Creeping temps at idle

[davepl]

My bone-stock restored 402 (which means every bit of fan shroud, heat riser, carb stove, choke, clutch fan, weatherstripping, etc is new and back in the factory spot) has been heating up at idle.

It runs around town straight on the middle of the gauge. But if you have to sit for 20 minutes on warm/hot day, it creeps without apparent limit - only I intervene once it gets to the "Red" zone of the gauge.

But it cools perfectly underway, so I though the only difference is airflow, and the clutch fan was original, so I replaced it. Not with the $23 Rock Auto unit, but to be sure, a brand new GM $100 clutch. But no change.

Yesterday I drove it about 5 miles, full operating temp, then parked and let it idle. Within 20 minutes it was up to touching the red. Turned it off for 10, drove it, and back in the center of the gauge within a mile or two.

It's not like the light would have come on, but I doubt these ran hot on the gauge in the olden days. Timing is more advanced than stock, so its not that (about 10 degrees vs 4 indicated, but that makes it cooler, not hotter).

Could be leaner than stock I guess, but it runs and idles OK. Can the idle mixture actually cause overheating?

Any ideas?