There is no "lean mix" screw. The two adjusting screws are the throttle stop and pilot airbleed. That second one adjusts the amount of air entering the pilot (idle) circuit. Out/CCW increases the amount of air in the pilot circuit, leaning-out the mixture. It's only effective from about 1/2 turn to ~3 turns out from seated. Less than 1/2 out, the airflow is cutoff and the pilot circuit goes dead...no fuel or air. But the important issue is that pilot (idle) circuit operation...and by extension all adjustments to it...are only effective up to about 1/8 throttle opening. Beyond that, it's all up to the main metering circuit. So, if you're expecting those two adjusting screws to have any real impact on normal riding, you're going to be disappointed.I happen to know the original owner. I’m the second owner of the bike. He ran it until it quit and parked it. So yeah. Odd carb. But stock. I’m seeing that 1979 not so great.
Got the bike to run without choke yesterday by messing with the lean mix and idle screws. But it’s boggy. Weak. So I guess I need to look up and figure out how to adjust the mix properly. I will probably pull the carb again and take it apart and clean it using my ultrasonic cleaner.
What you need to do is figure out why the main metering circuit isn't delivering enough fuel. There aren't many adjustments on this type of carburetor. Float height is fixed and, I believe that jet needle height is also. Basically, the carb has to be in perfect working condition, or the engine goes lean; that was the emission-compliance solution. It's kinda lame, imo. Surprisingly, these emissions-era carburetors do seem to respond well to a proper cleaning. If you're sure that the emulsion tube orifices are all flowing equally, there aren't many other places to "look". It is possible to adjust jet needle height in a non-adjustable carb; use thin washers as shims. I'd expect total usable adjustment range to be not much beyond 1/8". You'll know if you go too far; the mixture will go pig-rich.
How rusty is the inside of the tank? There's plenty of garbage in pump fuel. Rust flaking inside the tank can clog up the works. You've not mentioned the internal fuel screen on the carb...and there should be one, unless this carb is different from the earlier and later versions.
You can always throw a new reproduction carburetor at this. The early style, from trailbikes.com is a faithful copy of the early Keihin, has all the adjustability and uses OEM parts. But...before doing that, I'd want to know that the engine itself is healthy. If it isn't, carburetion won't correct the problems. How's the compression? Do a cursory compression check, crank the engine by hand...you should feel strong resistance via the kickstart arm. Make sure point gap hasn't narrowed, that can really degrade performance. Is the intake port clean? If it's blackened, the intake valve is leaking and the reversion is wreaking havoc on the incoming air/fuel mix.
