What is interesting about the Saito exhaust residue, is, it is my understanding a Walbro carburetor is NOT an ideal fuel distribution device for a radial or any multi-cylinder engine.
That said, NONE of my Walbro equipped gas four strokes spew the goo like my Saitos.
I have many flights on a RotoMotors 85 inline twin, running 30 to 40:1 Amsoil, and the engine exhaust is dark, but dry... VERY clean when compared to a Saito.
Dittos my Moki radials.
Goetz Vogelsang has said the Saito carburetor is superior to the Walbro for a radial... WHY does it not provide a mixture that burns more efficiently?