Scored Pistons. WTF!

[Mr. Sir]

In the last year or so, I've had 4 or 5 saws die on the job. When we opened them up, the piston and cylinder are badly scored. I suspected improper fuel mix, but the guys insist that they mix it right. We use avgas, 100LL, with Stihl brand oil mix, and Sta-Bil fuel stabilizer.

Now, I have one ground guy that likes to warm up the saws by running them at full throttle for like 5 to 10 seconds with no load. I've told him several times not to do it simply because I don't like the sound of it, but could this be damaging the saws?

 

[CurSedVoyce]

Did you switch to Av gas after running them on unlead prior?

 

[Mr. Sir]

Yes, for some of them.

 

[woodworkingboy]

Brett, you mix the gas.

I hate the sound of prolonged WOT too. My guess is that it doesn't do harm if the saws have proper mix and are adjusted correctly.

 

[Reddog]

My guess is that it doesn't do harm if the saws have proper mix and are adjusted correctly.

+1
Lean lube or Lean fuel setting.

 

[PCTREE]

plus you dont "warm up a saw" by running it wide open, that could imo be the problem.

 

[CurSedVoyce]

Agreed... not a good practice....
Also have to watch your carbon when you switch to the Av Gas... Avgas can knock it loose.
Also check your scoring.. it could just be lead residue.
Did the saws actually loose compression or did the plugs just get fouled and you peeked in the cylinder.
Good night all I don't feel too good...

 

[SouthSoundTree]

I like to warm 'em up by letting them idle for a minute after starting.

What types of temperatures is this guy warming them up in, at WOT?

 

[Mr. Sir]

No chit, we're in Floriduh!

 

[Reddog]

If they were water cooled sure it would set them right up. Air cooled usually not a problem. So in FL not much chance.

How you do set top RPM? Tach? Ear?
Who adjusts that, you or the crew?
How large a batch of fuel is being mixed at a time? Is it mixed every time before it is put in jugs?

 

[sotc]

Carb settings?

 

[Mr. Sir]

The saws are set up by the dealer when purchased, and whenever they go in for service. Otherwise, we don't mess with settings.
Fuel is mixed in a 5-gallon jug, then doled out into 1-gallon jugs. We always shake it up before pouring.

 

[Reddog]

Fuel mix system sounds good.

I would purchase a tach and check the wide open throttle rpm. Makes me think they are being run lean.

 

[woodworkingboy]

Dealer settings...hmmm. Every once in awhile run wide open and listen to it. More often than not, a bit rich I find. A little turn with the driver will do ya.

 

[Jed]

I wonder if filtration problems cldn't be to blame.

Like Jay--as usual--I think that the ground-guy's (really dumb) warm-up method is not necessarily to blame for the scoring. I'm a huge advocate of the white Max-Flow filters as opposed to the stock HDs.

 

[Al Smith]

The question is which side was the piston scored on .Intake side would be dust ingestion .Exhaust side either lousey oil or lean run .

Now do what you want or run whichever oil you choose too but were it I in the Florida heat I'd personally use a tad more oil in the mix .

It could be other causes too,all that heat and plugged up screens will cause problems .

 

[Peter]

Cold seize?

 

[Al Smith]

Well maybe if it fell though the ice on Lake Erie in Febuary .I somehow doubt that would happen in Florida .

 

[Magnus]

Is it scored all the way around?
I think they run a bit lean and got hot. Perhaps in combination with overload or air leaks.

I never advice fuel change unless saws are taken down, cleaned and new rubber parts.

 

[Magnus]

Starting saws in above freeze temp is not complicated. Run it up and down a couple times in rpm and off you go...
Same thing with cooling too hot saws, never shut down or Idle to cool as there is not enough airflow around engine. Flip throttle without load and it cools faster.
Most important is setting, so it doesn't hesitate on accelleration, EVER regardless of temp!

 

[SkwerI]

Brett, do your saws still have the spark screens? Restricted exhaust equals higher operating temps. And how often do they keep pushing with all their weight on a dull chain cutting wood in 95 degree heat?

And 'dealer adjustment' usually means just turning the high speed jet smack against the limiter cap and saying "that's good enough". They are usually more concerned with keeping it within mandated limits than actually getting it to last without burning up. I've never had one reset the limiter caps to actually give me any real adjustment.

 

[Al Smith]

Here's another thing that dawned on me .Do those guys run those saws until they die on the vine from running out of gas in the tank ? Not good if they do because done enough that way and given enough time it will take it's toll on the engine .

If they are lax about checking the fuel the best advice I can give is the first time that saw hiccups to get off the throttle .It will have enough fuel left in it to idle a tad ,then shut-er-down and gas it up .

 

[stehansen]

Could the way that they start them expand the piston faster than the cylinder expands and cause the scoring?

 

[Magnus]

Not unless they are set lean.
I thgink that is the culpit in this deal.

 

[CurSedVoyce]

Speculation.....