In my M103 experience going back to 1988 I found that resistor plugs created a slight idle roughness (that I could feel) caused by excessive misfires, and this also increased HC during emission testing to the very ragged edge of the cut points back with the old two speed idle/2500 rev no load test that was used in California prior to ASM dyno testing.

OE equivalent M103 caps and plug wire boots have 2K RFI resistors built in. Adding additional resistance alters the voltage wave form and lowers spark energy, which is what causes the increasing misfires and high HC.

"Lean" idle mixtures (stoichiometric) and relatively high exhaust gas dilution due to the M103's relatively high overlap are tougher to ignite than rich, dense mixtures (WOT), and need a high energy, long duration spark.

In my experience M103s need non-resistor plugs to operate properly including emission tests.

Duke