Hi John,
Just a few "bubble" opinions from past Model A cooling experiences.
If your head gasket is not leaking compressed air into your coolant and causing bubbles and foaming ....................... very good chance your semi-clogged radiator is in most cases the problem ....... but "nobody" like this scenario so they install thermostats, change coolant to Sierra, plus try a laundry list of Obama-Care type solutions for overheating.
First, having foaming coolant with bubbles means coolant is full of air ....... and "air bubbles" will "never" cool your engine, with or without anti-freeze.
With lots of Model A radiator "air bubbles", guess what? You got it !!!!!!!! You have instant Model A overheating.
Like explaining how to obtain lung cancer, rather than how to prevent lung cancer, how to make Model A coolant "air-bubbles" in a Model A cooling system may better explain "air bubbles" than how to prevent Model A "air bubbles".
1. One way is if coolant is low, and one goes uphill at about 45 mph, coolant seeks its own level and the high point near the radiator cap has "no" or very little liquid coolant ........ water pump cavitation begins, just like a boat propeller without a cavitation plate making: ................. bubble, bubble, bubbles.
2. Another bubble making way, on level ground, is to rev up the Model A engine and the Model A water pump rotates faster; however, if the radiator is semi-clogged, the faster turning pump cannot get enough water, so it begins the cavitation scenario, again, like a boat propeller begins making bubbles .......... bubble, bubble, bubbles.
3. Many in the past with bubbles in a semi-clogged radiator cut off part of the water pump impeller fins to impede coolant flow ................. others install a thermostat, (some with drilled holes, some without drilled holes), to slow the water flow so the semi-clogged radiator, (depending solely on "gravity" flow through semi-clogged tubes) can keep up with a "now" slower flowing water pump.
5. When Model A's rolled off of the assembly line with full flowing radiators, there were "no" bubbles ................ wow, not many think of that ....... and today, many guys with clean radiators have no bubbles.
6. Later, when Model A radiators became semi-clogged, "nobody" wanted to hear the truth.
Appears it is just like if one has a constipated horse ..... give him a laxative ...... don't cut his ration of oats in half ....... he will never function as well.
Hope this helps future horse owners ...... appears Model A guys today just don't like to clean radiators or even change head gaskets.