I must be doing something wrong

[DrinkMan]

I'm getting confused over my working on my Model A (late '28) and the engine. When I got the car, it would only go 25 mph and backfire. My wife and I have been slowly going over everything to fix it. But have stalled in our efforts to make it a reliable driver.

Thus far:

• Ignition system needed a lot of work - problems with getting spark. Turned out wire from pop-out ignition switch was bad inside the cable. Decided that we did not need the pop-out style and put a new style switch and cable.
• The evil wire from lower plate to upper plate gave me fits. Replaced with the modern points per Les Andrews book.
• Carburetor was not in good shape. We rebuilt it (that was actually a very fun project because I've rebuilt many a Solex, countless Webers, and even a Holley, this was my first Zenith and my wife got to do most of the work and we made it a fun project for the two of us).
• Checked compression - 50 psi cold, 65 psi hot. Very little variation from cylinder to cylinder.
• Way too much rotational play in the distributor cam - almost 90 degrees (no side-to-side). Found the lock washer missing. Still more backlash than I would like - almost 45 degrees play. Very hard to get the timing right but finally got that light to come on consistently at about 2 notches on the lever.
• Adjusted carburetor per Les' book.
• Other things that were needed - water pump, cooling system flush (with vinegar per this website). Radiator is evenly hot after a drive.
• Note - new distributor cap, rotor, coil, coil wire.

Some pictures:



Now - our end result: Easily climbs a local small hill accelerating at 35 mph (prior to this work, it struggled up that same hill at 10 mph), runs smooth at almost 40 mph. Never backfires. BUT - that is about it. It seems to top out at 42 mph even with the timing lever all the way down. And after only 5-10 miles, water starts coming out of the radiator overflow and then steam hissing. And just like another thread I read here, I can't restart it after 15 minutes, I have to wait about 40 minutes (vapor lock?).

I'm not looking for a high speed car - I have plenty of those. I just want to be confident to drive it to a car show 20-30 miles away without fearing overheating or breaking down. I'd also like to be able to drive it 45 mph for a long time and maybe a short distance at 55. Are my expectations too high and what have I missed. I feel rather embarrassed that this simple little car has me more stumped than any of my other cars. I'm "The Guy" that others come to help troubleshoot their car.

Where else should I look?