Compression Question

[Farmer_Nate]

Some say hold a saw by the cord to get a general idea about compression.

One saw in particular of mine falls slowly when held up by the start cord, but will seem to almost break the pull cord if given a hard, sharp, quick yank. It seems to me compression at higher speed is more important.

Jenning's stuff online about rings shows pressure above the ring and some pressure on the interior ring face, forcing it outward against the cylinder. My thought is that a sharp pull on the cord forces the ring outward into a more realistic position, while simply hanging the saw by the cord could be misleading.

I guess that is how compression test are done, by pulling quickly on the start cord?

 

[Ted Jenkins]

I would answer that yes and no. Not sure that would help though. What is needed is a compression gauge to have a basic understanding as to the condition of the motor. Yes a OP can move the piston slowly with out much effort. Move the same piston rapidly and a whole other condition seems to be evident. In racing engines this is really the case when single ring pistons are used. They can be turned over easily by hand but not rapidly. One method is to look through the exhaust port to see if there is excessive blow by. Or just ask some one to borrow their compression gauge for a couple of hours. Often a reading better than eighty five LBS will indicate a usable engine. Eighty five LBS or less would indicate it needs attention. I have seen engines with 90 LBS run for quite some time but would prefer them to be at a hundred and twenty five or better. Thanks

 

[old guy]

I had a saw run on only 90lbs, that is unusual, just lately I had 2 that would not run with 100lbs.
I have a 372xp that was down to 125, it ran well but lacked power, it has a 2 ring piston, I replaced the top ring with a Caber ring & after initial run it was up to 159lbs, got the 372 snort back.

 

[ZeroJunk]

If you have a few running saws, pull the rope on them and see how it feels and how it sounds.
You should be able to tell whether a saw will run or not once you get a feel for it.
Now, is it 130 or 160 ? Who knows.
The drop by the cord test is not accurate by any stretch, they will all drop eventually.
So, then you get in to how fast does it drop and all that BS.

 

[ballisticdoughnut]

The “saw hang” test is a way to get a general idea of the mechanical condition, it’s by no means the be-all-end-all standard. What your really looking for there is how much pressure is leaking past the ring gap, or potentially stuck/broken ring, etc.

 

[Farmer_Nate]

Thanks y'all!

 

[lone wolf]

Holding an idle and not stalling and making good power in the cut and just overall running well is a sign of good compression.

 

[ZeroJunk]

Holding an idle and not stalling and making good power in the cut and just overall running well is a sign of good compression.

Yep. If a saw will sit and idle indefinitely there is nothing wrong with the compression.

 

[lone wolf]

Yep. If a saw will sit and idle indefinitely there is nothing wrong with the compression.

Pretty good indication the carb is well also.

 

[Farmer_Nate]

Never knew behavior at idle was an indication of compression. Nice.

 

[Farmer_Nate]

Saw in question runs and idles fine, by the way. It has been refreshed with a new ring and seals, so I was curious if compression goes up over time and how to tell...without a guage. From what I read, guages and methods of measure are not all that consistent nor accurate, anyway. Saw runs fine, so I don't care so much now.

 

[lone wolf]

Saw in question runs and idles fine, by the way.

Good to go then.

 

[lone wolf]

Saw in question runs and idles fine, by the way. It has been refreshed with a new ring and seals, so I was curious if compression goes up over time and how to tell...without a guage. From what I read, guages and methods of measure are not all that consistent nor accurate, anyway. Saw runs fine, so I don't care so much now.

Dont worry about it. It only gains about 5 PSI or so after ring break in ,you wont notice that difference by feel.

 

[Lightning Performance]

After decades of messing with engines one of the best ways to test your ring seal imho, besides a leak down tester is this, use a washed clean engine to test compression and then add oil directly to the cylinder and test again. Straight gas is a good wash down fluid to use that won't damage anything. Laugh all you want but it is the best method we found besides a leak down test with gauges and high pressure air feed. Both perform the same task one requires only a compression tester.

 

[Farmer_Nate]

Thanks y'all. I'll go run my saw while y'all do all that testing! Ha!

 

[Ted Jenkins]

Thanks y'all. I'll go run my saw while y'all do all that testing! Ha!

Or as most folks do is to run the saw until it will not start then decide to find out why. When you pull on the starter cord to discover this can be done with out much effort then it might be time for an overhaul. Thanks

 

[Farmer_Nate]

Yeah, gotta squish that air and gas to get a good boom.

 

[Lightning Performance]

Some say hold a saw by the cord to get a general idea about compression.

One saw in particular of mine falls slowly when held up by the start cord, but will seem to almost break the pull cord if given a hard, sharp, quick yank. It seems to me compression at higher speed is more important.

Jenning's stuff online about rings shows pressure above the ring and some pressure on the interior ring face, forcing it outward against the cylinder. My thought is that a sharp pull on the cord forces the ring outward into a more realistic position, while simply hanging the saw by the cord could be misleading.

I guess that is how compression test are done, by pulling quickly on the start cord?

View attachment 1045628

Checked a 7310 last night. No b&c, full of fluids with WCS big dogs. It takes 15 seconds cold to roll over hanging by it's starter rope. The weird part was it started right up ;-)

 

[Lightning Performance]

Yeah, gotta squish that air and gas to get a good boom.

No boom, faster burn is the desired effect and the only desired effect inside your engine. Firecrackers and guns go boom or bang. Gasoline burns rather nicely most times.

 

[Lightning Performance]

Free pro tip
If you hang it by the starter cord and it never rolls over worry.