MOVE the antenna as far away from the generator as possible.
If that doesn't work then you will need to determine of the noise is ignition noise from the gen engine or AVR/brush related noise from the gen head.
Ignition noise is very common on small engines, typically most small engines do not have resister plugs (which suppress ignition noise). Check the plug number to see if it a resistor plug on not, if not then replace the plug with the resistor plug equivalent (you can find plug crossover information by searching the wobbly web).
If not ignition noise then it may be the AVR which uses brushes, worn/dirty brushes can cause excessive RFI but might not be easy or cheap to fix..