The engine runs 155 to 160 with a 180 degree thermostat, unless idling at long stoplights.  At the lower temp idle is perfect, smooth, 600 rpm.  It is pretty cool, actually relaxing with a point in there where a deep base note rolls through.  When the water temp gauge hits 180 the idle changes, very predictably.  Same thing each time...  rougher, uneven. 

I have noticed the coil is hot.  It is an Accel with internal 1.4 ohm resistence without ballast, which was the coil Accel called for to go with their pointless ingition if ballast resistor wasnt going to be used.  Coil is mounted in standard location over intake, with 1/2 in. spacers holding it above intake.

Intake manifold gaskets are wide open at the crossover, and they and the manifold are temporaries waiting for Mummert's release.  Thin stock phenolic spacer between intake and carb.

So what do you think is the cause in idle change?  Heating causing a change in idle mixture or an overheated coil changing its characteristics?  Idle returns to normal immediately after temp drops. 

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.