MTFCA Forum

The sparkplug gap does not have a big influence on running temp, but running lean sure does. I think that is your problem.

I'll reset and adjust mixture next time I run it.

Motometer’s aren’t perfect either, especially if you’ve got an original one. Some of the liquid can settle or unsettle and affect your reading. New repro Motometers I have run seem to very quite sensitive to temp, and I can readily manipulate their reading by changing the fuel mixture.

Just tossing another hypothetical condition into your situation, not that you needed one!

Clean the bugs out of your radiator! The reproduction motometers will get stuck at the top if they go all the way up. I have two which did so and nothing I have tried has brought them down. I tried bouncing them up and down on a rubber mallet. I tried putting in refrigerator, I tried boiling water. Once they go all the way up they are stuck. I drove into town about 2 miles each way yesterday to get some gas. Coming up hill to the house is Ruckstell hill. When I got home I was just at the bottom of the circle. It was about 75 degrees outside.
Either spark too retarded or gas too lean can cause some overheating. Also clogged radiator or sometimes on an old radiator the joint where the fins meet the tubes gets corroded and not a good heat conductor. The fins must get hot as the tubes for the radiator to work efficiently.
Norm

I drained the radiator this spring and put distilled h2O in as usual. I had it boiled out 10 yrs ago. Maybe it's time to do that again.
It might be possible the mixture was a tad lean. Timing was right on. No issue there.
Older radiator so maybe the fins are shrinking away from the tubes too. Lots of variables....

I'm sure others will tell you that a 50/50 antifreeze mix will cool better than pure water, and it helps prevent rust.

Yes, I'd prefer antifreeze. Distilled water cost is less. I'm cheap.

Another plus for the antifreeze is it has a higher boiling point than water so it will not boil away as easily when you drive around. Here it rarely gets below freezing in the winter but I have seen it get to 18 above once during our 50 years at this location, and when you are on a tour sometimes in the mountains or desert it can drop below freezing at night and be shirt sleeve weather during the day! So you never know when you car could freeze especially on tours.
Norm

I'd try flushing the radiator and block with a garden hose and sweeper nozzle, then add 50/50 green antifreeze and distilled water. If you have a water pump, make certain that it is not leaking air into the system when the engine is running. Rag seal water pumps are capable of sucking in air at the seal at higher engine speeds, even if they don't leak at other times. Air in the coolant makes rust and steam pockets, and it can mimic boilover.

Fever Thermometers had to be shook down after use.

You can do the same with one of those thermometers, if you tape it securely (top toward the axle) to a wood spook wheel and take a short ride or use a rea wheel and jack up that wheel.

don't go too fast and sling the thing off though.

Others have tied it to short a rope (lower end out) and swung it around their head.

A short trip to the freezer (bottom down) will also lower it.

Water transfers heat better than antifreeze. I usually run mine a little on the lean side. I used to have a John Deere model M that would run about 220° no matter what. I put pure water in it and it would barely raise the needle.

I know it sounds odd because it boils at a lower temperature when the mixture is lean, but according to my experience, it actually makes the engine run cooler overall.

A number of factors affect the way coolant, whether it's water or antifreeze mixtures, behaves in a cooling system. The temperature of the coolant at the water outlet does not tell the whole story of how the coolant is performing in critical areas, such as around the exhaust valve ports, or how well it resists foaming, forming insulating bubbles on water jacket surfaces, preventing air entrainment, preventing corrosion, surface wetting properties, and more, including water pump seal lubrication where a water pump is in use. A quality "green" antifreeze at 50/50 does a very good job in a Model T system at very low cost per mile. That's true from -20F to +115F. If your car overheats with green 50/50 under any normal operating situation, the cooling system needs service, and perhaps operator procedure needs modification as well.