Issue is couple of weeks back when I started I was thinking exactly the same. Hence I started wit a 14x4 and after I took off, we could hear the high pitched sound of the engine overrevving. My 14x6 is on the FS91SII so I put in a 14x8. Dont think that the prop will make that much difference. Remember, I have the very same problem with the 55ax before this and I used all props from 12x4 to 13x6 (12x4, 12x6, 12x8, 13x4, 13x6) but couldnt resolve it. Was using the same batch of fuel though

Ameyam