The Carter Thermoquad carburetor is a very versatile unit, but you need a special tool to adjust the preload spring for the secondary air door to unlock its full potential. This is an improved tool over other designs and has a very precise fit into the lock screw on the TQ carburetor.
I found the original tool (like the Carter 109P-397 or the Miller C-4152) had limitations when you get to a locking nut that has damaged corners. Those are usually the ones that people have tried to loosen off with a normal screwdriver or one of the cheaper tools and it just slips off, rounding the corners.
This design has a ring that centers the tool in the nut, and has a very positive engagement, even on damaged locking nuts. This tool WORKS where others do not.
Built from steel and coated for durable long lasting service. Comes complete with screwdriver.
Thermoquad Air Door Spring Adjusting Tool