you'll notice that idle speed increases slightly when fwd/rev knob is halfway, as there's obviously less drag, as moving less mass around. If the clutch drum isnt driving anything, and as it rotates freely around a roller bearing that's attached to the driven part of the clutch, then eventually it will pick up speed.
Your idle is too high-get the tickover smoother first-here's how;
make sure in neutral, engine at operating temperature
turn idle screw down to lowest possible speed'
turn one of the mixture screws in or out to achieve the highest possible smooth idle (you may need to turn the idle speed down as you do this)
now for the other mixture screw-again, turn the idle down to lowest idle speed possible, and repeat the above.
when done, set idle to 500 rpm
also;
lubricate the fwd/rev linkage
make sure gearbox oil level is correct. remove and make sure gearbox oil filter element is clean, and that the correct grade of oil is used.
terry