Stihl MS171 cylinder compression

[lone wolf]

What would be the minimum compression to start ms171 chainsaw. Story is I acquired above saw, compression was very low and found stuck piston ring (piston seemed in good shape ie noscoring etc). Replaced ring and compression went to 90 psi, saw wouldn't start so cleaned carb etc still won't start, put a bit of carb cleaner down plug but still wouldn't start, checked spark plug all good, still wouldn't start so now I'm thinking if compression is just too low and maybe needs new cylinder, piston etc. What would your opinion be. Thankyou

Forget minimum it should be 150 - 160, did you pull the saw over till the gauge stopped rising or did you stop at an arbitrary number? Is the gauge known to be correct?

 

[lone wolf]

That is an abbreviation for capacitor and I was thinking it might be some helpful trick testing spark.
We have testers at the store that have something like a nixie bulb that lights with the fire. Not sure if it is a Stihl tool, but I have not seen any on line that looked quite like it. Or, is flexible enough to use where they hide some plugs.

I have been known to just hold the plug wire, but I'll admit I was pretty sure they didn't have fire.

Me too and no shock certainly proves it.

 

[cheekyBob]

Forget minimum it should be 150 - 160, did you pull the saw over till the gauge stopped rising or did you stop at an arbitrary number? Is the gauge known to be correct?

Thank you for your reply, yes I pulled over till gauge settled, gauge is pretty accurate, done most of the suggestions quoted on this thread. Now ordered new oem cylinder kit im pretty optimistic that this will sort saw issue out.

 

[lone wolf]

Thank you for your reply, yes I pulled over till gauge settled, gauge is pretty accurate, done most of the suggestions quoted on this thread. Now ordered new oem cylinder kit im pretty optimistic that this will sort saw issue out.

In the meantime do a pressure - Vac test while waiting.

 

[benelli7d]

Me too and no shock certainly proves it.

You are both nuts! Getting shocked is one of the things I hate most. Hell I even hate carpet shocks. Luckily, I had a friend who would stick his finger in my plug cap 3-4 times a day if I was having trouble with my bike when I used to race. Thanks for good, all be it crazy, friends.

 

[lone wolf]

You are both nuts! Getting shocked is one of the things I hate most. Hell I even hate carpet shocks. Luckily, I had a friend who would stick his finger in my plug cap 3-4 times a day if I was having trouble with my bike when I used to race. Thanks for good, all be it crazy, friends.

I don't do it on the regular just when I have one that is really extra annoying.

 

[cheekyBob]

Just a quick update on ms171 not starting, installed new oem cylinder and piston and saw is now running as it should. Thankyou for all your replies and suggestions.

 

[J D]

Just a quick update on ms171 not starting, installed new oem cylinder and piston and saw is now running as it should. Thankyou for all your replies and suggestions.

Good to hear.
If you haven't already, I would highly recommend doing a leak test... cheap insurance you won't end up back at square 1 with another scored up top end

 

[rupedoggy]

Just a quick update on ms171 not starting, installed new oem cylinder and piston and saw is now running as it should. Thankyou for all your replies and suggestions.

Yea what is the compression now?

 

[cheekyBob]

Not checked compression with pressure gauge, but it feels a whole lot better when I pull start cord, I will put gauge on and post results, it would be interesting to see what it is now.

 

[irhunter]

Just a quick update on ms171 not starting, installed new oem cylinder and piston and saw is now running as it should. Thankyou for all your replies and suggestions.

Cool.

Mind telling us what that P&C cost?

Roy

 

[champagne_charly]

Not checked compression with pressure gauge, but it feels a whole lot better when I pull start cord, I will put gauge on and post results, it would be interesting to see what it is now.

Ok so it's been a year. Did you ever get the compression with the new pot and piston? i just tested mine and am only getting 105PSI.

 

[buzz sawyer]

Ok so it's been a year. Did you ever get the compression with the new pot and piston? i just tested mine and am only getting 105PSI.

Does your tester have the valve in the end that screws into the spark plug hole? If not, you are adding volume to the combustion chamber - giving a low reading. That's what happened to me so I machined a valve to fit in the end of the adaptor. btw, I am only getting 130 -135. It runs fine.

 

[champagne_charly]

Does your tester have the valve in the end that screws into the spark plug hole? If not, you are adding volume to the combustion chamber - giving a low reading. That's what happened to me so I machined a valve to fit in the end of the adaptor. btw, I am only getting 130 -135. It runs fine.

Hi. good point. No, it doesnt have a valve inside the plug adapter. Only at the end of the hose in the part that fits inside the plug adapter. What type of valve did you use?

 

[stihltech]

Returning to the original issue- did you break the glaze on the cylinder when installing the new ring? Needs a little roughness to seat the ring to the cylinder.
Had this exact issue on an 044 a customer brought in. Barely started and no power. They had put a piston kit in. I hate checking compression, but it was just 90.
Broke the glaze in the cylinder. Immediately started. After a short run in, compression was now 130. Saw idled and performed well.
Break the glaze. Basic engine rebuilding.

 

[champagne_charly]

Returning to the original issue- did you break the glaze on the cylinder when installing the new ring? Needs a little roughness to seat the ring to the cylinder.
Had this exact issue on an 044 a customer brought in. Barely started and no power. They had put a piston kit in. I hate checking compression, but it was just 90.
Broke the glaze in the cylinder. Immediately started. After a short run in, compression was now 130. Saw idled and performed well.
Break the glaze. Basic engine rebuilding.

I dont think the guy who originally posted has been back for a year or more since he installed the new ring. On my HS I think its the case as ring looks pretty mint. I'll create a separate thread for that. . But I don't think its the case MS.