The fuel is significantly under "nitro'd", under oiled and the engine is under propped. YS engines run best with 20 - 30% nitro (Hence the name 20/20 YS Fuel). Less than that and they become significantly harder to tune and run. The fuel should be a minimum of 20% oil, preferably all synthetic. Prop it to run 11,000 to 11,500 on this fuel and it will be propped correctly (most people seem to find that it means 12X8 or 13X6 APC, the main thing, though is to make the engine run in the right range). Reread the instructions, it mentions all of this. YS engines generally cannot be fueled and tuned the same as others. One other thing, after reading the other posts, use an OS F plug.

Edit for addition.