so this could be an adjustment thing too. I had a mower with a throttle/carb adjustment cable coming down from the handles. It looks like this pic below. If you move the cable up or down it alters the position on the carb to open/close the air flap. I found mine had come loose from vibration probably over the years. I adjusted the cable up/down until it was running properly.
View attachment 66847
I looked up the rpm rating for the mower in the manual, and then confirmed it with a gauge like this. You just wrap a cable around the spark plug wire, and it'll tell you the rpm it's running at.
Amazon product ASIN B07PMJH7QVView: https://www.amazon.com/Maintenance-Tachometer-Replaceable-Motorcycle-Paramotors/dp/B07PMJH7QV/
Your air flap on the carb may not be adjusted 100% if the cable isn't in the right location? No clue if you know the model # of the mower or if you have a manual?