I had a similar issue with a 4BTTS using AT Racing 25% with only 8L, see my post below.

http://www.rctech.net/forum/12899346-post6293.html

If you read from pages 420-422, you will see the issue I had chasing the tune from the begining. Everyone agreed it was the fuel, you can see in the pics the engine was filthy!!! Ive changed brands so see how I go.

I did end up cleaning the motor out with Autosol and the parts came up shiny and bright but in my frustration in trying to get the sleeve out, I used a pair of vise grips and ended up pinching the top by 1/2mm and couldnt get the button in