Quantum Models - and it's a .60 not a .61. I have a lot of Fox Engines including 5 of the Eagle .74 engines (larger version of the .60). All of my Fox engines run well. Just follow the instructions. do not run over 10% nitro and use 18-20% oil with some castor.
Here is a link:
http://www2.mailordercentral.com/quantummodels/
Dan