I think I need to modify the spark gap a bit . Tonight I set up some 'almost' empty paint cans and shot them , with the sparking circuit leaning up against the can . I had the sound activated trigger connecting the sparking circuit through its relay and had to unplug the battery each time - I think the load is a bit much for that relay and it sticks a bit .
I shot with the D90 in high speed mode at iso 200 , F9 and 1/1000th sec and only got one real fireball out of the lot , none of the others ignited . Since it was raining I stood in the garage with the can just out of the shelter ....

I should have had a wider lens - I never realized it would creep along the floor .