Compression questions

[Crocodile Lyle]

I'm new to tearing down saws and rebuilding them but bought a cheap 038 magnum that wouldn't start, I started with a compression check and was around 115psi. After further inspection it had alot of worn out parts so I just completely tore it down to the crankcase. The cylinder and piston are oem and looked to be in very nice condition so I bought new Caber rings to raise the compression. I took my digital caliper and measured the width of the old rings and compared that measurement to the new ones and it was clear they needed replaced. I put everything back together and replaced pretty much everything except the case,bearing,crank,piston and cylinder. Got it running but kept dying at idle after diagnosis I discovered I had a bad coil, replaced it and got it running and dialed in the carburetor. Now the saw after warm will only restart on half choke. So I redid a compression check and I'm now at 100psi. I let this saw idle for at least 40min and did some test cuts and was running fine. What bothers me is I lost 15psi in compression and still after warmed up only starts at half choke. I vacuum pressure checked and it's holding vacuum and also vacuum checked the carburetor and it holds as well. Should I be looking at replacing the piston and cylinder or try some oem stihl rings?

 

[Frogfarmer]

Cabers are good. If it's strong in the cut your guage may be off. Don't condemn it on numbers alone.

 

[Crocodile Lyle]

The gauge I'm using is a snap on compression testing kit which I know is good and gives accurate readings. I feel this 038 should be at least pulling over 150psi.

 

[Frogfarmer]

Send it to Randy. You'll have it by Christmas and it will blow 150 all day long.

 

[Crocodile Lyle]

That's funny because it is going to him in july. I'm just trying to get it running like it should before then.

 

[kpkustoms]

Put a couple drops of oil in the cylinder at tdc and recheck compression if it gets better compression is seeping by rings and would say there not run in or seated broke in what ever u call it

 

[Al Smith]

FWIW it takes some time for new rings to wear in on an old cylinder .Run the pizz out of it .It will eventually wear in .

 

[drf256]

After 3 months, it should be broken in.

 

[Al Smith]

It took me about that long cutting down to size about 10 cords of oak at 15 minutes at a time over the course of an entire summer .That praticular 038 mag btw now wears a D-handle .

Why it took that long I have no idea except the fact it had a lot of scuff on the intake side of the piston evidently from running a faulty air filter .Never the less when it finally came in it came on strong .So what I'm saying to the OP is don't give up the ship yet,give it some time .

 

[Al Smith]

Here's another thing .On one of my 125 Macs which uses tool steel thin rings since I relined it with with a cast iron sleeve it hasn't had enough run time to seat the rings .It cuts good but you can start it without using the d-comp .quite unusual on a 125 if you ever pulled one over except for one factor .You have static compression and dynamic compression .Under power the comp will and does go up .

 

[Landmark]

Have you compared you compression readings against other saws you have? Even though its snapon i would ck and recheck that guage. Is it new or old? Hiw long have you used it etc......

 

[ny15]

My Snap On gauge let me down a few months ago, schrader valve was shot and needed to be replaced. Bought the kit from the Snap On guy and it was back to reading accurately again.

 

[XP_Slinger]

I occasionally check my guage using my air compressor using a rubber tip for the air gun. If the pressure on the compression guage matches the pressure on the regulator guage it gives me a pretty good indicator of how accurate it is. Not precise, but better than assuming.