Detonation is a term used to describe combustion that's occurring too early and The popping sound you describe sounds like it. If everything else is normal and correct, a timing adjustment(s) will be required to correct this condition and since there is no ignition adjustment on our engines it must be done by shimming the head, using a lower nitro content fuel, or using a cooler plug.

With that said, I know plenty of folks running 30% and even 33% in their TM's with no issues, so before you start working on your engine, make sure everything else is working as it should be. If your fuel and exhaust systems are OK and you have no binding in your driveline my first guess would be old fuel. Our fuel is hydroscopic, meaning when exposed to atmosphere it will absorb water. Water is 2 parts oxygen and more oxygen means a hotter burn. A small amount of water absorption is inevitable and won't matter, but fuel left in the tank, or a partial jug stored for any length of time will take on enough water to make a difference in running temperature. The old fuel may not be something you are doing, it could well be too old when you bought it. Many shops buy their fuel in large lots to save money and if sales are not brisk it sits on the shelves for months on end waiting for a buyer.

Last of all, when the revo first hit there was an issue with chips of material left in the tank from threading the fittings that was causing fuel flow problems. Traxxas even had a bulletin on their website detailing this issue and recommending the tank be flushed prior to use. These chips were found as far upstream as the banjo, but having the same problem with two new engines would seem to discount that as a possibility . Stranger things have happened so it might be worth looking in to.