Hi!
There is your problem! Tank is to high ...or carb to low!
But why on earth use 15% nitro when all it takes to have nice throttleing /running engine is 5% nitro.
Do not use a warmer plug!
If you use as much nitro as 15% you could use a colder plug than the OS 8,but not a warmer plug and certainly not a an OS F plug. Warmer plugs are used when you run less nitro or no nitro at all...or have less compression.
The OS 8 or Enya 3 or 4 plug will work good with 5% as well as 15% nitro.
So...Lower the tank and set the low speed needle and you will see that your engine will work just perfect.
