How reliable is the starter rope comp test?

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Kurtsaw

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Hi I bought a stihl 026 a few weeks ago and it seems to run ok but I tried the hang saw by starter handle compression test and it fails. So that's my question how reliable is that test and should I be worried about it or just keep running the saw?
 
Pull the muffler and look at piston through exhaust port, that will give a good idea of condition of top end. Letting the saw drop from starter handle can give u a very basic idea of compression, but different starter configurations (pulley size, spring assisted) can make it seem like a saw has low compression
 
Wow thanks for the quick responses I will pull the muffler when I get a chance and post pics because honestly I'm not sure what I'm looking for. Again thanks for the quick responses
 
I would say it would be reliable if you know what one model feels like with a piston and cylinder in good shape as it goes down the rope.

I can feel a big difference on models that I have rebuilt and ones I pick up to do the rope fall test....

As others have said, pull the muffler and check the piston and cylinder from the exhaust side for the best indicator, next to a compression test.

Also, if you don't see scoring but the compression is low, the saw may have a lot of hours and the ring may be worn. Installing a new ring may be all that is required to get it back to healthy. Sometimes the piston will be worn and sloppy as well, but the cylinder will be fine. In this case, install a new piston and cylinder.
 
Very true, which is why looking at the piston through the port and a compression test combined is the best indicator of condition.
 
You would be very surprised what will make good compression and run. 2 strokes are extremely forgiving to imperfect cylinder walls, I've put together snowmobiles with jugs that felt like washboard after being honed and had 150+ after breakin.

As for replacing just the ring; while it seems to be a common practice, unless you have a good idea how thick the skirt should be or measure wall clearance, it's taking big chance.

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This is an 800 Arctic cat that was ran about 500 miles with an aftermarket kit and tune that was too lean, but not meltdown lean. The other side didn't fare too well.

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Rings are not stuck, and it still leaked down good, there was too much shrapnel to do a compression test, it wouldn't make a full revolution. The side that didn't pop had no visible scoring like you would think you should see when a piston is that worn. Total mileage on the engine was 2200.
 
I've had a few says come in that felt like almost no compression but actually still had 100+ and run fine.

Buy a guage, they are cheap.
 
I know the pictures aren't the best but can you guys see anything that is obviously not good? Sorry I just haven't seen enough good or bad to know the difference.
 
The last two pictures looks like you have a little bit of scoring or at least a saw with considerable run time. If you have something like a dental pick or maybe even a tooth pick see if those marks can be felt, could just be long term wear or carbon scoring. If the scoring is near the rings or if the rings get stuck you will lose compression. Measuring the piston skirt and piston cylinder clearance are probably only the 100% positive methods to know how much life the piston has left. A new ring can get you more compression, not sure if there is a rule to how many rings can be run on a piston. I guess if the piston is not worn out it will run another ring. Measuring the dimensions of ring and the end gap when placed in the cylinder will tell you about wear of said ring. If you have the cylinder off and it was questionable you would probably be better off replacing the piston and rings. I have a stihl that I bought new and it has a lot of use. Also I have the same model of saw that is new. If you went by the rope pull method you would think the older saw was shot, but it still runs fine.
 
I see some mild scoring that will clean up. I would check the compression with a guage. If under about 130-140, I would pull the piston, clean it up and put a new caber ring in it.
 
Thanks again for the quick responses I'll have to get a compression tester and see what it says. Any special way to test these saws for compression or just screw it in and start pulling?
 
Buy a tester with a schrader valve on hose end that will hold your highest pulled psi. Choke open and pull until the needle no longer rises. About 10 should be ample. I have a craftsman that has lasted for yrs.
 
Ok rested the compression with a tester while engine was cold and got 135-140 psi not sure if it would be higher warmed up. So with that much compression do you think it's worth tearing down or should I just run it as long as I can?
 
I've got an old ryobi (one of the better ones, pre-pullin') . It starts ok but after it gets hot and stalls it won't start for love or money. At that point the pull is extremely easy so I suspect that the compression is way off, probably because I leaned it out too much.(still learning). I wonder if rings will fix it? Is that typical that the compression goes down after they get hot?
 
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