Arduino Uno R3 ISR does not work on pin 2?

Question

I have an Arduino Uno R3. I have a paddlewheel waterspeed sensor that I use on an ISR on pin 2. The power supply is approximately 8v stepped down to 4.92v with a circuit yanked from the phone charger. I have also tried this with the arduino 5v.

The pin returns the correct values with digitalRead(), so it's cycling between 1 and 0.

The ISR works fine on pin3. Why does it work on pin 3 and not pin 2 ISR?

void itest() {
    Serial.println( "test" );
}

void setup() {
    attachInterrupt(0, itest, FALLING);
}

Answer 1

never use Serial inside an interrupt handler, increment a (volatile) variable and print it in the loop().