recheck all your plugs and make sure they are tight...one of the signs that a spark plug in a 2V head is backing out is a ticking noise. This is a common problem with these heads...they sometimes eject plugs and strip threads. Add some antisieze on the thread and make sure they are tight but don't over tighten...

 

Also, if the boots are properly seated onto the plugs, you could have an arc between the contact and the plug. This should be hard to do with the design, but you can't rule it out 100%.