I posted this before but never got a definite answer, so I though I'd try again.

When I start the bike (NC30) on choke, the No.1 downpipe stays cold. As soon as I can shut off the choke, and give it a blip it instantly starts to heat. I've checked the plug and leads by switching them around but it always stays the same.
Is it a blocked choke circuit or something. Carbs are clean & balanced.

Any advice appreciated.

Yup id say itl be a blocked choke circuit. I had it on all 4 carbs though so the bike would die on the choke. Poke some fine wire down the 3rd brass jet thing (the one that isnt the pilot or main jet) and see if that helps. I know it did for me

Good luck