First time post. Had this 70s all metal 020 av for 50 years. Always able to keep it going. Just started being unable to power up. Starts and idles great. Had carb apart a couple of times with rebuild kit. If I have the patience and power up slowly, it will sometimes power up and cut fine. Pulse tube fine, everything is clean, new coil/points, plug. Almost like main jet clogged. But then why would it power up occasionally? Confession - don't really know how to get into the main jet. Advice would be appreciated.