We have been searching for a good communication system and have tried several systems/radios but we don't seem to get a clear transmission. It seems the sound from the atv corrupts the signal.  

it is very hard to find a good atv - atv wireless system.

does your system have an attenuation adjustment?

some of them you can  tune so they work halfway decent.

but at best it's going to be spotty.

there are so many ring frequncies that the engine of fourwheeler is putting out (especially with 2 cycle) it's hard not to find one that doesn't receive these frequencies.
they haven't put much technology in these.
you might try one of the small handheld walkie taklies with a headset.  still you have to push the button but they'll probably work a little clearer.  

My system does not have an attenuation adjustment. I got it in a site in Singapore (ixecessory.com). It is basically a harness with speakers and a microphone that go in the helmet and connects to the radio (Motorola). This system woks fine.

The problem I believe is the Radio transmission. The noise from the ATV is captured by the microphone and the signal goes out corrupted.

I have been looking for other alternative in the net, but do not seem to find one. any recommendation  

does the microphone have a windscreen?  if not put one on it. that may help cut down on outside noise.

try to find some way to encapsulate the microphone with just a small hole to pick up sound.
it sounds like the mic is just too hot.

to see if this will fix it use some aluminum foil, wrap it around the microphone in the above fashion.
it should cut down on engine interferance.  

have you tried a throat mike? regular microphones pic up vibrations in the air. Beacause of this it can pick up additional sounds such as the engine and air rushing by. But, a throat mic picks up vibrations from your vocal cords, eliminateing basicly all noise.