Losing performance after about 10-15 minutes

[Y-Blockhead]

Originally Posted by Marshall V. Daut

Capital letter with an "a". A capitol with an "o" is a building...
Marshall

Thanks, that didn't look right.

[Marshall V. Daut]

Shall we move on to "principle/principal"? O.K., O.K.!!! Enough already with the grammar lessons that we all hated in the 8th grade!! And it continues to haunt us in our "Golden Years".
In the meantime, the poor OP's Model A is still in need of help. If the electrical suggestions made so far don't pan out, I think Patrick L. is on the right track. In this hot weather, if the gas cap is not venting properly or if a radiator cap has been installed instead of a gas cap (which CAN be done on '28-29 threaded filler necks), the OP may be experiencing the dreaded and banned word on this website: V-a-p-o-r l-o-c-k. Funny how we rarely see these kinds of complaints during the winter. If the gas cap is wrong or marginal, it will become more obvious during hot weather months because of the hot air expansion inside the gas tank restricting the gravity fuel flow at the worst possible time. The OP can test this theory by removing the gas cap and driving around to duplicate the circumstances when the engine began to fail. Of course, the gas level should be down low enough so it won't slosh out. If the problem can't be reproduced, the gas cap was at fault. You might also try filling the tank and with cap in place, test drive. The additional gas will replace the air and vacuum accumulated inside the tank so that the gravity fuel system will operate properly. As the tank level drops, the problem may return.
If removing the cap or filling the tank doesn't fix the problem, try blowing back through the fuel line into the gas tank with the cap removed. That will force any accumulated gunk in the on/off valve temporarily back into the fuel supply so that a steady stream of gas will flow to the carburetor. That gunk will eventually settle back inside the on/valve and restrict fuel flow again, but at least this way the OP can isolate the problem as being fuel flow. If the tank is dirty inside and/or the on/off valve was not cleaned during restoration, bits of fuel gauge cork, sediment and impurities in gasoline will find their way into the on/off valve. There will be enough fuel flow to drive for a while, but as higher speeds are demanded, the crud inside the valve will not allow the fuel to flow fast enough to keep up. Then the engine dies. While the owner is scratching his head standing beside the car, fuel begins trickling through the on/off so that the engine "magically" starts again. The cycle will repeat itself until the gunk is blown out of the valve.
Try these suggestions beginning with the gas cap removal as Patrick L. recommends. Then move on to my suggestions if the cap removal doesn't do the trick.
Marshall

[pgerhardt]

And now add to our spelling woes - auto correct - that corrects your spelling by inserting a correctly spelled, but completely wrong word that screws up the meaning of your sentence!

[Marshall V. Daut]

Ain't modern technology great! And even though your third grade teacher told you that there is no such word as "ain't" in the English language, note that Spell Checker didn't squawk about it here! So, it MUST be a word after all!
Marshall

[Y-Blockhead]

From the dictionary on my Mac...

ain't |ānt| informal
contr.
am not; are not; is not: if it ain't broke, don't fix it.[originally representing London dialect.]
• has not; have not: they ain't got nothing to say.[from dialect hain't.]
usage: The use of ain't was widespread in the 18th century and is still perfectly normal in many dialects and informal contexts in both North America and Britain. Today, however, it does not form part of standard English and should not be used in formal contexts.

[Cornishman]

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Originally Posted by Marshall V. Daut

Cornishman -
You are right/wright/rite about that!
Marshall
By the way, Cornishman, I have always been impressed by your command of the English language, which I am guessing by your location is a second language to you. Articulate, the correct words chosen and properly spelled, and very, very few grammar errors, certainly no more than the rest of us make. Are you native French or an American/Englishman living in France? I suspect that because of the "Cornish" part of your Fordbarn name, you are from Great Britain? I wish I were as fluent in my chosen foreign languages of German and Russian as you are in English!

I have to admit that, despite my location, Cornishman is a clue to my roots. (mes racines sont en Cornouailles) I was born and educated in Britain. I left there in 1986 and although I live in a French environment my first language is English.

[JBill]

People mean "etcetera" but say "ecsetera," and not even professional speakers and writers seem to know the difference between "lie" and "lay."

[Joef21]

If all the above does not find the problem , when it gets warm, run a compression test.

[Tom Wesenberg]

Originally Posted by JBill

People mean "etcetera" but say "ecsetera," and not even professional speakers and writers seem to know the difference between "lie" and "lay."

Seems even a former president told a lie about a lay.

I agree with George that the first thing to do when the car starts to fail is to check the spark from the end of the coil wire to a head nut. You should have a snapping blue spark when jumping a 1/4" gap. If it's a faint yellow spark, then I'd replace the condenser first and try it again.