I'm betting that you don't have a resistor plug. If you do, make sure that the ignition is as far apart from the radio components as possible. Another fellow had a similar problem, and it was because he cut off his ignition cut off wires on his magneto type ignition, and just tucked them away. If you have a mag type ignition, run a cut off switch with it. As mentioned above, make sure there is no metal to metal contact anywhere, such as in the throttle linkage, that can vibrate together. This can cause severe radio interference.

Good luck,
AV8TOR