The PD42a/b carbs (yours might be the "b" model) have a lean idle circuit by design, and the idle mix adjuster screw's operation is the key to engine speeds below 1800 RPM. There is one critical item re; the idle mix screws' tiny O-ring(s) on that long needle of a screw. This must be installed above the tiny washer that these screws press on as you adjust them: they always seat snugly and then back out loosely, with the spring pressing the washer upward to help seal these tiny O-rings. In many PD42[n] carbs these tiny O-rings are hard (aged) if not replaced, or could be in upside-down ( i.e., on the bottom, the wrong side of the washer): both situations will cause the bike to not idle at all when cold unless the choke is on, and after it warms up it tends to not idle below 1700 RPM - it just dies instead. I find many of these tiny O-rings to be still and flattened, which makes them unadjustable in operation.
Another thing that will cause this sort of low-RPM instability is the ignition timing, specifically too much advance at idle. I've seen many Dynas that would not cooperate with this, because the pickups were too far away from the rotor's magnet. The pickups must both be the same distance away from the rotor-magnet's face, too. A timing light will help with this, although the critical setting is the one you're also struggling to get: the timing marks "1-4 F" and "2-3 F" must align on the indicator at idle, and not jitter back-and-forth. If they are jittering around, then the points advancer shaft is bent more than 0.006" out-of-straight and will need to be trued. Turning the engine backward with sparkplugs installed, using that tempting big nut by the points side, will bend the shaft, nearly every time!
