First off.... I'm sthil Silver Certified Tech


Stop throwing money at it. Stop guessing. Stop listening to all the ideas.

If this shop really is a Sthil Shop have them do a Sthil small engine check list. They can print it off of their dealer page, and all mechanics should know what it is. The check list will find the problem every time.

I'm 100% positive you have a air leak.

The intake boot, crank seal, carb, impulse line, decompression button. Somewhere is an air leak they are missing.



Either way this shop is a poor example of a repair shop. You should never just rebuild a burned up saw without determining the cause first with the stihl checklist.

I would agree with the getting air accesment , china parts are a bad idea. I see your from Brazil do you know who d & d enterprises is.

Spark plug in all the way?
Seal between head and crankcase?(that what y’all call crank seal?)
Adjustment screws on carb still set right?(didnt vibrate out of position)?
Carb gasket have a hairline crack?

When you get it back ,go buy a cheap vacuum pump and learn from YouTube on pressure checking your case ,its pretty simple , by the way check the little rubber line from the bottom of the carb to crank case ,them will get soft and go bad just like fuel line. by the way china pistons and jugs are not much.

Seal between head and crankcase?(that what y’all call crank seal?)

Crank seals are on the crank shaft.
There's also a head gasket.

Unless you are thinking of a clamshell motor..... A 660 is not a clamshell, those are usually homeowner saws with stranded plastic crank cases

did they vacuum test the saw and it held?


here is the parts https://www.baileysonline.com/nwp-b...r-stihl-066-ms-650-ms-660-chainsaws.html

Mityvac vac and pres pump costs less than one hour dealership time.

I have an old fs 80 calling me right now, it ran good twenty years ago.
A dealer would quickly charge more than its worth.

Crusty rusty husky tech, not for hire.

Not sure if you have read the entire thread, but what is your opinion on the muffler causing the whole thing? I don't think it is.

Yes muffler mods can ruin it if the air/fuel has not been adjusted to it. your messing with your air flow .

Clutch side crankseal most likely. You said they told you they checked it , but did they really ?

Unless there are huge holes drilled in the muffler or it's half rusted off.
I have worked on many saws that people have modified the muffler, and have not seen issues with that. Removing baffles, adding extra exhaust holes isn't horrible unless its over done. I'm not sure on the proper measurements, but I know it has to do with the exhaust port size on the jug and the size of the holes in the muffler (Basically back pressure like a car).



I personally don't think the muffler modding is worth the out come. Unless you've had someone do major work on the saw like messing with the timing I don't see a benefit. Why spend money on porting and hot roding an old saw when you can buy a new one like a fuel injected 500i.

I stopped building my own saws last year and bought a new 400c with Mtronic computer carb, and magnesium piston. That thing out cuts both of the 361 I own, and one of those has a big bore kit and a few other power mods i did. I have even tuned the 361 with the computer using a program that Stihl dealers have. The computer found 1000 RPM on the top end that my Ears and a TACH couldn't find.

Excellent advice and much appreciated!

Is it a common thing for stihl dealerships to use aftermarket parts? Typically when I have a saw go down like that I buy another new one and use the old one for parts.