Pull-cord kickback when starting a STIHL MS 660

[morten]

During a two day softwood (Spruce) milling session, I recently experienced severe kickbacks from the pull-cord, when trying to start my MS 660. The saw was warm at the time. At every attempt to start the saw, the pull-cord handle was nearly pulled out of my hand. Very unpleasant and painful. I used the decomp valve, and after every kickback, the valve was pushed out, indicating that ignition had taken place.

 

The kickback repeated itself once during the two-day milling session, but otherwise I havn't experienced the problem.

 

Any of you guys know what causes this unpleasant pull-cord kick-back, and what I can possibly do to prevent it? I can only guess at premature ignition, causing the piston to cause the crank to move backwards, thus pulling the cord. But why?

[Rich2484]

Exactly what you just said.

 

Kickback can happen if the flywheel key has gone or the flywheel has moved. This will cause kickback more than once or twice bit every time you try to start it.

 

Check the plug.

 

I would at a guess say clogged exhaust. And the exhaust gasses are travelling backwards into cylinder rather than making there escape.

[morten]

Exactly what you just said.

 

Kickback can happen if the flywheel key has gone or the flywheel has moved. This will cause kickback more than once or twice bit every time you try to start it.

 

Check the plug.

 

I would at a guess say clogged exhaust. And the exhaust gasses are travelling backwards into cylinder rather than making there escape.

 

Funny that it seems to have cured itself then. Could it be a problem that only manifests itself occasionally on a warm start?

 

I'll check flywheel key and spark plug regardless.

[muttley9050]

My saw did it when the bolts on the cover were a bit loose.

[Graham]

A bit of glowing carbon when the saw's really hot causing pre-ignition?

[spudulike]

The kickback is being caused by ignitiion takeing place too early on the upward stroke - this is called too much advance in the ignition because it is too much in advance of top dead centre.

 

Generally it is caused by the flywheel shearing its key but is also worth making sure the coil is a business card thickness away from the magnets on the flywheel closer and it may cause this issue.

 

Also check the coil you have fitted is the one with a number of levels of ignition timing - early 066/064 would kickback badly because the ignition had fixed one position timing, later ones had built in retarded ignition on starting that then advance over 3krpm and then retarded again at near max revs - the part the newer MS660 should have is - 1122 400 1314 ....other coils may have variable timing so let me have the number if different and will see if it is correct.

 

If the saw has been fine and just started doing this it probably is the key but the coil may be begining to break down - it is easy to see if the coil is advancing OK by attaching a timing light to the plug, starting the saw and shining the light on the top of the flywheel and revving it.

 

Don't ask me how I know so much about ignition timing:blushing::lol:

[Big Beech]

my 064 does this very occasionally. I just hold on tighter and pull harder. I presume we are talking about the same thing here

 

I've not yet experienced this on my 090av. that would probably break my shoulder

 

s

[morten]

If the saw has been fine and just started doing this it probably is the key but the coil may be begining to break down - it is easy to see if the coil is advancing OK by attaching a timing light to the plug, starting the saw and shining the light on the top of the flywheel and revving it.

 

The thing is: The saw has been working fine, suddenly started acting up for about an hour, or so while milling, and has been fine ever since (a week, or so). So whatever it is, it has rectified itself. I.e. a periodic failure. That is what makes it difficult to diagnose. But if I can get some ideas of what can be the root cause, I may be able to prevent it from happening. Could it be caused by dust build-up in the flywheel housing?

 

I doubt it is carbon build-up, as I have only been running the saw on Aspen, but I'll check the spark-plug. Haven't been able to for the past week, as I have been lying on a beach in Mallorca

[Rich2484]

Had a nasty episode from a 088 once. Would just kick back and really hurt. Would snap the pull handle clean through the fingers.

 

It stopped one day. Just didn't happen anymore. Anyway the next time it went out to work the flywheel failed. Lost its magnetism and lost spark. Took some testing to find the cause.

[raker]

I had this on an 064 once when the main bearings were on their way out.Not all the time, but the coil would catch on the flywheel.Sometimes sheared the key. Most of the time it would run but every now and again the worn bearings would come out of alignment causing a kickback on starting.Have a look for scuffing on your flywheel.