Common things to try:
If possible, route VHF coax away from the wiring loom coming from the motor.
Try jumping engine block to negative battery terminal with a heavy piece of wire. Noise reduction points to bad grounding.
Re-terminate the PL-259 connector on cable.
Try a resistor spark plug of similar temperature. Should cut down on spark interference.
Try running VHF from a separate, isolated power source while running the motor. If that fixes/reduces noise, it points to a grounding problem.