ST50 Idling issue...how to solve this one?

Hi everyone.. I Recently picked up a 1970 ST50. I am having trouble getting this bike to run right. Normally, people are asking me how to solve issues but this is a strange one for me. Maybe someone has an idea.

FIrst, let's state the obvious...points, plug, rockers all adjusted. Getting plenty of fuel. Wiring is all sorted and everything is in order on the bike. I replaced the intake gaskets and smeared them with a little yamabond...did the carb cleaner and propane test...no leaks there. Throttle cable adjusted. Jets are clean.

I am really confident this is a Carb issue...but I'll look for input outside that as well.

The carb is one of those aftermarket carbs that looks exactly like the original CT70KO carb...it's not the CHP version, not sure what it is as it has no markings. The carb is jetted 35/60 which is rich for the application...should have a 55 main in there.

The bike will not start unless I give it a little gas but it comes to life, seems to run well and revs out pretty cleanly. It will idle really low when cold. Screw doesn't do anything...it is all the way in. Goes 35mph which is correct for an ST50. It will not idle when it warms up. It will idle if i turn the throttle a little bit and hold it, but I can put the idle adjustment screw all the way in and it doesn't raise the idle. I can take the spring out and put the idle screw all the way in and it will actually hit the slide and still the idle does not increase.

So...i have never experienced that before...that you cannot get the idle to go up even with the screw all the way in. And, think about what I said before...I have to have the throttle twisted a little bit to start and maintain an idle...so...it sure seems like a mechanical issue but maybe you have thoughts?

Are you able to visually see the idle screw moving the slide up with either the carb off the bike or the air box off? Maybe the wrong carb slide or some mix of oem and aftermarket.

From your description it sounds as if the slide is so low in the carb that it isnt getting enough air/fuel.

Yup id check to see if when the idle screw is all the way in and with throttle closed, check if there is an air gap between the carb and slide while looking through by the carb venturi

How about the air passage that goes past the A/F screw...in the casting... If that were plugged, could it cause this?
Or the tiny hole that's in the bottom of the slide tube/throat?

I think that the PO used a combination of old parts and new parts and it's just not going to work and I'm not going to muck with it. So rather than spend hours screwing with the carb, I just went ahead and purchased a 51mm cylinder, matching piston, touring cam, Mikuni VM20 carb, and fabricated a supertrapp exhaust for it and will top it off with a completely rebuilt TRX70 head I recently completed.

Things escalated quickly didn't they!