Try a new plug. Saitos plugs gives you the best high end, perhaps a bit worse idling than OS type F though. Also check tubing. Use thick tubes and clamp them in all ends. Maybe the oil mixture is on the rich side. I use 15% synth and 10% nitro with excellent result. Try an APC 13x8.

Well thats what I can come up with straight away.

You have checked the valve clearence that's good. Maybe a ball bearing is bad, does it make any strange noise? It should spin like a kitten on idling very smoothly and roar on Wide Open Throttle operation.

The Saito .91 is a real puller so something must be radically wrong.

Good luck!

/Fredrik