Don't undertand what you mean about the choke. On my mc18, I only use the choke for cold starts. Fire it up, with the choke on it should idle around 3k. I let mine warm up on the choke for about 1 min then close the choke and the bike will idle at around 1200 rpm. You shouldn't need to touch the choke again. Are you saying that when you apply choke when the bike is hot the revs drop? Does the bike idle in the first place? Have you got a fuel filter in-line? I had this problem, whithout a fuel filter, some gunk from the 20year old tank would clog up the float needle. Carbs off, clean, run fine, more gunk, carbs blocked again. A fuel filter fixed it no worries. Have you checked your coil connections? Pitted or corroded/loose connectors can produce some funny symptoms. Checked your battery voltage? Should be 12.5 v rising to around 14.5 volts at 2000rpm. All of these can make an NSR hard to start. Good luck!