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

 

[NWCS]

i can not speak for an 066, but i have an 036 pro with a decomp valve and have run some VERY abused 084s with decomp valves and have never had any problems from them, none have been replaced. the only decomp valves i have had issues with were on my Mcculloch SP125C's, and the issue is not breakage, but leakage from the vintage design. i would consider it a fluke, i have never heard of a decomp valve breaking on any brand saw!

 

[Gypo Logger]

Hi Greg, although I have never had any similar problems with decomp valves, I do replace them with plugs sometimes.
Here's the plug that Stihl offers for this very purpose. The part # is 1122 025 2200
I don't know the actual size of the plug, but get the OEM or you run the risk of doing damage to the cylinder.
John

 

[1953greg]

thanks guys. gypo, my plug looks bout like that except there is a 1/4" screw inserted in the top. thanks for the pic

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? plug is still mostly white.

any comments appreciated 1953greg

 

[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???

any comments/info appreciated 1953greg

 

[thompson1600]

I'm not sure, but is it possible that the valve is blocked from releasing when the saw starts, and then is damaged by being left open during the pressure/vacuum cycles?

Tom