This is due to either a vacuum leak, or not enough fuel. There could be a vacuum leak at the base of the carburetor, intake manifold, or any other place that uses vacuum. Use either propane or carburetor cleaner to spray around the areas that have vacuum. If the RPM smooths out, you found the problem. Fuel pump low on pressure - should be 3-4 lbs. Float out of adjustment. Needle & seat sticking. Make sure the low speed jet is clean. A plugged jet will cause a bad idle. If the carburetor has been sitting for any length of time, then it will need to be taken apart and rebuilt - cleaned. Run wire down all of the small passages. Ethanol will leave residue in the carburetor cleaners will not get out.