I'll dishout the piston to bring the static compression down to something more mangeable like 120 psi. It makes for an easier starting saw, while simultaneously increasing the amount of time I get to spend running my beloved chainsaws, because it takes longer to get anything cut. Perfect concept.

Thats what I do,

Sam

Thanks for the great idea, I'm gonna lower all my saws compression.
I think Ill just drill a small hole in the top of the cyl, this compression stuff sucks!

blsnelling · Dec 9, 2009

Pressure may be pressure, but the chainsaw will move much less volume, making it easier to hold back.

Slamm · Dec 9, 2009

mowoodchopper said:
Thanks for the great idea, I'm gonna lower all my saws compression.
I think Ill just drill a small hole in the top of the cyl, this compression stuff sucks!

You're too late I already have the cordless drill with an 1/8" drillbit into the top of my 084 ........ whoa, wait ................ There done............................................. Now lets try it, <Pull <Pull <Pull .... Hey this is great, it pulls over a lot easier now.

What a great idea,

Sam

mowoodchopper · Dec 9, 2009

Slamm said:
You're too late I already have the cordless drill with an 1/8" drillbit into the top of my 084 ........ whoa, wait ................ There done............................................. Now lets try it, <Pull <Pull <Pull .... Hey this is great, it pulls over a lot easier now.

What a great idea,

Sam

Dang I was gonna patent that!!

stipes · Dec 9, 2009

On a 80cc saw...

handyrandyrc said:
Just talked to a two-stroke guru here at work. Now, he mostly works on snowmobiles and dirt bikes, but he said the principle is the same. At this altitude, he says he likes to have 120 psi for 'good' compression on a 2-stroke.

He told me the easiest way for him to do a quick test is to pull the spark plug and turn the crank over. If he can hold his thumb over the plug hole, he knows compression is dead. If air is pushed out, he knows compression is good. Now he said he's used to 200-400 cc 'lungs' so there is more volume of air. However, our thinking was 'pressure is pressure', right?

120 psi is 120 psi whether it's 50cc air volume or 400cc volume, right? Or am I missing something else here?

At the GTG I went to and a friend tryed to drop start my sp80 without the decomp on and before I yelled stop,,,,it about flew outta his hands....Thats compression...

stipes · Dec 9, 2009

Be like me...

Drop starting Brad's 090....End up with a broken arm,,or black eye......LOL!!!

blsnelling · Dec 9, 2009

stipes said:
Drop starting Brad's 090....End up with a broken arm,,or black eye......LOL!!!

I do it

You just have to "mean it"!

mtngun · Dec 9, 2009

bcorradi said:
120# of compression may be good on a snowmobile, but not on a saw.

The OP has the misfortune of being in Rexburg, Idaho, at 4800 feet elevation. The rule of thumb is you lose 3% compression/power per 1000 feet, so a saw that blows 120 psi in Rexburg might blow 140 psi at sea level.

Force on thumb = pressure times area of spark plug hole. As long as the spark plug hole size is the same, force on thumb at a given pressure will be the same regardless of engine displacement.

But, I prefer to use a compression gage.

Rookie1 · Dec 9, 2009

handyrandyrc said:
Well, that answers my question on the compression on this Poulan 4200 I am working on.

I can hold my thumb over the hole and rotate the flywheel by hand. I feel pressure build under my thumb, but I can definitely hold it over the plug hole without leaking any.

I'm gonna have to pull the head off and see if piston/rings can be done.

Heres a backyard test Randy. I assume you poulan wont start because of low compression right. Take out the plug and squirt some oil into cylinder. The put plug back and try. Oil will temporarily seal rings and give good compression.

StihlyinEly · Dec 9, 2009

mtngun said:
The OP has the misfortune of being in Rexburg, Idaho, at 4800 feet elevation. The rule of thumb is you lose 3% compression/power per 1000 feet, so a saw that blows 120 psi in Rexburg might blow 140 psi at sea level.

Thanks for that info, gun. I cut for about a year at 9-10,000 feet in Colorado and could never remember the elevation compression tradeoff figures.

gee_dubya75 · Dec 12, 2009

where should I get a compression tester?

Is this one at Baileys decent?

http://www.baileysonline.com/itemdetail.asp?item=G+320HD&catID=

I'm thinking about buying a used saw this next week and would like to check the compression before I buy it. What kind of compression should a Makita DCS 601 have?

Where are some places that one could by a compression tester locally? Does Autozone carry something like this?

handyrandyrc · Dec 13, 2009

I picked up this one from Amazon for 11 dollars. Tried it out today and have 130-ish PSI on the old Poulan. It has a rubber nipple that will fit in any spark plug hole, and a valve that holds the pressure reading.

Link to Amazon

Poor man's compression test?

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