066 decomp woes

[1953greg]

has anyone experienced this?  the decompression stem sheared while the saw was running.  i shut it down as soon as i could (2-3 sec) after hearing the unmistakable "choof choof choof".   motor stopped w/ a clunk and was locked up til i rotated the flywheel backwards.  there is a grove in the stem that 3 ball bearings detent on.  snapped at the grove.  inside part went into chamber.  after removing muffler, sparkplug, and what was left of valve,  best i can tell it looks like everything is ok.  the seat of the stem has 2 flat spots on the sides maybe .010 deep.  i have removed the top of the decomp (everything above the hex shoulder) and tapped the valve guide and screwed a 1/4" screw from the top w/ jb weld to seal off the valve guide and wil install the plug tomorrow.  maybe i dodged a bullit.

   this is the only saw i have w/ decomp.  is this a fluke or is it a common failure?

any comments appreciated     1953greg

[jokers]

WOW! Never heard of that happening before Greg.

I think that I`d want to be sure that no small parts are trapped in the intake port of the cylinder just waiting to be re-injested by the engine. I`d pop the cylinder off and have a look. Just be careful that nothing falls into the base of the engine as you pull the barrel.

Russ

[ehp]

I  had an 066 do the samething but I seen it before starting the saw so nothing got hurt, most saws have the hole in the jug smaller than the tip of the decompressure so if it breaks it doesnot get into the motor , 99% of the decompressures are 10mm by 1  for the tread and around here every hardware store has bolts with that thread , so I just cut the bolt to the lenght I want and tighten it in

[1953greg]

put the saw back together today w/ the plug. everything is ok except my fingers. an 066 is a son-of-a-gun to cold start w/o a decomp valve. not too bad after its warmed up. bout like a cold 440. anyhow, i wil make a trip to the dealer for a new valve. really just wanted to get it running to see if everything is ok. looks like i dodged the bullet.

another question. what should be the 066 exhaust temp be while milling? checked it today and it looks like about 500*. my thermometer only went to 450 so i was guessing at the rest. im running 93 octane w/ echo oil @ 40:1. hi speed jet is 1/4 turn more than normal. can i get it too rich if it still runs smooth?  richer is cooler, right? plug is still mostly white.

any comments appreciated 1953greg

[StihlDoc]

Engine temperatures are normally taken at the spark plug gasket and with a probe inside the muffler at the exhaust port. Sounds like you don't have the proper temp gauge and you should not worry about taking engine temps anyway. Milling lumber is a demanding application for a chain saw. You can richen up the high speed (H screw) to a point just lean enough before the engine starts to "blubber" at wide open throttle. Or, if you have a tachometer (or a good chain saw dealer) set the engine speed to 12,800-13,000 r.p.m. Your engine heat will be well within safe limits. Also suggest you use STIHL 2-cycle mix oil as it has a high percentage of bright stock. Bright stock is an additive that does not easily combust. Its purpose is to provide additional bearing protection and reduce the risk of piston scuffing and seizure at higher temperatures and engine loads.

[1953greg]

guys you wont believe this but,

i installed the new decomp valve this afternoon and milled using 2 tanks of fuel.  took the saw to the bench to check tightness and the button was gone!  yep, sheared off just like the first one.  i used it on the coldstart and didnt notice anymore.  no damage as the lower half was still in the guide.   

what the heck is going on??  the button is recessed in the coweling and i am milling in my yard so there is nothing nowhere to touch them!  i have had this saw for 2 years and never han any problems with anything til this. 


still dodging bullets. dont know how many more i can miss.

any ideas???

stihldoc,
temp was taken bout 4" inside a dual ported muffler w/ a probe thermometer.  at nitetime i can see a little redness in there.  is 500* bout right?

any comments/info appreciated    1953greg

[StihlDoc]

Exhaust gas temp inside the muffler at 2" from piston face is approx 1,200 degrees F. You have something unusual going on with the deco valve. There is a plug available that replaces the deco valve but then you lose the benefits of the valve. You may want to take the saw into your STIHL dealer so they can check for any issues that may be leading to the deco valve failure.