Stihl 066 tuning issues.

Bob Hedgecutter

Bob Hedgecutter

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So, over the last wee while I have been rebuilding an old 066 that has been kicking around here in the form of various parts
Original flat top non-decomp cylinder, new OEM piston & rings, donor tank, new bearings, seals and rubber components- original carb.
Nothing out of the ordinary in the build- all pretty stock and what anyone might do to an early 1990's saw to freshen it up a bit for another few decades.
Had some issues matching up a coil to the flywheel- but had a few options to choose from and got that sorted- for some reason the crankcase was sitting with the old original alloy flywheel fitted and a three screw poly flywheel coil bolted in place- but I digress.

After borrowing a couple of pieces off my MS460 I got the saw ready for initial fire up this morning.
Popped on full choke, started on high idle, blip up to running position and all seemed well, WOT sounded good for initial start up on a build oiled cylinder.
Idle seemed a little low, low circuit sounded a bit rich.
Shut it down, tried starting up on "run" - no go, dropped back to high idle and the saw ran, blip to run and sounded rich again, idle slow.

Now the fun begins- LA idle screw adjustment makes very little difference to the engine revolution per minute rate, adjusting the L screw lean from the one full turn out fails to find that high rev position to back off 1/16th to a 1/4 turn rich for that nice position, richen it up from one turn out from lightly seated- saw bogs and stalls.
Should add- no ethanol in our fuel over here, saw vac and pressure test passed, carb is "clean" internally, had some new bits internally before the use here- but not totally kitted.

I have a likely candidate for what might be wrong, but who is willing to hazard a guess or provide an explanation for the lack of adjustment to the low circuit?
 
Bob Hedgecutter

Bob Hedgecutter

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LA is for idle speed adjustment. You would turn it in until it makes contact with the throttle linkage, plus maybe another turn or 2 just to let some air in.
L is for idle mixture adjustment.
Yep, got that bit sorted- but would you not agree that turning in the LA should increase the saws revolutions per minute, have an effect on chain speed/rotation? In as started condition- it would not. ;)
 
huskihl

huskihl

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Make sure that your idle speed adjust screw is making some sort of contact with the throttle linkage. Somewhere along the line maybe a different idle screw was put in its place. It’s a stretch, but anything is possible I suppose
 
huskihl

huskihl

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Yep, got that bit sorted- but would you not agree that turning in the LA should increase the saws revolutions per minute, have an effect on chain speed/rotation? In as started condition- it would not. ;)
I got ahead of myself and misunderstood what you wrote. Once I reread your post, I realized I was way off and deleted it.
 
Bob Hedgecutter

Bob Hedgecutter

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Make sure that your idle speed adjust screw is making some sort of contact with the throttle linkage. Somewhere along the line maybe a different idle screw was put in its place. It’s a stretch, but anything is possible I suppose
True that, it is making contact- just hard to increase an overly rich mix to a point where it will alter things with the LA screw if other factors are in play that alter the air fuel ratio?
 
SteveSr

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Does removing the carb, shoving a bit of old fuel line on the inlet nipple, blowing through it and manipulating the metering valve count?
No. A good carb must hold 7-10 psi for an indefinite amount of time. If it doesn't the fuel pump will push excess fuel through the carb possibly causing trhe issue that you are seeing.
 
Bob Hedgecutter

Bob Hedgecutter

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No. A good carb must hold 7-10 psi for an indefinite amount of time. If it doesn't the fuel pump will push excess fuel through the carb possibly causing trhe issue that you are seeing.
Exactly- but where is the main culprit?

My idea behind this post was for some of the folk here that do not own mightyvac's or have much of an inkling as to how carb's work, if they strike the same kinds of issues- where might they be advised to look.
 
giver

giver

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Did you accidentally put the diagram on the wrong side of the gasket in the carb? I've done it before and it sounds like what is happening with yours
 
SteveSr

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Exactly- but where is the main culprit?

My idea behind this post was for some of the folk here that do not own mightyvac's or have much of an inkling as to how carb's work, if they strike the same kinds of issues- where might they be advised to look.
Some things just cannot be properly tested without the proper tools. A mightvac alternative would be a sports inflator with a gauge on it. Just have to make sure that your connections don't leak.
 
Bob Hedgecutter

Bob Hedgecutter

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needle and seat issue I bet it is leaking
Winner winner chicken dinner.
A simple test of an old fuel line (or suitable tube) on the inlet nipple, hold the needle valve lever up lightly to seat the needle, blow into the tube and (if the lower portion of the carb is primed) fuel is emitted from the valve gallery- the valve is NOT seating, allowing fuel into the combustion chamber way in excess of what the low circuit would normally supply.
Was my number one suspect from the get go and was rectified this morning with a new needle valve and baking soda qtip polished seat.
Saw now starts WAY easier, idles properly and is tuneable on the low circuit.
A fault that can be simply proven with simple tools, no gauges nor pumps needed.
 
Bob Hedgecutter

Bob Hedgecutter

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Just as a matter of interest (or maybe not! :laugh: ) tuned to where I thought it should be by ear- tach read 2730 at idle and 12,750 at WOT.
By my ear, it was set a little rich at WOT for a (maybe 5 run hours) run in period and will probably be close to 13,000 once the rings bed in.

Some say 12,500 is tops for an 066, some say 13,000 and idle should be around 2500- I say when it sounds right to me, it is good enough and that kind of theory has not let me down in the past.
What say the masses? Should I set to the manual recommended speeds? Or stick to what I have been doing for years and just 4 stroking out of the cut and cleaning up in the wood?
 

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