Hi!
It's no good to have the hole inthe props bigger than the axle on the engine.
That's a problem with using metric size props and using inch size axle.
To prevent this from happening I always turn my own aluminum spinner nuts, with a flange that goes inte the prop to center it corectly.
I also recommend that you use the Tettra line of "bubbleless" fueltanks that the pylon guys are using. These tanks can't have the fuel foaming at all. Dave Shadel of Performace specialties in Nevada (US YS importer)sells them.
As for props, a 8x6 3blader seems a bit small for a 4.0 cc engine (.25), you must get around 13000-14000rpm using one of those? And that is a bit too much. I would try a 9x6 APC 3-blade prop and if this is to much (my MVVS .28 engine using the ultra quiet MVVS silencer has no problem spinning one of those) I would file away on the prop, either lessen the diameter or reducing blade area by filing away some at the trailing edge.
For a 2-blade prop I would try a APC 10x5 or a 10,5x 4,5 APC
Regards!
Jan K
Sweden