Causes of low Compression

[fidiro]

I just received another tool and it's a compression tester.

I went testing through my saws I have around the house and first up was the 7311 with a new Caber ring I just installed. I have started the saw before and ran it about 30 minutes total with new ring. Tested this saw completely cold and read at 180psi, wow is this a little high? Didn't test it warm.

Next was the 390 and I can't believe it tested 130psi, it works great but will probably need a ring. The 440 was 110 psi, but this one will be getting a rebuild/restore so it will get new ring or piston and ring when apart.

Now the one that I hope to get more info as to why the compression came up at 90psi is the 029 Super. Looking through the exhaust port cylinder looks like new and piston just the same even has those thin wear lines still visible on exhaust side, no scoring anywhere.

Would a worn ring be the culprid of the 029? What would be the first easiest thing to check as to why low compression before teardown because there is no damage to cylinder/piston? Had to be the clam. Hope I can take one of these apart if it comes to that. I guess it's better to learn on the 029 so that when it comes to the 390 I'll be a pro.:laughn:

 

[fidiro]

Could impulse line do it?

Edit: I have not used this saw since I did the carb kit. When I rebuilt carb I went ahead and replaced the fuel line but not the impulse. Fuel line was a little stiff so I wonder if the impulse is just the same. If the impulse line woulc cause low comp., I'll go ahead and change it. I don't know just thinking outloud here of the easiest things first.

Edit is not working for me. I tried to edit above post and two times it failed to post it so I'm trying the reply now.

 

[oscar4883]

The 440 and 029 are real low. I would pull the mufflers and check the pistons.

 

[oscar4883]

I can't see an old impulse line causing huge comp. loss like you have. Does the gauge have a schradr valve in the tip? How many times did you pull? IMO most saws that need top end work have been damaged in some way and are not just worn out.

 

[fidiro]

I just received another tool and it's a compression tester.

The 440 was 110 psi, but this one will be getting a rebuild/restore so it will get new ring or piston and ring when apart.

Now the one that I hope to get more info as to why the compression came up at 90psi is the 029 Super. Looking through the exhaust port cylinder looks like new and piston just the same even has those thin wear lines still visible on exhaust side, no scoring anywhere.

Would a worn ring be the culprid of the 029? What would be the first easiest thing to check as to why low compression before teardown because there is no damage to cylinder/piston? Had to be the clam. Hope I can take one of these apart if it comes to that. I guess it's better to learn on the 029 so that when it comes to the 390 I'll be a pro.:laughn:

Thanks OSCAR but I did look through the exhaust on the 029, everything looks like new.

The 440 I really don't worry about, it's getting a restore/rebuild so I was going to replace whatever it needs as it got back together.

 

[fidiro]

I can't see an old impulse line causing huge comp. loss like you have. Does the gauge have a schradr valve in the tip? How many times did you pull? IMO most saws that need top end work have been damaged in some way and are not just worn out.

Gauge has valve on bottom tip of hose and the release valve up by the gauge. Pulled many many times in hopes to see higher numbers. Just looking for options before I take it all apart. Thanks again

 

[fidiro]

Post editing is not working for me

Sorry to yell out the post editing but wondering if I'm the only one having the issue with editing.

Anyway, could it be rings sticking? Only way to find out I guess is to pull clam apart. Maybe I'll swish some fuel in there and dump.

 

[oscar4883]

Maybe it is worn out, or like you mentioned, the rings may be sticking. If you push gently on the ring it should have some give to it and be able to be pushed back into the groove slightly. May be that it is worn on the intake side due to debris ingestion. See what that looks like.

 

[brages]

Test the gauge against a bicycle pump or air compressor...

IMO, if it starts easy and runs well, it IS good...

 

[fidiro]

Maybe it is worn out, or like you mentioned, the rings may be sticking. If you push gently on the ring it should have some give to it and be able to be pushed back into the groove slightly. May be that it is worn on the intake side due to debris ingestion. See what that looks like.

I'll have to pull muffler again and try to see if the rings move any. Intake side cylinder wall is super clean and shiny, no scoring. Piston/rings on that side I'll have to pull carb to see.

 

[oscar4883]

Bradges has a good point. How does it run? If it runs well I wouldn't even re-ring it just because of a gauge reading.

 

[ZeroJunk]

If you have one saw test 180 and another test 90 it would be hard to suspect the gauge.

If the piston looks pristine could be a broken ring, bad decomp if it had one, poor seal around the spark plug hole with the gage.

 

[nmurph]

you tester sounds erratic; high compression on a new rebuild and low on running saws. i would replace the shrader and make sure the check valve is clean. you may have a weak spring or trash in the hose.

 

[fidiro]

Test the gauge against a bicycle pump or air compressor...

IMO, if it starts easy and runs well, it IS good...

Funny that you mention if it runs. It started and ran excellent with great throttle response until I opened up the high needle to feed more fuel to richen it up a little and now it's getting hard to start and when it does start it runs like crap now. I tried leaning the H a little to get it back to where it was and still runs bad and will not stay idle now. I'll check to see if rings move some and I'll pull carb to see piston from that side.

I don't know if the impulse would cause the low comp. reading but I think I'll replace that first this weekend and go from there.

 

[nmurph]

your impulse has nothing to do with compression. it uses a small amount of vacuum to move the diaphragm. most of the air comes through the carb throat with a small amount coming through the muffler. that is why it doesn't matter if you pull the throttle when you are checking compression.

 

[brages]

Impulse line won't affect compression readings at all...

 

[fidiro]

I just got the gauge new. Not the best one but I did test the saw a few times removing the hose and reinstalling and got same reading. Tested the 7311 again and still got the 180 reading. Tested the 390 again and still 130sh. So I don't think gauge yet.

Saw is not running properly now. When I do get it started it will run poorly and I have to slightly hold throttle to get it to idle, if I let go it dies and can't get it restarted. I'll try to post pics of piston/cylinder of at least the exhaust side. I mean they are flawless from that side.

 

[fidiro]

Impulse line won't affect compression readings at all...

I guess I can stop thinking of that being the culprid. I'll have to see the carb side to check piston and cylinder from that angle.

I guess a split ring could also be what happened. It is two rings right? I think I saw two rings, don't remember. But if one breaks would the other hold enough psi?

 

[boutselis]

I have not rebuilt to many top ends but I do remember looking at an echo (I didn't bother rebuilding) and the ring would move on one side of the piston but was stuck in the other half so that only half of the ring was "springy" and the other side would have not been pressing the cylinder. I assume that would lower the compression.

I have no idea how it could have gotten like that though

My point is, although I bet its rare the ring could look fine on the exhaust side and maybe stuck on the intake side.

 

[redunshee]

I guess I can stop thinking of that being the culprid. I'll have to see the carb side to check piston and cylinder from that angle.

I guess a split ring could also be what happened. It is two rings right? I think I saw two rings, don't remember. But if one breaks would the other hold enough psi?

If it has two rings my understanding is that the bottom ring does most of the work. I've increased compression by simply switching ring position i.e. top to bottom etc.
Bob