Its NOT a vacuum leak. When symptoms like this occur, its nice when it is that easy. When a popping up through the carb is detected, it is often something else. Exhaust is being forced into the intake. The cause is one or more worn EXHAUST lobes on the camshaft. During normal light duty cruise, cylinder pressure is low. The limited opening of the exhaust valves poses little problem. When you lay into the throttle, cylinder pressure increases. The cylinder cannot evacuate the exhaust as normal. The popping you hear happens when the piston is on its upstroke during the exhaust cycle. When any cam lobe is worn, the valve opens late, lifts less and closes early. In your case the piston is still rising. when the intake valve opens , the trapped exhaust gases are forced into the intake tract. this makes you lose power .
Remove the valve covers again and run the engine. Pay close attention to the rocker arms to each exhaust valve. Look for one or more to have less movement than the intake rockers next to them. You should find the problem there.