912 ULS Rough Running Low EGT - Solved!

1974 RV12, 320 hours.  SW Florida hot season. This plane always ran smooth with EGT's very close (10-30F diff.)

Several weeks after first experiencing rough running, problem persists.  Runs rough IN FLIGHT ONLY, and RIGHT EGT ALWAYS goes very low.  Much worse at high power.  Carbs have been synched twice.  Right carb removed and reinstalled 3 times finding no defects.  All plugs replaced, right side plug wire ends remade, no water or debris found in carbs or in gascolator or on gascolator screen.  Probes, R & L are correctly wired, and R just replaced and still shows identical behavior.  Test flight today:

Conditions 85F, high humidity. Field elevation 30 ft.1. 4000 rpm runup, ignition check drops: 140 L/130R.2. W.O.T. on ground: 5170 rpm, drops within 30 rpm.

Climb out on takeoff immediately after above numbers: 5230 rpm, 1370 L/1240 R.  Pattern level flight at 4700 rpm, 1400L/1315 R.  Climb starts rough running: at 1800 ft, 5100 rpm climbing, EGT's 1420L/1211 R.  Level flight 2000 ft, 5200 rpm 1420L/1211 R Rough running.  Full throttle immediately runs even rougher.  However, reduced throttle, level flight 1900 ft, 4800 rpm, egt's level out some: 1390/1320. Not smooth, though.

After this flight, on ground 4000 rpm ign. check: egt 1220/1257, mag drops 130/90. WOT 5130 rpm, 1200/1220!

The right carburetor is most likely, but what could cause this?  Obstructed main jet?  Leaking diaphagm?

Thanks for any help.