RE: Parts
Sometimes the throttle barrels will vary a bit. What happens when the engines are built is the gasket is not installed and the barrel is lapped into the carb bore so its a perfect fit.
The reason your stuck is likely the concern...Did it get some debris in there and bind it up....
Another option is that the rear carb bolts are warping the carb body and causing it to twist binding up the barrel. These bolts should be tight but don't torque them down...If you loosen the carb bolts slightly and the barrel frees up try tightening it even and not too tight on the bolts sometimes this works.
There is also the option for you to lap the barrel in the carb body. You need to do it when the carb is bolted up tight to the backplate of the engine.
This is something I recommend YS parts and service taking care of for you.
the parts break down show different parts for the 91AC and the 91FZ. This was a change that I guess was made between the two engines.
91AC throttle barrel is YSE1960
91FZ throttle barrel is YSE4310
I don't know the specific differences from these parts numbers. If you contact YS Parts and Service Richard can swap you out for the proper part number. Also I'm sure Central Hobbies can help you also. But they don't work on the engines everyday Richard at YS parts and Service does.
Troy Newman
Team YS