The hesitation you are talking about is generally caused by a lean condition, where there is too much air and not enough fuel DURING THE TRANSITION when you step on it. The color of your plugs is indicative of steady state driving. You may have a problem with your accelerator pump or secondaries. It's interesting it happened right after changing spark plugs. Try putting in a set of the original and see if the problem goes away. If it does, it may mean that your engine had enough fuel with a cold plug, but with a hot plug, it is burning all the fuel and needing more. I don't think this is the problem as much as the carburetor power valve or accelerator pump or maybe even timing.
Here are some things I found on line.
Hesitation or Stumble When Accelerating
Hesitation is a classic symptom of a lean fuel mixture (too much air, not enough fuel) and can be caused by a dirty or misadjusted carburetor, or one with a weak accelerator pump or worn throttle shafts. Rebuilding or replacing the carburetor may be necessary.
The accelerator pump squirts and extra dose of fuel into the throat of the carburetor when the throttle opens. This helps offset the extra gulp of air that is sucked in until fuel flow through the metering circuits can catch up to the change in air velocity through the venturi (the narrow part of the carburetor throat). The accelerator pump may use a rubber diaphragm or a rubber cup on a piston to pump fuel through its discharge nozzles. If the diaphragm is torn or the piston piston seal is worn, the accelerator pump may not deliver it's normal dose of fuel. Or, if the discharge nozzles are plugged with dirt or fuel varnish deposits, it can restrict fuel flow.
The operation of the accelerator pump can be checked by removing the air filter, looking down into the carburetor, and pumping the throttle. You should see a jet of fuel squirt into each of the front venturis (barrels) of the carburetor. If no fuel squirts out, or the stream is very weak, or only one of the two discharge nozzles on a two-barrel or four-barrel carburetor are working, the accelerator pump circuit has a problem.
Fuel usually enters the accelerator pump past a one-way steel check ball. The ball lets fuel in, but is pushed back against its seat by pressure inside the pump when the throttle opens. If this check ball is stuck open, it acts like a pressure leak and prevents the accelerator pump from squirting fuel through the discharge nozzles. If the check ball is stuck shut, it will prevent fuel from entering the pump and there will be no fuel to pump through the discharge nozzles.
If the carburetor jets are coated with fuel varnish deposits, or there is dirt inside the fuel bowl, this can restrict the flow of fuel causing a lean condition. Cleaning the carburetor with carburetor cleaner can get rid of the dirt and varnish deposits to restore normal operation.
Air leaks elsewhere on the engine can also lean out the fuel mixture. Air can enter the intake manifold through loose or cracked vacuum hoses, emission hose or the PCV system. Vacuum leaks in the carburetor base gasket or insulator, intake manifold gaskets, power brake booster or other vacuum accessories can admit unwanted air. Air can even get into the manifold past badly worn valve guides and seals.
A defective EGR valve that fails to close at idle or when the engine is cold can be another cause of hesitation.
Other causes may include a defective distributor advance mechanism, a weak ignition coil, carbon tracks on the coil tower or distributor cap, bad plug wires, worn or dirty spark plugs that misfire when the engine is under load, or even an exhaust restriction. Even bad gas can cause hesitation problems. So before the carburetor is rebuilt or replaced, these other possibilities need to be investigated an ruled out.
Hesitation Under Load
A hesitation, stumble or misfire that occurs when the engine is under load can be caused by a faulty power valve inside the carburetor. A carburetor uses intake vacuum to pull fuel through its metering circuits. As engine load increases and the throttle opens wider, intake vacuum drops. This can reduce the flow of fuel and make the fuel mixture go lean, so the power valve has a spring-loaded vacuum-sensing diaphragm that opens to increase fuel flow when vacuum drops. If the diaphragm has failed or the valve is clogged with dirt or fuel varnish deposits, it must be replaced. A new power valve is usually included with a carburetor rebuild kit.
Hesitation or misfiring under load can also be caused by a weak ignition coil, or cracks in the coil or distributor cap, or bad spark plug wires.