While i'm driving my atv will bog down as soon as i go 20kmh and on my 2nd atv same atv it shuts off after going the same speed 20kmh, so what could be causing these problems?

 

Probably dirt in the carb on both machines.

Most quads also have a rev limiter for reverse, so, if the CDI thinks it is in reverse, it will bring in the limiter. It is possible a faulty gear indicator switch circuit could cause this.