Change:
_counter + 1
To:
_counter = _counter + 1
Also, add:
goto "loop1"
before your line with exit.
This will work exactly how you'd like it to, assuming that the first element in the array that you are passing to your script has a value of 80.
For setVelocity, did you read the description of it in the Com Ref? Don't just guess at how to use these commands!
vehicle setVelocity [x, z, y]
Operand types:
vehicle: Object
[x, z, y]: Array
Type of returned value:
Nothing
Compatibility:
Version 1.8 required.
Description:
Set velocity (speed vector) of vehicle.
Example:
_flare setvelocity [0,0,1]