Good luck. I do hope its not just because the cooling system needs burping.
Thanks to ALL for those great comments. I'm really grateful for the time taken to help me. Thank you so much.
I didn't know that reading spark plugs requires an off-idle shutdown. I'll try that later.
The spit-back occurs on part throttle. Not WOT or idle. Not a lot of spitting at all, but a couple of pops every few minutes.
I'll work on having the carbs set up properly, but I hear you all saying it's not likely the cause of my heat problem. Message received.
Frankly, I still think improper bleeding is a likely suspect, but I've read every thread on the topic and can't identify anything I'm doing wrong. I filled up the system as prescribed using both the bleed screw on the water pump and the one on the water manifold. If I crack either of those screws now, only coolant comes out. I've rinsed and repeated that whole process three times!
It'd be so much easier if all those coolant hoses were transparent...