If a model engine that's been properly broken-in will not throttle-up without sputtering, it likely has the idle mixture set too rich. You may also have configuration difficulties, such as how the fuel system's set up, the type of exhaust system used, and so forth. Your best bet would be to "hook up" with another heli flyer to help debug the difficulty. Because you have two engines that have the exact, same problem, we'd look for something procedural or systemic. It would not be something with the engine, itself, other than adjustments.