I'm glad you're biggest problem appears to be this. You've got a very nice bike if that's the case.
As a general comment, don't get too hung up on the perfect operation of these bikes. They were a little rushed into production, they were barely more than prototypes and a novel engine in a completely new application. They are quirky, a little odd and frankly, rarely ever completely perfect. Remember, Norton fiddled their rotary for nearly 20 years before releasing it.
This may help you. It specifically refers to your issue (this is the brass screw angled at 45 degrees on top of the carb):
From Bulletin 9, July 11, 1975, page 9.
19. Setting the idle mixture:
a. Turn in the primary pilot mixture screw until the needle lightly bottoms. Back the screw out approximately ½ of a turn.
b. Thoroughly warm up the engine and set the idle to 1200 rpm.
c. With the bike on the center stand quickly open the throttle grip to bring the rpm’s over 5,000 then quickly close the throttle grip.
d. If the exhaust has a popping sound or afterfires on deceleration turn the primary pilot mixture screw out approximately 1/8 of a turn and repeat step c.
e. Continue this process until there is little or no hesitation when the throttle is quickly opened and no popping or afterfiring when the throttle is quickly closed.
f. Reset the idle to 1200 rpm if it has changed.
Thank you. The main service manual has a big disclaimer - "Do not disturb the pilot screw . . . factory setting ... based on individual flow measurements" I was just wondering if something may have changed to cause this but I take it as just little idiosyncrasies? I'll get it good a warm, then see how it its, then maybe tweak and 1/8 turn. And yes, the frame is chromed. That's how I got it. And with the Lester wheels too. There is a thread on this bike here as it was when I got it. Just search for Lester Wheels.
