Thanks guys; I'm hoping that you're right.
1. I do use Water Wetter... so maybe that is the source of my brown spots in the radiator fluid. That would be great! I had no signs of engine overheating (no high water temps on the gauge) when the car started acting up. I have an original fan with an otter switch that has always been sufficient (along with my oil cooler.) I just went to start the Sprint for my Sunday morning drive, and it misfired so badly that I barely got it out of the driveway before I turned around. When last parked the Sunday before, it ran no where nearly as bad. I hardly ever get it into 4th gear on my Sunday drive, but I don't sit in traffic either.
2. I reworked the exhaust clamp at the header/exhaust pipe joint, and now it's sealed... but I still get some backfire.
3. This weekend I also replaced a clear fuel filter that resides in the trunk (boot) between the gas tank and the recently-professionally-rebuilt mechanical fuel pump (with filter screen.) There was some linty-looking crud in the existing in-line trunk filter, but nothing to the point that it would hinder fuel flow. My misfire problem persisted even after replacing that filter with a new one this weekend.
4. I also sprayed WD-40 around the o-rings with the engine running... no discernable change in engine function. I don't think my o-rings are a problem.
I had a Fiat X1/9 years ago that blew a head gasket while being driven, and when that baby blew, there was not doubt about it! White smoke blowing out the exhaust like a crop duster! My Sprint has had no such episode, and when last parked, it ran well (except for some backfire on decelleration.) It was only on start-up a week later that it became totally undriveable.
I do have Miles Wilkin's book. I also had a professionally done valve job done about 4 years (about 4,000 miles) ago. I have about 75K on the odometer. I'll check for bubbles in the bottle and do a leak-down test. I really appreciate the forum sticking by me in what may, or may not, be my "darkest hour."*
Gary
'71 Sprint FHC
Miami, Florida
* (gratuitous Churchillian reference)