031av compression

[BoiseSaw]

i checked the compression on my 031... i was 130 after three good pulls... is that good?

 

[Lakeside53]

Not great, but good enough to start and run. Does it increase with say 5 or more pulls? I just pull away until it won't rise any more...

 

[BoiseSaw]

do you think it needs rings? i have had the muffler off and the piston and cyl looked fine.

 

[Lakeside53]

do you think it needs rings? i have had the muffler off and the piston and cyl looked fine.

Probably not. Does it rise if you pull it over more times?

 

[BoiseSaw]

just a touch.... id say maybe 135... but no more.... should the check be done cold or hot?

 

[Lakeside53]

just a touch.... id say maybe 135... but no more.... should the check be done cold or hot?

Hot/cold - doesn't really matter - might vary a little. If the saw was in great condition, it MIGHT get to 150. 145 is typical of these old saws.
if you're going to restore the saw, then rings for sure, but if all you want to do is cut wood now and then, I'd say use it. I see lots of 031 that run just fine with 135..

 

[BoiseSaw]

thanx Lakeside53

 

[RaisedByWolves]

I often wonder if people know you should hold the throttle wide open when doing a compression test.

Throttle at wot vs at idle will make a huge difference. 20-50 psi

Hot or cold is simply hot or cold, though cold off the shelf after sitting for a day or week will matter.

 

[Lakeside53]

I often wonder if people know you should hold the throttle wide open when doing a compression test.

Throttle at wot vs at idle will make a huge difference. 20-50 psi

Hot or cold is simply hot or cold, though cold off the shelf after sitting for a day or week will matter.

Huh? You want to explain why holdling the throttle open will make any difference?

 

[RaisedByWolves]

If you dont hold the throttle open, the saw cant breathe. If it cant breathe it will have nothing to compress.

 

[TimberPig]

BoiseSaw, don't forget you are not at sea level, you have some altitude at your location. Normal compression loss is 3% per 1000 ft of elevation. Since you are at ~2700 feet, this gives 2.7*0.03 = 1.081* the compression reading, to give sea level compression. This means at sea level, you would have 146 psi.

Also, on some engines, the carbs do provide a significant restriction to air intake when closed, and can cause significantly lower compression readings if not held wide open. Others may not be as affected by whether the throttle is open or closed.

 

[Lakeside53]

If you dont hold the throttle open, the saw cant breathe. If it cant breathe it will have nothing to compress.

Never seen it on a stihl... even a "closed" carb breathes enough to compress the crankcase air... and replenish the crankcase so there is no vacuum.

 

[RaisedByWolves]

Here, read these while I go get some actual numbers from my 925....

http://www.volvoadventures.com/howtodocompressiontest.html

http://faq.f650.com/FAQs/CompressionTestingFAQ.htm

http://www.geocities.com/SiliconValley/2072/carcomp.htm

http://www.dansmc.com/compression_test.htm

 

[RaisedByWolves]

"K" Im back.

PHEW.... Man that thing is a biotch to crank when its cold.

Ok, First I warmed the saw up to get rid of any latent sludgy oil in the rings. Without doing this Im getting 225psi:jawdrop: of compression.

Then I put the guage on (the screw in type, not the silly one with the rubber nipple) Pulled her over vigourosly 4-5 times and got 175psi.

Next I straddled it with one knee on top, my foot in the handle, my left middle finger on the trigger to hold it open fully and my free hand between my legs(At this point I look like Im playin twister by my self in the gerage)(mostly cause Im on that funny mat) And pulled vigorously again to get a reading of ........TADA!! 195psi.


Stihl or no stihl, when the throttle is closed, most of the air thats being compressed is coming in through the muffler,Rember, theres only reeds on one side of the engine. traveling down the boost ports and entering the crancase thusly.

When the throttle is closed the engine draws a vacuum on the throttle plate, witch causes less air/fuel to enter the combustion chamber (After its little detour in the crancase) thereby allowing the engine to run at different speeds depending on how much air/fuel it is getting.

If the engine recieved the same amount of air/fuel at all times it wouldnt be able to run at more than one speed.

And, Yes, I realize the examples I posted are of four strokes,(First things I found, sorry) but the procedure remains the same for even wankel engines, it just differes in importance on different types, ie on two strokes you will get a lower reading, on 4 strokes you may wreck your guage.

Eric

 

[retoocs555]

I'd say 130psi is pretty good for an old 031 and not to worry

 

[TimberPig]

RaisedByWolves, just so you know, most chainsaws aren't reed valve engines, they are mostly piston port motors. The piston blocks the intake port, to take the function of the reed valve to allow crankcase compression to push the air/fuel charge up into the cylinder through the transfers.

 

[RaisedByWolves]

Only speaking from the experience that I have as I prefer the old school saws.

Most of mine are reed engines, see sig.

I do wonder if this isint even more important with the piston ports, however I suspect that port timing between different saws will cause things to vary somewhat also.

Lots of variables I guess.

 

[Pollock777]

With 130 lbs. compression the saw should, with good care,run for a few more years. Depends how much you use it. I've had an 028 with 130lbs. and I bought it that way 3years ago and it still has 130 lbs. compression and I use the hell out of it. I'm waiting for it to go so I can put a new piston and jug on it. It has 20" bar I don't cut big stuff with it. I mostly run 20 and 24 inch bars. All my climbing saws 14 inch and my bid saws 36-42 inch. And it still cuts great. If it's cutting good and your happy Run It.

 

[Lakeside53]

Only speaking from the experience that I have as I prefer the old school saws.

Most of mine are reed engines, see sig.

I do wonder if this isint even more important with the piston ports, however I suspect that port timing between different saws will cause things to vary somewhat also.

Lots of variables I guess.

Just for fun I pulled over a bunch of saws today with and without the throttle open. Couldn't see any difference at all over 5 pulls. All maxed out the same. Guess I can keep doing what I've always done.