Don't usually like to dispute what other have said, but my experience runs contrary to what has been posted here.
First, I run all my four stroke engines (OS 120 Surpass III with pump; OS 70 surpass II blackhead; OS FL 70 and Magnium 91 RFS) on less than 10% nitro. The nitro percentage is around 7.5%. To further stir up the pot, i use quite a significant bit of castor oil; medicinal castor oil. This was not necessarily out of choice, but out of necessity at the time, as I as based in Dhahran Saudi Arabia where fuel was pretty expensive, so had to go for an alternative.

The Magnum 91 and 70 surpass II are both inverted and idle just great (~2100 rpm). So do the others, which are at the 7 o'clock position.
One thing though; tank height was very important. I had major issues with the magnum until I lowered the tank about 1/3". Also they all have OS F type plugs in them.