Not sure I've mentioned this in prior posts, but neither myself nor my daughter have owned or ridden a motorcycle before...so this is all new to us. My daughter however, did take a course last Fall and has her license, but still has no riding experience outside of that.  All the guidance thus far has been appreciated.

We're really struggling with the idle. As you all know, there are 2 screws...the smaller is the pilot air screw (right in pic) and the other is the throttle stop screw (left in the pic). drive.google.com/file/d/1SSNAUoKuAmZrRdz...JPp/view?usp=sharing.

The manual says to turn in the pilot air screw until it slightly seats and then back out 1 1/2 turns. Then says to adjust the throttle stop screw to get the idle to between 1300 and 1400 rpms. Doing this and only messing with the throttle screw, we can get it close to 1500, but then sometimes it will die randomly. Also, we have to really let the clutch out (in 1st) before it begins to move...and then it usually dies. I've read it shouldn't be necessary to hit the throttle when releasing the clutch in 1st as the idle speed should be enough to move the bike...and wouldn't think we'd have to let the clutch out as far as we are? I'm fairly certain we cleaned the carb well and put everything back to where it was supposed to go  .  We also haven't messed with adjusting the throttle cable as of yet either.  

Is it as simple as getting these 2 screws adjusted correctly...or could something else be going on? That's the frustrating part...not having the knowledge... we're just guessing.

 

You will need revs when pulling off - only someone with great clutch control could do this at tickover.

Have you adjusted the clutch/cable as per the manual (both at the lever and at the left side of the crankcase)? Although the manual advises otherwise, you could do the lever first to see if it helps - you should have 2-3mm of slack.

 

Good morning and welcome by the way! I read much more than post as there are always many more knowledgeable members around than me. Plus, I can’t type as fast as I think!
Anyway, I have a and b models of your bike and got them to run well. In regards to the stalling at low rpm, that points to the pilot circuit being a bit dirty still. I would clean your carb again and give that jet the stink eye. Doesn’t take much to act up like you describe.
As to the idle rpm, mine doesn’t like to deviate from 1500 either. The mounting bolts, when even slightly over tightened, can keep the slide from moving freely.
Don’t remember what you said for sure, but have you adjusted cable free play per the manual? Hope that gives you some things to check. You have a great bike to work on. The smiles per miles when right is great!
One mantra stated here has been, clean your carb and then clean it to again. And then clean it some more. Good reasons for this being written. Good luck!

I wouldn't get too hung up on trying to get it down to 1400 rpm,. I keep my CT1 about 1600-1700, because it would die occasionally if I had it lower. Also, do you have a resistor spark plug in it? If so, and your cap is a resistor cap, you may have a little bit too much resistance, and I found that could make mine die. Often when you order a non-resister plug, you end up getting a resistor plug, that has an R in the number.

When taking off you DO have to give it some throttle. If I just let my clutch out slowly at idle it would die as well. Completely understandable if you haven't ridden before, but you have to get used to the delicate balance between the throttle and the clutch. It's the same technique used when driving a manual transmission car or truck. These days I know many people have never driven a manual transmission, but for we older folks, it's what we grew up on, so it was very easy to transition to a motorcycle. After a while it just becomes second nature,.

Thanks all. Tinkered a bit today and couple of few things.

1. Clutch free play - there seems to be a discrepancy in the documentation regarding the distance in mm. We have the DT125B Owners Manual (little booklet) that references a distance of between 2-3 mm (this is what we've been using). I reviewed the online manual found on this site, and it references 5-8 mm. Which is right?  online manual --> drive.google.com/file/d/1qaKdtFfhUxEWTmG...ZUR/view?usp=sharing
booklet --> drive.google.com/file/d/1mHI_U5paXg2TuNk...nz9/view?usp=sharing

2. Carb leak - actually noticed a leak the past few days and thought we resolved by swapping out the fuel line. It definitely was leaking, but we now believe there is another leak between the float chamber body and the rest of the carb body. We disconnected the carb last night and I was able to tighten the 4 screws connecting the two slightly. Seemed to fix it temporarily last night but seeing some gas today after starting up for a bit. Could it be the aftermarket gasket we bought? The original was toast. Could the leak have an impact on the idling?
 

Mine leaks, leaning on the kickstand, with the tank valve on. Remedy, learn to turn the valve off while the bike is running for a minute or two, then kill the engine… no more drips.
Sneezles61

I believe I have the correct transmission oil, but looking for confirmation --> drive.google.com/file/d/1LTJlCG-28jm2eGj...pq6/view?usp=sharing

Also, I also attempted to make the recommended clutch 'mechanical' adjustment per the service manual. I'm not able to loosen the lock nut...I'm guessing it's seized. Any ideas on how to unseize it or should I just let it be? Because it's seized, I can feel it pulling the cable when I try to loosen. drive.google.com/file/d/1vvU2AxK9hOS9cfQ...1fd/view?usp=sharing

 

Just let it soak with some penetrating oil like Liquid Wrench or Gibbs Lubricant,, or anything that says "penetrating oil".

Yes Trans oil is fine are you holding the screw with a screwdriver and using a wrench to loosen the nut sometimes they are difficult use Tom P suggestion also

Just tried with wrench and screwdriver…no luck, but thanks for the tip. I’ll get some penetrating oil and let it sit and then try again.