I have completely flushed Popeye's block and radiator with vinegar. Lots of crud came out, was worth the effort. As measured by my infrared thermometer, the temperature drop from the top of the radiator is now about 75 degrees (185 at top, 110 degrees at bottom. So I believe the radiator is doing its job. I installed the new drain petcock and it seals nicely.
The car starts and idles well when cold. After the first few minutes I can advance the spark to about the 8-or-9-oclock position and achieve a smooth idle.
However, when the engine heats up to operating temp (185 degrees by my thermometer), it begins to run rough and I can see bubbles appearing in the radiator water (no bubbles when first starting). The car won't idle smoothly at that point, will die if I try to lower RPMs to 'normal' idle range. No change in rough running condition if I adjust the carburetor.
Compression measures a consistent 40 pounds per cylinder for all four cylinders - so I don't believe valves or cracked block are a problem. There is no water in the oil. No oil slick on the radiator water.
I theorized in another post that the head gasket could be the problem because I was seeing the bubbles in the radiator, but hesitated to touch that until all other possible problems are eliminated.
So before I lift the head, does it sound like head gasket to you experts? I have a new copper head gasket on hand and it doesn't seem like a difficult job except possibly the rear bolt - would like to avoid having to raise the body in order for bolt to clear, but will do it if necessary.
I'm beginning to be very discouraged with this car. Every time I think I will have it running well enough to drive with confidence, something else happens. I don't have any basis for comparison as I've never driven another Model T, and have not had the benefit of the expertise of another T owner.
Much more of this and I may just dump it and go back to another Model A.
So please, what's your considered opinion? Head gasket? Or is there anything else you can think of?
Uh Oh, I posted in the wrong spot. Will re-post on the forum.