I know this may sound crazy, but I was dumb and this happened to me.
Make sure the slide is not in upside down/backwards.
My R3 wouldn't run right when I bought it, "rebuilt carbs" by the guy I bought it from but he couldn't get it to run right. When I rebuilt the carbs I just put the slides back in how they came out, not even paying attention. Turns out they were upside down. I struggled with everything you are dealing with too, same exact problems.
Tried troubleshooting spark, fuel, air, literally everything. Starter fluid, all that. Couldn't figure it out, took it to a mechanic and he realized pretty quick when he took the carbs off that the slides were wrong.
Try flipping the slides haha.