RE: Saito 150 problems
I think you misread the print on the previous prop... it was probably a 15x8... my 150 does nearly 10K on a 15x8 and 10% fuel.
An 18x8 is WAY too much prop for this engine. A 16x8 makes a very good "all round" prop. Aim for the low 9000's when selecting your prop.