that prop is to small of a load in flight for that engine. I run the 7.4x7.8 to 7.4x8.0 on my short strokes. The long strokes like the 7.4x7.5 and 7.4x7.6.

since you are running that light load it will unload more in the air. get some 7.8 props, run 15% nitro. I dont set by a tach but get them close to a good sounding peak and then back off slowly till hear the rpm drop ever so slightly. it seems to me with the 7.8 to 8.0 props that it like to be very close to peak on a Q40 model. the LS need to have a larger rpm drop. The LS with stock timing runs good on a 7.6 prop but can goto .005 on sleeve and .008 more on the head and rune the 7.5 prop for more rpm. this should bring timing to 196 or so.