Sounds like you have petrol flow problem. Could be petcock or it could be carbs. We'll need to do some
testing and
measuring to narrow down exact problem area.
Numbers will tell us if a part is good or bad.
1. Test petcock flow. Unplug fuel-hose from carbs and aim into measuring cup. Set petcock to ON and attach vacuum-source to petcock and apply some vacuum. Some people attach hose and suck on it. Or use syringe. Let petcock flow for 10s, how much petrol did you collect?
2. Test petcock flow in RESERVE. Repeat test and let petcock flow for 10s in reserve position. How much petrol did you collect?