I have a Stihl ms660.
It's probably around 20 years old. I got it on a trade from a logger friend of mine 5-6 years ago. Last fall,it got where it wouldn't idle down. I changed the intake boot,all the fuel lines and put a new Stihl carburetor on it. No improvement. I took it to a Stihl repair shop that has a really good reputation. They said the piston and cylinder were shot. They put a Chinese replacement in. 2 months later, I get it back and in 1/4 tank of gas, piston and cylinder are trashed. They said "must be a fluke". They put another on, same thing. They put a third on, they blew it up testing it. I had holes drilled in the muffler that they welded up and they are blaming the muffler modification for the issue. Personally, I don't think they know what to do. They are looking for a new muffler, but I don't think that's the problem, it's ran 20 years with the old one. I'm stumped. Any suggestions? BTW, it's been in the shop since October.

Did they check the crank seal ? Two strokes dont like to idle if the crank seal is leaking

If it won’t idle down it is sucking air somewhere, did you check the decompresser? Sometimes they stick.

They have checked the crank seal,shaft and bearings and everything is good. They are blaming it on the muffler saying it's making it run too hot. I'm around $500 deep in this and I am losing patience, LOL.

Everyone I have talked to says the muffler is NOT the problem.

Sounds like too much air for sure. I would NOT think the muffler is the issue.

Seems to me we have a member who runs a Stihl shop...maybe OK, KS ??

If it was mine I wouldnt stick another dime in it. Wont be long and you could have bought a new one. That said, it a possible muffler problem. If they get carboned up the saw wont run right. Does it have a spark arrestor ?

Other obvious problems could be a clogged air filter, or improper carb adjustment.

In my experience if the spark arrester is an issue the saw will not accelerate.


What happens if you choke it ?

My guess it’s the intake boot, all it takes is a small crack in it. I’d double check if they really replaced it. Also if there is a spark arrester screen in the muffler, remove it, they plug up and the saw will not run right.

I have changed the intake boot and all seals are good. When they put the new piston/cylinder on it runs for about 5 minutes then it burns them up. I don't see how it's a muffler issue??

They are saying the holes in the muffler are the problem, it's ran 20 years like this and now it's an issue?!?!

Its not the muffler. I run my 661 with a stripped muffler and have ported it and run the snot out of it with zero issues. You are getting air into the mix some place. Its running to lean, that makes it run hot and melt pistons and such. Also those pistons and jugs made out of chinesium are not the same quality as OEM stihl parts. If a Stihl dealer and shop messes around with those parts, I would stay away from those folks from the get go. Its different if you like messing with things like I do and get a saw in just parts and then build it yourself, tweaking those parts and machining them as you go along. But for a Stihl dealer / shop to just slap chinesium bits into a stihl saw, it smells ratty to me. Did those folks ever bother to see how many ripums that thing runs at ?? Or is it to those folks just a case of the louder it screams the better ??

You are thinking the exact same thing as I am. I say it's junk Chinese parts? I'm going back next week to get my saw and hopefully some of my money back. I have learned a lesson, for sure!

A still dealer should put in real parts!

Did you request the aftermarket parts?

They said it would be 6+ months minimum getting OEM parts, another lesson learned.

If they put junk parts in the jug, odds are the put in a junk carb also.

This shop sounds like a shop I wouldn’t do business with at all. Stihl dealer putting junk piston and jug on multiple times. Even a cheap jug and piston is going to last longer then five minutes. Let’s get real lots of Chinese two stroke motors out there made with cheap internals but last a few years. Find a better shop that can actually tune a carb in. You can rebuild it as many times as you’d like but breaking in a rebuild with a out of wack tune is going to end up the same. You said they check crank seals and bearings and said they were good? I’d get another shop opinion on that probably one that wouldnt put non oem parts on your saw. Hope you get it fixed up them ms660 are nice saws.

I understand what you are saying, but I put the OEM carb on myself.

Check Ebay, you can get OEM parts usually cheaper and much faster. Even aftermarket Meteor rings are supposedly good.