Those hdc carbs are finicky. Period. For starters, new seals (no crankcase leaks) go a long ways in providing the impulse needed to maintain steady idle and transition to throttle. The capillary style high speed check valves can curl a bit in the pocket they reside. This can lead to lean idle symptoms. Use a piece of fuel line and gently blow/suck with it sealed against the screen under the metering plate. It should only pass into the Venturi and check (well) on the reverse. As well, there is an additional impulse port that is opened upon throttle. It is indexed through the throttle shaft on the rear flange of the carb. Make certain this port is clear and any new gaskets are punched out in that area. You should be able to blow cleaner or wd40 through the screen on the flange and have it move out into the metering area when the throttle is opened. From what I gather, this may act similar to an accelerator pump. Helps with throttle and the L can be adjusted fairly lean.
The idle circuit is not independent of the high speed. I've noticed such also An adjustment on the H jet will really impact the low. (especially if the saw is idling fast) Set the high first and leave it. (right) Then adjust the L as lean as possible for throttle response. (and start slowing down the idle with the idle speed screw so as to lean out the intermediate idle port instead of backing out the L idle jet to slow the idle speed)) I have found on many of them, that if the idle speed screw is set in too far it with richen up and die. Likely a combination (both ports intermediate and low feeding the engine) of the throttle shaft impulse opening slightly and the intermediate idle port being opened by the butterfly.
