I'm gonna take a punt and say, the gunk on the needle jet was probably the culprit  

Glad to hear it all worked out  

Members here have mixed feelings about fuel filters. There's already a filter on the fuel tap, so an inline filter is like adding another place where things can go wrong without any real benefit.

FWIW, I spent a lot of time tuning my carb (as a newbie). I also like that little 'crack' when I drop the throttle. It goes away during winter and comes back in the spring - we're heading into spring here now. I haven't heard the crack for a few months. Then we had a 10 degree (celcius) jump in temperature yesterday ... and there it was