Stihl 066 Operator Said, "It Stopped Dead."

[Wood Doctor]

... Then he called me. He said the saw went through three or four tanks of fuel. Then he could not even pull it over and restart. I carefully took it apart, hoping the top end was still OK. It wasn't. Here's the piston from all sides:
Exhaust:


Intake:


Left of Exhaust:


Right of Exhaust


So, what caused it all? I have my ideas, but I thought I would not bias your opinion. I'm rebuilding the top end as I write.

 

[dall]

hows the bearings and seals

 

[Redfin]

Tore apart a 460 this week looks just like that. Carb inlet plugged.

 

[czar800]

Welcome! Looks like you have a stash of Stihls under your bench!

The forensics team will be a long soon!

I'm going with straight gas?

 

[Wood Doctor]

hows the bearings and seals

Fine. Just checked them all. Heck, at first I thought the rod was gone. Engine could not even turn over. I'll run a compression test after the rebuild. just to be sure.

 

[beaglebriar]

Fine. Just checked them all. Heck, at first I thought the rod was gone. Engine could not even torn over. I'll run a compression test after the rebuild. just to be sure.

You mean leak down test right?

 

[Wood Doctor]

Tore apart a 460 this week looks just like that. Carb inlet plugged.

This was a brand new carb. However, guess what I just discovered, The hi-speed setscrew was closed all the way. Is it possible that the engine ran lean at WOT end for about three or four tanks of gas and then produced this complete shut down?

 

[Wood Doctor]

You mean leak down test right?

Yes, of course. See above post. I think the carb was running a really lean mixture at WOT. Owner says the same fuel is being used on other saws that are running OK.

 

[jmssaws]

This was a brand new carb. However, guess what I just discovered, The hi-speed setscrew was closed all the way. Is it possible that the engine ran lean at WOT end for about three or four tanks of gas and then produced this complete shut down?

Yes.

 

[beaglebriar]

It was definitely lean. Sounds like you found the cause.

 

[dall]

if running so lean i would think there is bearing damage also to seize it up

 

[czar800]

This was a brand new carb. However, guess what I just discovered, The hi-speed setscrew was closed all the way. Is it possible that the engine ran lean at WOT end for about three or four tanks of gas and then produced this complete shut down?

That would do it!

 

[jmssaws]

It was lean and seized, it don't need bearings and seals it needs a piston and the carb tuned correctly.

There won't be anything wrong with the bottom end,you found the problem and it's a wonder it lasted 3 or 4 tanks.

I don't think the operator knows saws very well cause it would have been screaming lean.

 

[czar800]

Got a picture of the plug? It's got to be white as snow.

 

[ft. churchill]

The bottom end bearings will be hardened steel against rolling ball bearings that are very hard, the piston is soft aluminum against hard nicasil cylinder wall and sliding not rolling friction. Ball bearings can get by on very little lube, pistons, not so much.

 

[Wood Doctor]

Got a picture of the plug? It's got to be white as snow.

It was. I forgot to take a Pic of it before I cleaned it up. I've got some snow in the yard that might be a bit whiter than this plug. This baby was running hotter than a $2 pistol. One or two of the cylinder fins are missing, but I doubt that would have produced this.

The carb has these plastic limiters on it, so it's possible that the setscrew was not all the way closed. Regardless, that worries me. I'm ready to pull out that limiter. These things drive me bonkers.

Oh, and I just checked the exhaust port on the jug. On the inside at the exit, there are little sharks teeth coming down. I've seen top end failures before, but none produced enough heat to cause that. That might be worth a Pic by itself.

 

[darkimpulse]

That thing must have been screaming like a dying cat. I hear you on the limited coils, hate them too. I always have to set the high when cutting into wood. But that's just me, the ole firewood hack. Glad you found the problem and are getting it running again.

 

[Deets066]

Exhaust side failures are usually caused from a lean condition, operator error here.

 

[drf256]

He probably thought it was a very fuel efficient saw.

 

[Backyard Lumberjack]

Exhaust side failures are usually caused from a lean condition, operator error here.

lesson to be learned here...

since too lean is detrimental to chainsaw piston life... maybe check that H setting more often. each tank full, at least the slot's position? the mixture adjustments can vibrate inward... my 019T started acting like an unfriendly cousin... I wasn't happy! so, started the research... finally got into the LA, L and H... etc... tweaked the L and the LA... and got it back on course... then H. now once running will sit there and idle with chain just creeping... pop-pop-pop! with an "Let's get this show on the road!" attitude... and throttle response is crisp up to cutting rpm... each to his own, but I never run my saws on a steady WOT when cutting... oh sure WOT but I momentarily lighten cutting load and back off, just to change rpm, let piston cool bit, cyl oil up, skirts oil up... then back to work... etc. I was taught this procedure by my Dad at a young age... he was a well-trained very experience engine mechanic... A&P type... and was an air-cooled engine specialist... very mechanically inclined! not saying it's gospel, just sharing what I do.

darn!, guess I will loose the chainsaw race! lol

too lean hurts: I picked up my 044 from an owner/operator/arborist... used it every day. day after day! over two years he owned it! ran fine. one day all day, fine. next morning on the job 15 mins and stopped. had no idea what was wrong with it, and sold it. I got it next day still with full tanks of fresh fuel and oil for chain/bar... back at shop, when I got the muffler off, i could see he had scored bad the piston exh side and stuck the rings hard! that saw had had it. lol... no doubt he had not checked H lately, and it leaned out on him and in the excitement and noise of running hard... Murphy showed up! as I got into it, was relieved the cyl looked very serviceable once cleaned up... speaks very well for Nikasil - but alas, not so the piston and rings!

this 044 piston has seen better days!



rings on exh side stuck hard!



intake side; rings still loose in lands...



and the winner is!: