I have been competing in trials for the past 4 years and stalling fives are ruining my scores. Apart from a faster tick-over I have been avoiding my back break hindering my riding in other ways.

Any advice please.

if riding without using the clutch, work the motor against the rear brake and practice doing very tight turns until you get it all working automatically

if riding with clutch, same as above but also control the amount of clutch slip at the same time

A flywheel weight will help,as will setting the idle mixture a shade rich.300s do seem to stall easy if you let the revs drop really low,my sons 300 stalls easier tham my 280,you have to really concentrate on your clutch/back brake control

+1 on above suggestions. I would have another GG300 rider try it and see if it's the bike or rider. If the bike, and your jetting is set ok, a low comp head insert/spacer would also help alittle.

+1 as above

Make the idle mixture a little richer to help a bit, or retard the ignition 3mm on the ignition base plate, or lower the compression ratio, or polish the top of the piston and head ......or all four...... I did on a 270 Beta and found it a lot smoother and I didn't notice anything missing from the top-end power.

Thanks, that gives me something to work on.