[murphyv310]

Are you drawing the spark from the top cap of the DY86? If so and you are using a decently sized screwdriver and only getting a 3mm spark, I would think the line stage is in distress!
Check the HT, the line drive preferably with a scope, one thing comes to mind. How old are you?
If like me you have a few grey hairs I would doubt you should hear a 625 line whistle so if its way low and sounding like 405 that will possibly be your problem!
Let us know your findings.
That 21 inch 110 degree tube will need about 16 kv EHT a spark of 10-12 mm is what I would be looking for.