It doesn't sound like your mechanic did proper tuning on the carb. If so, he should refund about 80% of what he charged you. Anyone can swap out a carb, but you have to know what you're doing to tune it. That's what people are paying you for when they have you replace it.
The base tuning involves setting the most basic and important adjustments on the carb. They'll be things like float level adjustment, metering rod and spring sizes, jet sizes, idle mixture and accelerator pump adjustment. If you don't do base tuning first, you'll have all kinds of issues that you will never fix. You'll simply chase them around the carb, adjusting one thing to fix one symptom and creating another symptom in the process. The very first and most important thing in a base tune is your float level adjustment. Do this before doing anything else. While I'm not certain what issues a low float level might cause, some of your runnability problems, rough idle, stuttering and the like are most likely because of that.
Because it wasn't done, you should spend some time in the manual (link is in my signature) verifying that the other base tuning items are correct as well. I haven't done much tuning on these Keihin carbs, and it's been a loooong time since I worked on one, so you'll have do some reading on general carb tuning, then look through the manual to see how much of it applies to you. Carbs can get built a lot of different ways and include some tuning items that other carbs don't. It's complicated.
Here is a basic guide on carb tuning (
https://www.yourmechanic.com/article...r-by-ed-ruelas). It's generic, but you get the jist. Edelbrock has an excellent carb tuning guide for their carbs (
https://www.edelbrock.com/pub/media/...ers-manual.pdf). Most of it won't be applicable to you, BUT you should definitely read the theory of operation portion to understand what's happening with all your adjustments and what the various symptoms tell you. This is vital, IMHO. You shouldn't have to make any dramatic changes because the car is generally running well, but these adjustments are so important, it's worth making sure they are 100% perfect. You only have to do them once and you won't have to worry about them again.
Once that is done, I suspect you'll still have the low idle at stops. My guess is it's a vacuum leak, like others have already said. Don't rule out a gasket, even if it's new. New gaskets can still get damaged during installation. Goodness knows I've done. You can learn to find vacuum leaks with a Google search.
I somewhat alarmed that the fuel level rose when you raised the fuel filter. That shouldn't happen, but if the car is running well, then whatever.
Good luck on your adventure and keep us posted on your progress/questions.
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