Probleme mit dem ADC beim Mega8

#include <avr/io.h>

#define F_CPU 8000000UL

#include <util/delay.h>

unsigned int adc_value1;

unsigned int adc_value2;

unsigned int adc_value3;

int main(void)

{

  //DDRC=0b00000000

  DDRB=0b11111111;

  DDRD=0b11111111;

  ADCSRA = (1<<ADEN)| (1<<ADSC) | (1<<ADFR)|(1<<ADPS2) |(1<<ADPS1) | (1<<ADPS0); //ADC_Enable, _StartConversion, _FreeRun, Prescaler: 64

  while(1)

  {

    PORTD=0b00000000;

    /*ADMUX=0b00000000;

    ADCSRA |= (1<<ADSC);

    //while (ADCSRA & (1<<ADSC));

    adc_value1 = ADCW;

    //adc_value1 = 50;

    _delay_ms(10);

    PORTD=0b11111111;

    ADMUX=0b00000001;

    ADCSRA |= (1<<ADSC);

    //while (ADCSRA & (1<<ADSC));

    adc_value2 = ADCW;

    //adc_value2 = 120;*/

    ADMUX=0b00000010;

    ADCSRA |= (1<<ADSC);

    while (ADCSRA & (1<<ADSC));

    adc_value3 = ADCW;

    //adc_value3 = 80;

    if (adc_value3 > 500) PORTB |= _BV(PB0); else  PORTB &= ~_BV(PB0); // gelb

    //if (adc_value1 > 300) PORTB=0b00000010; //else PORTB=0b00000000; // gruen

    //if (adc_value1 > 500) PORTB=0b00000100; //else PORTB=0b00000000; // rot

    //_delay_ms(100);

  }

  return 0;

}