G'day Zippy, and welcome to the ODK Forums.

The mixture screw on these only affects the idle, and the correct initial setting is 1.5 turns out from closed. Note that this is an air metering screw, so it's clockwise for richer, anticlockwise for leaner. I'd say you will have to adjust it with the air cleaner off the engine.
The idle speed screw initial setting procedure is to back it out until the throttle butterfly is completely closed, then turn back in 1.5 turns.

It has been found that the Mikuni carbies on these engines are particularly susceptible to getting gummed up from old fuel [89 octane unleaded is the worst for this], and this commonly causes surging/hunting. They are a bit fiddly to clean out properly, but doing so usually has good results.

These past threads discuss this in some detail: https://www.outdoorking-forum.com.au/forum/ubbthreads.php/topics/49918/

https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=50677

Cheers,
Gadge

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