MS171 Compression

[buzz sawyer]

I'm examining an MS171 that has external parts damaged from a drop. Saw looks like it was run very little - P&C look like new through the exhaust, Pressure/Vacuum holds 7psi and 10 "hg for at least 20 seconds, dropping very slowly. Compression is only 90 - 100 using two different testers (snap-on and Lyle. I don't believe it has run for a while. I shot a little oil in the cyl and comp. is 125 - same as my 180 and 170.

Is this a normal reading for this saw with small cc engine? Next step would be to open it up and take a closer look inside.

 

[buzz sawyer]

bump

 

[lone wolf]

First, are you pulling the saw over until the gauge stops rising?
The compression should be 150-160 lbs.
Second, that pressure is dropping, so you need to `spray soapy water and find the leak or dunk the whole thing underwater while adding pressure.

 

[buzz sawyer]

Yes, pulling till the gauge stops - 15 or so pulls. I had trouble sealing the second (upper) intake port. Saw some bubbles there so I figured that was the leak. When I say it dropped, I mean like 7 to 6 in 30 seconds. If there was a crack affecting compression, it should leak a lot faster than that. I'm just going to pull the engine and check it out.

 

[xtremez]

It sounds like your reading’s are low because of an automotive tester with a long hose and schrader valve not close to the cylinder since your other saws are showing similar readings

 

[buzz sawyer]

It sounds like your reading’s are low because of an automotive tester with a long hose and schrader valve not close to the cylinder since your other saws are showing similar readings

I used two testers, both valves within an inch or so of the cylinder, same reading on both. I was getting higher readings (175) on other saws with larger engines - will retest. My big question is - does anyone else have a reading from another 170,180, or 171?

 

[stihltech]

PLEASE_ PLEASE_PLEASE forget the compression tester!!!!!!! Hang it by the starter handle. Does it hold or release very slowly? Probably OK. The pull muffler and check piston condition front and back. Internal visual inspection is MUCH more important.

 

[Vintage Engine Repairs]

PLEASE_ PLEASE_PLEASE forget the compression tester!!!!!!! Hang it by the starter handle. Does it hold or release very slowly? Probably OK. The pull muffler and check piston condition front and back. Internal visual inspection is MUCH more important.

Hanging from the rope not the best method for testing compression. Recoil size relative to the flywheel, the location of the pawls too makes a big difference to felt compression. A perfect example is the difference between the 076 and the 076 super. They did away with the decomp in favour of a larger recoil mechanism.

A good compression tester, suitable for small engines is a fantastic way to test the life of the rings.

 

[lone wolf]

If that compression tester gives higher readings on other saws and a low reading on the one you are working on, there is a problem with the piston or cyl or both. You have a big air leak also, and I told you to spray with soupy water and find it already ,so fix that .
@dmb2613

 

[buzz sawyer]

The leak mentioned before was from the upper carb rubber port - my bad trying to seal it. I pulled the engine, sealed between the rubber intake assembly and the block - no leak pres or vac. I did turn the crank while testing. No sign of impact or cracks on the body. I pulled the piston and other than a hint carbon on top, the crank, rod, P&C look like new out of the box. Ring is free, no carbon anywhere.
My 361 reads 165 psi using the same compression gauge. LONE WOLF, I agree, still doesn't look right but in light of the readings on the 170 and 180, I think it's the nature of the beast. I was hoping someone with the same saws could give me their readings. I was going to sell this but now, since I have it apart, will wait till I find a 171 with bad engine. If it runs, it runs. If not, I can still part it out. Thanks for all the input.

 

[cookies]

The leak mentioned before was from the upper carb rubber port - my bad trying to seal it. I pulled the engine, sealed between the rubber intake assembly and the block - no leak pres or vac. I did turn the crank while testing. No sign of impact or cracks on the body. I pulled the piston and other than a hint carbon on top, the crank, rod, P&C look like new out of the box. Ring is free, no carbon anywhere.
My 361 reads 165 psi using the same compression gauge. LONE WOLF, I agree, still doesn't look right but in light of the readings on the 170 and 180, I think it's the nature of the beast. I was hoping someone with the same saws could give me their readings. I was going to sell this but now, since I have it apart, will wait till I find a 171 with bad engine. If it runs, it runs. If not, I can still part it out. Thanks for all the input.

Get new rings if the piston and cylinder look good and stuff it back together, they do wear out over time with use. I know I have saved quite a few different saws with new rings, sometimes they simply loose their ability to hold tension.

 

[buzz sawyer]

The more I think I think about the condition, I wonder if this is even broken in. The guy said it hadn't been run much before the damage. Not a bad idea on the ring. Can't hurt to try.

 

[buzz sawyer]

Just called the local Stihl dealer. They said on that saw 110 should be ok.

 

[Vintage Engine Repairs]

Just called the local Stihl dealer. They said on that saw 110 should be ok.

No it’s not, they’re wrong. It won’t run with 110psi. Though your readings all seem low which means more than likely you aren’t using a small engine compression tester.

Edit: engine cc doesn’t have any relation to compression ratio. Just because you have a smaller saw, doesn’t mean the compression will be lower.

 

[buzz sawyer]

From the Stihl website - FAQ. See last one.
I still say this saw hasn't run enough to break it in. There is nothing wrong with the Ring, P&C, they look like new. it's not the first saw I've been into. Still hoping someone else with one of these will chime in.

 

[xtremez]

I could check my 180 when i get home from work but idk how much help it’ll be since i only have my automotive tester atm

 

[buzz sawyer]

I could check my 180 when i get home from work but idk how much help it’ll be since i only have my automotive tester atm

Thanks. My automotive tester showed the same reading as the Lyle unit I bought.

 

[lone wolf]

Thanks. My automotive tester showed the same reading as the Lyle unit I bought.

Have you tried to start it after fixing the leak?

 

[buzz sawyer]

Have you tried to start it after fixing the leak?

No, since there are missing/broken parts (handle, main housing, AF housing, I was just going to leave it apart while I look for other parts or a parts saw with bad engine. I was going to sell for parts but may just keep it in case I find another with a bad engine.

 

[buzz sawyer]

No it’s not, they’re wrong. It won’t run with 110psi. Though your readings all seem low which means more than likely you aren’t using a small engine compression tester.

Edit: engine cc doesn’t have any relation to compression ratio. Just because you have a smaller saw, doesn’t mean the compression will be lower.

Can you recommend an appropriate small engine compression tester?