Gearbox chirp while burping?

To isolate fault doing one step at a time, cut about 3/8" off each ign lead, all boots solid connection; ohm'd across coil primary, all are 15 +/- 0.1

aircraft in heated shop overnight with light preheat before start to get oil temp indication to move, oil cooler completely taped over (no oil thermostat), engine run with cowlings removed to measure exhaust stack temps (instant read IR thermometer), OAT low 20s:

 - Did initial temp reading (Test 1) as soon as oil pressure stabilized, front temps in direct prop wash with cowls removed 

 - did 4000 RPM check after oil temp reached 120F, engine much rougher on Lane A but still 600 drop and with noticeable bark, couldn't hold more than few seconds due to roughness, not temp readings, pull 'choke' for ~150 RPM rise and rechecked Lanes, Lane A still 600 drop (maybe slighly less) but no bark. Lane B drop unchanged at ~75 and very smooth.

 - pulled to 2000 RPM and switched to Lane A (Test 2), no bark and fairly smooth: ~200 RPM drop, temps as listed below:

did inductive ignition test on leads, all checked normal.

Question: does the change in 4000 mag check (slight difference in drop and 'Bark' without choke) imply a carb issue or noticeable lower OAT?

appreciate the help

bob

Replaced rotary ignition switch (old one required you to jiggle key to get it out so planned to replace anyway) no change, still 600 drop on "A"

Swapped "P" leads at 6 pin connector, 600 drop switched from "A" lane to "B" lane

Swapped 6 pin connectors without changing "P" lead position so "A" is back to "A" & "B" to "B", 4K mag check 75 on "A" no drop on "B"

After cool down checked drop, "A" very slight, no drop on "B"; went to shutdown engine would not quit.  Choke wouldn't kill it so selected fuel off and let it run out of fuel.

Swapped Red leads, 600 RPM drop stays on "A" circuit.

Swapped "P" leads, drop switched to "B" circuit; returned "P" leads to original position

Swapped modules, 600 RPM drop on "A"

checked trigger gap, all at 0.012 except A 3/4 which was 0.038"

 - closed gap to full travel of adjustment slot, gap 0.030".

- turned flywheel off interrupter: centered coil in center of adjustment slot; taped trigger coil with wooden hammer handle to get close to 0.016", used slot to fine tune gap to 0.012"

Groundrun: 50 RPM drop on "A", 75 RPM drop on "B"; problem solved

Original problem? light scuff mark on edge of mag interrupter, looks like a washer or something must have been picked up and bent tab of trigger coil

Appreciate technical support from the folks at LEAF, thanks all bob