Are you talking about a .61?

If a .61 an 11x7 or 11 x 7.5 or 12 x 6 is a good prop choice.
Don't screw the pump regulator screw in too far...just a little bit of pressure is all it needs...just bring the fuel to the carb, don't try to force it in. If there is too much pressure, the high speed needle will be almost all the way in (typically something like less than a turn open ) and will be very sensitive...so that will tell ya to back the reg. screw OUT some. (the reg. screw is like an Oxy/Acetylene regulator, in that you turn it IN to increase and out to decrease pressure )