hello, I new here.
But I got 4G13, the carb CO adjustment to me is not as easy as ABC.. u need much experience, listening to 'good' tuned one. Your ear and eyes could be a good focus one, particularly. One-time adjustment u cannot get it cleared, u need many times, adjusting then drive to feel how it runs. Then shut the engine, then start again. re-adjust. It also not easy to put your hand behind the carb, small space to the bay wall plus hot metals around. like touching hot pan. U need to really be patient.
That DIY special tool to adjust the A/F screw u just need to get a used spark plug, then use long nose pliers to pull up the ground electrode tip, make it straight. Then feed up that tip into the A/F screw (looks like covered by round ring). U need to have some experience also to 'feel' the tip slotted within the screw head and that moving ring, then you can start turning clock wise or counter clockwise SLOWLY. To bar ein mind, the A/F tuning when the engine running is tend to make u turn more, as the vibration of the engine will induced it. Again, this process is not so easy. Onces u FAILED to feel the engine lean or rich, u may lost that optimum 'peak' setting. This 'peak' condition practically is so 'narrow'. U easily will miss the position or far away from the 'peak' point. Then your engine getting worse, the may frustrated. I think don't attempt to do if u don't got much knowledge and experience, unless u want to see your car 'sick' after that. Juts a little touch will drag the needle inside.
Once you get the point, you will feel smooth silent idling (close all windows, then listen to engine, u will noticed in the cabin the engine sound like 'far away'), then step ur gas pedal untuk rpm 3k, it continues to smooth, no vibration, noise, then the back to idle position, it remain stable.
Just from my experience.. sorry if not relevant with others.