Author Topic: jonsered 670 champ carb  (Read 820 times)

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Offline Eccentric

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Re: jonsered 670 champ carb
« on: November 08, 2016, 02:43:44 pm »
If you're bringing an old saw back to life for use as a work saw you should replace the crank seals.  Like the fuel line and carb diaphragms, the crank seals are rubber parts with a finite lifespan.  Sometimes crank seals will leak (or leak more) when hot.  When they leak, the impulse signal to the carb gets weak.   That will cause the saw to lean out.   A weak impulse signal afects the ability of the pump section of the carb to consistantly pump fuel from the the tank to the metering sectiion of the carb.

People often 'tune out' an air leak by richening the carb settings (often without even knowing that they are compensating for an air leak).  This can work for a while (until the leak worsens), but is bad practice and will eventually lead to damage.

Contrary to popular belief, a 2 stroke engine often does not 'race' or refuse to slow to idle speed when it has an air leak.  Those symptoms do happen sometimes, but not always.  Same goes with changing tune/idle speed with tilt.  That is a good indicator of a leaking crank seal.......but it does not always happen when a seal leaks...
-Aaron

For older saws:
Tune the H side so that it 4-strokes (burbles) at WOT unloaded and just cleans up when under load.
When you lift cutting load, the saw should immediately revert to 4-stroking.  Fine tune the transition point for the wood you're cutting.
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