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
Stihl MS171 cylinder compression
- Thread starter cheekyBob
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When you give it the rope test, ie holding on to starter handle and letting the saw drop. What happens?
The saw does drop in about 6-8 seconds (bar and chain are absent) I'm sure cylinder is down but just curious as to what min pressure would start saw, the next thing Is a leak down test to check crank seals etc. I've also rebuilt the carby but still no joy. Thankyou for your reply Coffeebreak
Typically, with any 2 stroke the min. is 100 psi. However, I have had 2 pcs of equipment with less start and run "decent". These were string trimmers though.
If you just put in a new ring and still only got 90, time to look for other problems.
If you just put in a new ring and still only got 90, time to look for other problems.
Thanks for reply, think I'm going to to do leak test etc, so if seals are bad I can order some if I decide on new cylinder etc, saw probably not worth it but I just want to see it running again
You might check to see if one or two of your parts is the wrong size for this saw..
Pressure and vacuum test. Check spark with a clean plug and large cap. Check flywheel key. Also, does it have a working pulse to carburetor?
What do you mean by large cap ??
It's hard to get an accurate reading on a small displacement engine. Not that yours is reading low, but I have three and they all have a hard time getting pumped up.
Checking your flywheel is a good idea. They spin occasionally and break the key, so it may be firing at the wrong time.
Checking your flywheel is a good idea. They spin occasionally and break the key, so it may be firing at the wrong time.
Not pressure/vacuum tested yet but will do. Flywheel is ok, also sprayed brake cleaner around crankshaft seals etc, looking for leaks all OK. The pulse could be a problem as I took spark cap off and pulled engine around 20 times then pulled spark plug expecting it to be wet but wasn't, rebuilt carb (pressure tested 7psi to see if needle wasn't passing etc) carb passed test. Even tried new carb that I had available, same result. How would I test pulse line please. Thankyou for your time.
Squirt a thimble full of mix down the carb. It should light that up regardless of impulse, case leaks , or anything else if it has compression and fire. Putting carb cleaner down the plug hole is no good.
The world won't come to an end if you spray some starter fluid in it , although I'm sure some moron will come in and say it will blow your saw up and kill your cat.
Pure ether in a can that started all of that which was somewhat true at the time is long, long, gone.
The world won't come to an end if you spray some starter fluid in it , although I'm sure some moron will come in and say it will blow your saw up and kill your cat.
Pure ether in a can that started all of that which was somewhat true at the time is long, long, gone.
Thankyou Zero Junk, I will give it a try and report back what happened
Sorry big fingers. Large gap.
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.
What would you consider large gap please?
For testing a new plug out of cylinder, about a 1/4" gap. A good coil should jump that gap no problem.
If it does kill your cat I may have an extra..
I think what this site needs is a Saw Won't Start sticky. With a simple check list for beginners to saw repair. I will start making the list:
1. Look for gas in tank; check it has oil in it
2. Clean air filter
3. Pull muffler, look at piston and check muffler for flow
4. Check spark
5. Swap out plug
6. Teaspoon of mix down hole
7. Pressure/vac test
1. Look for gas in tank; check it has oil in it
2. Clean air filter
3. Pull muffler, look at piston and check muffler for flow
4. Check spark
5. Swap out plug
6. Teaspoon of mix down hole
7. Pressure/vac test
But here is the kicker...
I had China aftermarket coil on a Husky 55 show great spark on a plug out of the cylinder, but saw would never start let alone pop over at all. Had no spark under compression. Got a new OEM old stock coil off eBay and saw fired a ran perfect on 1st pull. It's hit and miss on aftermarket coils I hear.....
I had China aftermarket coil on a Husky 55 show great spark on a plug out of the cylinder, but saw would never start let alone pop over at all. Had no spark under compression. Got a new OEM old stock coil off eBay and saw fired a ran perfect on 1st pull. It's hit and miss on aftermarket coils I hear.....
