what input pin is it using?

Using Arduino Programming Questions
10

Joes:
would this work or am i barking up the wrong tree?

Wrong tree entirely. You have completely removed the interrupt handler now.

In your original sketch, this interrupt handler was called when you got a falling edge on pin 2:

void rpm_fun()
 {
      rpmcount++;
      digitalWrite(ledPin, HIGH);
      delay(50);
      digitalWrite(ledPin, LOW);
 }

These are the lines of code that turn ledPin HIGH, wait 50 milliseconds and then turn it LOW again:

      digitalWrite(ledPin, HIGH);
      delay(50);
      digitalWrite(ledPin, LOW);

If you take those three lines out it will no longer flash the LED and will no longer have that unwise delay() in the interrupt handler.