Overtightening the prop or using nylock prop nuts are bandaids. You need to solve the problem. You are experiencing pre-ignition caused by too much heat inside the combustion chamber. That is usually caused by too lean a mixture, but if your on-board glow is constantly on, that may be contributing to the problem.

Overtightening the prop can also fracture the hub, causing the prop to throw a blade. You don't want that to happen.

First, disconnect the on-board glow. If that doesn't solve the problem you will have to richen the mixture.

The first question is; At what throttle setting does the "backfire" occur? If it happens at less than full throttle, then your idle mixture is too lean. If it happens at full throttle only, then you high speed needle is too lean.

The high speed needle on a high compression four stroke like your Enya needs to be richened so that the engine runs 300 or 400 rpm less than peak rpm. The idle needs to be on the rich side of smoothest idle too, so that the midrange stays rich during transition.

If that still doesn't solve it, you have a fuel feed problem and need to look for air leaks in the fuel system or carb/intake manifold.

Good luck, Jim