I have been fixed this issues a lot on my two stroke model boats, the butterfly won't close perfectly is caused the engine for not stopping, that's why they used the choke to kill the engine.
A sticking butterfly or throttle cable can aggravate the condition and is ONE of the common findings but a sticking throttle doesn't cause the engine to diesel, and usually there are other signs of sticky throttle like higher than normal idle in water.
So yes, the throttle system needs to be checked to make sure it's not sticking, but this may not be the root cause in this case, there may not be a root cause aside from the engine having enough compression and the right combination of temperature, humidity and air/fuel mixture and perhaps even a small piece of carbon in the cylinder, just enough to coax compression ignition conditions.
Just by adjusting idle too high can coax the engine to slip into compresion ignition mode and run away, it's just like a vacuum leak. A too lean fuel mixture can cause it, perhaps the idle mixture screw(s) can be opened 1/8 turn to enrichen it a little.
Also, could be the carburetors need rebuilding, they may have gum in them. Gray colored fuel lines are the clue, these fuel lines corrode inside and plug the carburetors with gum, this restricts fuel flow through the tiny passages of the idle and low speed fuel circuits.
How does the engine run otherwise, is the throttle response crisp or are there flat spots/areas where the operator has to "nurse" the throttle, indicating carburetor issues? Not good if the answer is yes, the motor is running too lean and won't last long that way.
A 2 stroke engine can be destroyed if it's not running right b/c the fuel system is too lean, the pistons will overheat and burn the oil film on the cylinder walls, causing a seizure. A carbureted(specifically, wet manifold) 2 stroke engine receives it's lubrication by virtue of the fuel carrying the lubricant to the wetted surfaces such as cylinder walls and crankshaft bearings, if the cylinder is not being fueled properly then it also isn't receiving the lubricating oil and may seize.
