In the hi desert of southern california the guys back off of the nitro! Its one of the few areas you will ever see 12 1/2% nitro. Because of the altitude and the heat. You did not mention the temp but the hotter it gets the more sluggish the engine is. It's not like some more modern engines where a computer adjusts the mixture. Put the regulator screw back to flush or about 1/4 in at the most and richen up you low end. Stay with the OS F plug. I dont think higher nitro will cure the problem.
Be sure your fuel and pressure lines are coming out of the correct nipple.( sounds like they are but double check!) Otherwise contact the YS guru in the maufacturer section(Mr Shadel).
Good Luck!