greg bender
http://www.thisoldtractor.com/gtbender
Even if you don't have mercury carb sticks, you can still do a pretty good job of synchronizing the carbs by hand (both of the following methods are performed with the engine off). And, since the factory didn't include bosses in the intake manifolds to which mercury carb stick could be attached, they expected (and provided explicit instructions in the manual) the carbs to be synced without them.
One method (recommended by the factory) is to put a thumb and a finger into each air inlet and feel when each slider starts to move when you very slowly open the throttle. The goal is to make both sliders open at exactly the same time.
An easier and more sensitive method is to create two 6 inch straight pieces of wire. Place each wire in the air inlet with the end under the slider. The other end of each wire should come pretty close to meeting in the middle. Now, when you open the slider very slowly, you can simply view which wire moves first. Thanks to John Schwartz for this tip.
Regardless of which method you choose, use the throttle cable adjustment screw to synchronize the movement.