Sincere thanks for an interesting reply, Rovert, with some decent general suggestions regarding potentially stale fuel, lean mixture settings, and part-throttle-openings. Unfortunately they don�t quite seal the deal for me due to the number of years that the problem has taken to surface, the ease with which the machine still starts, and how reliably she still runs up until the point that the problem intervenes. Add to that the fact that I�ve already given the carburettor a thorough cleaning, to no avail.
That�s not to say that one of those factors you mentioned is not still the underlying cause, but unfortunately your suggestions remain too broad and unsubstantiated at this point. The issue is apparently not a common one, but the link to the other thread that I provided in my first post illustrates that it is not an isolated one either.
With regard to your comments that I should be running the unit at full throttle for heat-dissipation reasons, please note that I had a look through the Owner�s Instruction Manual, and there�s definitely nothing anywhere in there about needing to operate the unit at full throttle - even for just part of the time.
I did however discover the following paragraph amongst the General Notes on Operation:
�After long periods of full-throttle operation, allow engine to run for a while at idling speed so that the heat in the engine can be dissipated by flow of cooling air. This protects engine-mounted components (ignition, carburettor) from thermal overload.�
This is not an instruction to run the unit at full throttle, but rather an explanation by Stihl that idling the machine cools it better than does full-throttle operation.
I�ve got enough of the current batch of fuel left to run it again up to a decent temperature on the new mix-screw settings, and I will definitely run some fresh fuel through the unit after that if the problem persists in spite of the new settings, before pulling the pin on it. Hopefully all will be good again by then. For what it's worth, the fuel was bought from a very-high volume outlet.
Best Wishes,
Moby Mick.
