Troubleshooting lean condition on rear cylinders

[punchsponge]

Bone Head Rookie question:

I was just going through my new (to me) bike and I decided to put a vacuum pump with probe connected to a wide band oxygen sensor set-up on the exhaust to see where I stood. Exhaust coming from the front two cylinders was at 13.7-13.4 through 6,000RPM running 103 octane. Little rich, but good. However, the rear two cylinders were at 19.8-21.2. Very Lean. I pulled the rear plugs to see if the condition resided in one cylinder or two and they are the same, visually consistent with a dead nuts to rich mixture. Condition does not seem to lie in O2 sensor mechanism, as I have used it many times and am familiar with its idiosyncrasies. No fault codes or visible air leaks. So, can anyone give me a good troubleshooting decision tree? Start by changing the plugs and take another look? Is there a good way to get a look at the spray from each injector? Rear Pb sensor, of course, is under suspicion. Thanks for your time, JE