does the throttle slightly jam at that position or just doesnt seem to have enough spring?

if it just needs more spring you can change that or cut it a bit shorter to get a bit more force from it. but thats not a good idea if the throttle is actually jamming for some other reason

hesistation when you first press the accelerator often indicates a problem with the little throttle pump on the carby

if you brake and accelerate at the same time the engine will only rev up if the clutch is slipping or the engine power is overcoming the brakes... you need a fair bit of power to do that

98 octance fuel should allow you to run more timing safely not less, from 7 to 5 is a decrease