What you are looking for is used on several big concrete cutting saws. Been a long time since I've messed with them but target was one of the brands.
Stop the video at 35 seconds. You can see the RED knob pulled out. That's actually the adjustable throttle cable. You simply push the red button in and unlocks it. That's an old Target saw. If you go by a Hertz Equipment Rental or a United Rentals they can get you that part.
http://www.youtube.com/watch?v=1L8cebANF28