Looking for options to address mid-range burble in a Homelite 30cc magneto engine.
Top RPM with a Master Airscrew 18-6 prop is about 8,300.
It will idle forever at 1,500 rpm (see video below).

Fuel is 93 octane ethanol free, with 32:1 STP oil mix.

When reducing throttle on the ground, burble will start around 6,000 rpm.
In the air, the burble will start around 7,000 rpm.

Low end is leaned for good transition,
Further leaning of the low end produces too much hesitation.

I thought the problem was solved by leaning the low end on Sunday morning.
But as the day warmed up, the burble returned.

Since the engine has been so reliable, I can live with the burble if it can't be easily fixed.
Any suggestions will be appreciated.

Thanks!