Stihl 066 hard jerky start

[JerTree]

Hello,
I've got an 066 that has been getting progressively harder to start with some serious yank-back on the handle, like can't hold the handle/bruised knuckles kind of hard.

I've read some other threads with similar sounding issues, particularily this one:
https://www.arboristsite.com/commun...l-038-hard-to-pull.131853/page-2#post-4188286
with likely flooding issues, and this one:
https://www.arboristsite.com/commun...er-rope-like-last-084-post.44726/#post-604258
(leaky needle valve in carb for also flooding issues?).

So I thought this might not be too bad to fix, and have pulled the carb off entirely - however, it is still really hard to pull even with the carb completely detatched!

I have also removed the muffler - no help there.
When I unscrew the spark plug, it turns over easy - as soon as I have the spark plug back in, even just finger tight, it is hard to pull.
So what is going on here - am I fighting some sort of vacuum force in the piston that having the spark plug out alleviates?

This 066 does not have a decomp valve, just a bolt in there - should I consider getting one? Would it even help?

Thanks!

 

[pioneerguy600]

Spark timing advance can also cause a yank back condition but if its just being caused by too much compression then its either the cylinder is too wet from flooding or you really need a decomp to let off some of the compression to make pulling the recoil out much easier. You have to make a dedicated pull on my high compression 066`s, pull it like you mean it or the engine will rip your fingers near off, a decompression valve would help but I don`t like them, they are prone to leaking.

 

[ZeroJunk]

Maybe you are getting progressively older.

There are several different ignitions for the 066. Some of them have no advance and fire at 25 degrees or so before top dead center all the time.

So, if you don't jerk it hard enough to get through that it will kick back on you when it fires. Just the way it is.

 

[fearofpavement]

If you have a carbon buildup on top of the piston, that can actually cause an increase in compression. Take a peek at that to see if there's much of a layer. Some bigger saws like that, you really have to give a robust "all or nothing" pull to in order to keep it from making you say bad words.

 

[ncfarmboy]

If you have a carbon buildup on top of the piston, that can actually cause an increase in compression. Take a peek at that to see if there's much of a layer. Some bigger saws like that, you really have to give a robust "all or nothing" pull to in order to keep it from making you say bad words.

My 4200 Poulan comes to mind!
Shep

 

[ZeroJunk]

And yes, the decompression valve will help. When it fires the decomp will kick out. So, you will have to remember to reset it every time it hits until it actually starts. But, it does help.

 

[bplust]

Partially plugged exhaust port?

 

[KTM640]

JerTree, I'd check to make sure you have not sheared your flywheel key. I have not personally seen this happen but the only time I have had issue with a hard pull start was when the timing was off and it was because I had the flywheel in the wrong position. Good luck. Keep us posted.

 

[ZeroJunk]

JerTree, I'd check to make sure you have not sheared your flywheel key. I have not personally seen this happen but the only time I have had issue with a hard pull start was when the timing was off and it was because I had the flywheel in the wrong position. Good luck. Keep us posted.

That may generally be true. But, 064's, flat top 066's, early 044's are just nasty. Like take the meat off your fingers when they kick back nasty.
Newer saws start close to top dead center and advance with RPM .

 

[Evan]

My 066 tended to like to bite me at the end of a long day of bucking big wood. I ounce gave it a half assed im wore out pull. It did not start but it bit me so hard i left it there and called it for the day

 

[JerTree]

Thanks everyone for the feedback and sorry for the long delay in getting back to this post! I finally got some more time to work on this (and my decomp valve came in the mail at last).

I pulled apart the engine top and the piston head looked clean, if a little bit textured (but not really pitted or bumpy). No obvious scoring on the piston sides or cylinder.

Now I'm working on the reassembly (need to figure out what I've done wrong with the off/on/choke lever) and will update with results.

 

[JerTree]

Lol! Bad to worse - I sorted out the choke lever, put everything back together, and now I can't even turn it over once! Not even with the spark plug out.
Back at it tomorrow for another tear down.

 

[furb]

Check the top cover screw above the flywheel. If the wrong screw is in there it can hit the flywheel.

 

[Wood Doctor]

My 4200 Poulan comes to mind!
Shep

My Stihl 064 comes to mind also. No decomp to help, but when I get it running, it's a monster. Come to think of it, my Echo CS-670 is in the same league. Some of these saw engines have a mind of their own.

 

[Wood Doctor]

Lol! Bad to worse - I sorted out the choke lever, put everything back together, and now I can't even turn it over once! Not even with the spark plug out.
Back at it tomorrow for another tear down.

I don't like to scare you but you could have complete engine failure. The piston's skirt could be contacting the case at BDC (bottom dead center), blocking rotation. That happened to me with an 066 last month. No new pistons or cylinders could fix that malady. Let us know what you find.

 

[JerTree]

@furb you nailed it! I pulled off the housing to remove the decomp valve and I was able to pull over. Put the housing back on - the screw pinned the flywheel. I must have put in the short screw somewhere else - that's ok, I use a nut as a spacer and things are grand!

I've got the saw running again. Having the decomp valve makes starting is SOOOO much easier (even though it is still a beast)! Now to fiddle with the jets a bit so it keeps running at idle.

Thanks folks! You're all awesome and the reason why this is my go-to site for saw knowledge.

 

[Bridog]

Check the top cover screw above the flywheel. If the wrong screw is in there it can hit the flywheel.

Just did that myself a couple days ago... Maybe we can start a support group. Lol

 

[Wood Doctor]

Just did that myself a couple days ago... Maybe we can start a support group. Lol

I remember when that happened a few years back to me when an owner screwed in a bolt that was too long on an 066. I believe Stihl fixed that problem on the MS660. Sorry I missed that possibility for the rotation lock up. Regardless, I doubt that you would want to see that 066 piston failure I described that completely destroyed the skirt.

Good luck!

 

[furb]

Glad it was something simple.