I am fairly new to gas engines myself, so one of the more experienced guys can back me up if I'm mis-speak myself. The needle closest to the cylinder is the low speed needle, I believe. Both the low and high speed needles will affect the top end, but only the low speed needle has effect on the bottom end.

Most people I've seen use a carb with an 11.11mm Venturi on motors in the 25cc range. Walbro stamps a number on the inlet of theirs in 64ths of an inch. Your looking for a carb with a 28 stamped in the inlet. (28/64ths of an inch = 11.11mm) I'm not sure if its on the intake side or the engine side.

I have a 25cc Poulan that came out of a Weedeater trimmer that I'm running gas/glow in. Like you, I mix 1 part 15% Cool Power with 1 part HEET. I take that mixture and mix it with twice the amount of gas. i.e. 2 oz. Cool Power, 2 oz. HEET, 8 oz. Gasoline. I don't have any info on priming issues because I use a starter to turn it over. I get around 7500-7600 rpms with an APC 16 X 8.

I tried to find the carb adjusting tutorial I used to adjust mine by, but I can't seem to find it now and it's getting late. Hope this helped somewhat. I'm sure someone with more experience than me can answer you better. Good luck!

Mike D.