How exactly are you driving the BU807? I expect with an ignition coil you will need a bit more current than what the micro will provide. Also D2 needs to be placed across the coil not from VCC (which I assume is 5V) to the coil. I would say this circuit is unsatisfactory for driving an ignition coil for many reasons - what biases the transistor? A 1N4005 diode is probably not a good choice especially if you want high rpm out of the circuit. I suggest you use a circuit meant for driving an ignition coil - if you want it to be reliable.