If the old spark plugs you changed had pure white insulators with perhaps blisters on the center electrode then the system is running too lean. That eliminates the ignition system.
Get a can of starting fluid, and if you can find a tip that fits that takes a plastic straw. Set the engine idle to 1000 RPM. CAREFULLY squirt around the base of the carburetor.
bit by bit check the carb throttle shaft bores, Then around manifold fittings.
If there is ANY noise change or RPM stumble from the engine speed, you've found your culprit.
There are a ton of tiny vacuum tubes on the intake manifold and carburetor. Go to the parts store and buy a three feet piece of this hose.
Disconnect one-by-one the hoses that lead to the choke pull of diaphragm, the secondary of the Thermoquad and the vacuum advance on the distributor.
Suck on the hose. No resistance -- is the culprit. That's a vacuum leak.
No push-back on the brake pedal after pumping it several times then holding it down hard and starting the engine is a "tell".
Leaks in the air injection reaction system will cause a lean high CO2 reading.
A tachometer will "tell" when noise is too high to hear subtle engine speed changes.
All of this will take longer than an hour. I vigorously encourage the use of a tachometer. Take your time being meticulous. I used a vacuum hand pump with gauge instead of my mouth to test vacuum components.
Good luck to you.
