I tend to agree that you need to get the prop tight.

Read the manual carefully beginning with page 16 for initial needle setting and the next several pages have info about break-in and tuning.

I suspect you are running the engine too lean.
Your RC car engine turned many more rpm than your plane engine will, and the sound is different. For the plane, having a tachometer would make tuning easier. RPM will increase as you lean the engine, then it will decrease again when it is too lean and beginning to overheat. You can hear this but a tachometer makes it easy.

Page 37 of the manual gives specifications at 16,000 rpm. Chances are your engine will turn more like 10,000 to 12,000.