hello friends.yesterday my engine explode while idling(broke the shaft).after 20 seconds of running suddenly exploded(not overheated).i use for fuel, optifuel 12%nitro ,oil 18% for 2 stroke engines.os suggest 20% oil fuel.i do not think it was the 2% of oil difference that made this.i bought the engine new but from other person(not from shop).i used only 1 galon of fuel when this happened.the engine was almost new.i notice before the last flight some drops of fuel on the backplate(makes the airoplane very dirty).also the breath nipple tube was black(in the previous flights was clean).the engine could not keep good iddle.i use 12x5 prop.the airoplane piper cub 2.5 kg.i did not push the engine.i flew slow.i do not know what is the reason.it was my first fs engine.is it so sensitive this engine?it was the bad fuel(it was not for four stoke engine)?bad tunning?failure from the manufacture?what do you think(not make the same mistakes again)?