With eggs square on my face I see I was looking at specs on the Cobra 28-14. OOPS! I would still go to a smaller prop, even drop back to an APC 13 x 4.5 and see what you get. On that same 72 ounce pig of mine, I was happy with that one or APC 13 x 5.5, ESC at the time was CC Ice Lite 75. I know you have your own home built props but it might be wise to go back to some stuff we know works and go from there. With the 4.5 pitch prop you will need to be somewhere around 10,200 RPM.