Thanks for everyone's help! Some interesting results from looking at the bike today...
1. The gas test suggested by @
DannoXYZ was very interesting. I pulled the hose between the petcock and gas tank and tried collecting it into an empty container for 10 seconds; no gas/one drop came out. I thought it might be the fuel filters, so I took them all off and then tried again. Same result.
2. I spun the idle screw all the way inward and looked at the carb to ensure that it was actually moving. I watched the spring compress all the way, so the screw is working. The bike still had issues starting.
3. I checked the hoses (without taking the tank off, so I wasn't able to see the two closest to the airbox, but I know they're there). The fuel inlet and vacuum hoses were correctly attached to the petcock,and it looked like the coasting enricher vacuum hose was also correctly attached to both sides of the carb. The breather/vent between the carbs has a hose on it that isn't attached to anything and dangles underneath the seat.
4. I tried starting the bike up without the inline filters (there are two) but still had the same result.
5. Checked the carb boots; they looked airtight, at least from the sides.
Assuming that #1 means there's an issue with a vacuum, where should I go from here? Thanks again for everyone's help; really appreciate it
Here are some pictures of my carbs from the sides. Note: there is an excessive amount of tubing because of the two inline fuel filters. It was the shortest I could get without it kinking.
Description of photos:
8691: hose and inline fuel filters from petcock to carb
8692: finger pointed at the vacuum hose (in black) connected to the carb in the background
8695: picture from the right side of the bike of the coasting enriched vacuum hose connected (foreground) and the vacuum hose to petcock (background)
8697: right side of the carb, meant to show the boots to the airbox
8698: black hose is the overflow tube from the fuel tank, clear tube is from the breather from the middle of the carb