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Chainsaw Repair
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Return to: Carb Fixes and Mods
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#Post#: 90704--------------------------------------------------
Finally discovered the culprit of flooding
By: joe_indi Date: July 10, 2024, 11:19 pm
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I had bought several aftermarket metering diaphragms for
repairing carbs, especially brushcutter carbs Stihl FS 120 -
250- 400
What had been bugging me was that the carbs stall if left to
idle for less than 10 seconds. Throttle down and they idle fine
but gradually idle speed drops, runs as if L is turned out too
much , then runs as if on choke and dies.
Initially I thought it was because the metering lever was set
too high and used to adjust it for a leaner level. But all this
did was make the engine more difficult to start and stall when
throttled was opened up. I was left with twiddling with the L
and H and idle screws the the whole time.
But yesterday i did what i should have right at the begining,
pressure test .
Ok i did pressure test with the metering chamber open and there
was no pressure drop. But yesterday when i tested i found that
one the diaphragm and cover were placed in position there was a
pressure drop. So I bent the metering lever down. But then when
I had the screws tight the pressure dropped again. So i
readjusted the metering lever again. Same issue and the lever
was bent so much it wouldnt go down any further.
It was then i thought of examining the metering diaphragm,
alongside an original one.
See that marked bit? On the OE it is nearly in level with
therest of the metal.
And, flip it over and the other side too juts up and on a Stihl
carb with a pressure compensator this projection hits the cover.
In effect once the metering cover is tightened down the
diaphragm is nearly locked down with no movement.
I resorted to the crudest remedy. Used a file to bring things to
Stihl/Zama standards. And it worked! Smoothest idle and
seamless acceleration.
The curse of these carbs seem to be their primers/purgers, which
i solved by blocking the paths on the carb top and the outlets
with some gasket sealant. Ok, they need 2 pulls extra in the
morning, but i found this helped me salvage a couple of old
carbs . I intend to try theses fixes on my collection of
disacarded carbs now that imight have a fix in hand
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