Actually the higher nitro will retard the ignition! The YS's like higher nitro because it actually smooths them out. Whereas higher nitro on 2 strokes makes the needles touchy, it has the opposite effect on the YS. The needles become less touchy.

Jeff may have stumbled onto something. See if any friends have 30% heli fuel and borrow a tank or find a gallon of YS20/20 blend. Don't add anything to the fuel. Also if the age of the OS F plug is unknown then get a new one.
Let us know, Roy.