HELP! Stihl 661...wtf...

[S&S_Work_Saws]

This is a new one to me. I've never had this exact issue before and have this thing completely tore down and still haven't found anything out of the ordinary.
I've got a 661 that in high RPM situations, with or without a load, has a hard, dead miss. It's like you bumped the kill switch for a second and then it keeps cutting. It's not M-Tronic. Carb has been replaced with OEM, Coil replaced, and it's been vacuum/pressure tested. No leaks on testing. Carb and coil made no difference. I've tuned it by ear and tuned it with a tach. Tried tuning rich and a little bit lean...no difference. The saw is very low hour. Wires all appear to be in good shape from coil to multi function switch.
Spark plug, air filter, and fuel filter have been replaced also.

Any ideas on what it could be?

 

[redneckhillbilly]

bad fuel?

 

[S&S_Work_Saws]

bad fuel?

I thought that possible at one point also but drained the tank when I replaced the fuel filter. No change.
I have it tore down to the bare cases. It's going back together with new everything this evening. Hopefully that will fix it. Be nice to know what caused the issue though in case it doesn't fix it and for future knowledge.

 

[Wilhelm]

It's a Stihl, it needs no reason to misbehave!

The finest of German engineering, made in China - what could possibly go wrong.

 

[Loony661]

Have you performed a vac/pres test? Possible air leak?

 

[Mammyrat]

Bad/grounding switch?

 

[CAMike]

Or possibly a short within switch wire.

 

[HumBurner]

Bad/grounding switch?

Or possibly a short within switch wire.

These. And how's the flywheel? Any dents, dings, marrs, chips? No debris caked on or around the coil?

 

[hacskaroly]

I have it tore down to the bare cases. It's going back together with new everything this evening. Hopefully that will fix it. Be nice to know what caused the issue though in case it doesn't fix it and for future knowledge

I used to work with electronics (ground based aircraft navigational and weather equipment) and sometimes the process of taking it apart and putting it back together will solve the problem...frustrating when you don't know the why, but glad it works again when it does.

Good luck playing Sherlock!!

 

[Coupe]

I go with what the other guys say on here possibly a faulty switch or even a
ignition wire lightly pinched and with saw running catching every now and then
Pull the wire off the switch and try it, will probably have to choke it to stop her.
I had troublr with my jonsered 930 with spark a couple of days ago was all over the place
one minute I had spark and then it completely failed. Ordered $230 worth of coil and trigger
but went back at her again and the wire from coil to trigger under the outside wire cover
found the crimp on the wire had never been crimped and had been that way since new i
would think. Gave it a good hard crimp and put it all back together and a nice fat spark
and back in business. It these sill little things like this that can catch you out. As you
can see by the saw that,s why I wanted to keep it going pulls 160psi.

 

[hacskaroly]

The finest of German engineering, made in China - what could possibly go wrong.

Kind of like Jäger Chou Doufu....

 

[Sloughfoot]

It's a Stihl, it needs no reason to misbehave!

The finest of German engineering, made in China - what could possibly go wrong.

Distant 2nd to the finest English engineering made in India.

 

[S&S_Work_Saws]

The Saw is reassembled and ran great for about 15 minutes. All of a sudden it started running awful again, super hot and yanking the starter rope out of my hand. I knew that had to be timing related. Sure enough, it chewed half the keyway up and over advanced. Compared the new replacement coil with the original and the replacement was already a little bit more advanced based on the pickup location. Filed the flywheel keyway groove back into shape the best I could. I guess we'll see how it goes tomorrow evening making some test cuts. I've never seen a 661 have so many issues.

 

[Woodwackr]

Have you run all the wires and switches with a meter while jiggling them around? I had a harness that looked fine, tested fine but on the saw something kept cutting out. Turned out to be a clean break of a wire inside the sheathing…move it just right and no connection. 200t
Now I test wiring while moving them around

 

[East/West CADaddict]

It's a Stihl, it needs no reason to misbehave!

The finest of German engineering, made in China - what could possibly go wrong.

Shots fired!
Beware of the Creamsicle Army!

 

[S&S_Work_Saws]

Have you run all the wires and switches with a meter while jiggling them around? I had a harness that looked fine, tested fine but on the saw something kept cutting out. Turned out to be a clean break of a wire inside the sheathing…move it just right and no connection. 200t
Now I test wiring while moving them around

I haven't tried that yet and never even would have considered that. I've got several known good sets of wires laying around here. I'll swap them and see if it changes anything.

 

[S&S_Work_Saws]

It's a Stihl, it needs no reason to misbehave!

The finest of German engineering, made in China - what could possibly go wrong.

I've Still got my favorites hidden away...

 

[Wilhelm]

Shots fired!
Beware of the Creamsicle Army!

Don't forget to powder Yer cheeks before entering the battlefield!

 

[East/West CADaddict]

That's way my soap is ALWAYS on a rope. Stihl fan boys usual use liquid.... so their always bent over trying to pick it up.

 

[Wilhelm]

That's way my soap is ALWAYS on a rope. Stihl fan boys usual use liquid.... so their always bent over trying to pick it up.

I also heard they don't tuck their shirts in and wear pants 3 sizes too small!