Its an air leak or idol circuit way lean.The only way to kill a run away if you turn off the key and pull the plug wire and if its still going is to add fuel, Fuel cools. Pull up as fast as you can on the choke and then with KEY OFF wide open throtle. This will cool down the plug tip. What this does is by going
WOT (Wide Open Throttle) it fuel a larger amount of fuel in to the
Engine to mix with the large amount of air ( lean condition) that heated you plug tip to a point that it self ignited the mixture.You just simulated pre ignition. It's know time to count your blessing that you did not blow up your Engine doing the speed runs. Any time a stock ody run 70 theres a problem. Seals, o-rings anad plugs are cheap.
Note: If you experence a boggin off the bottom ( off idol) that was not there before In most cases the side case seal is leaking air. This leads one to faten up the pilot jet to mix more fuel with the increas in air. If the pilot jet is clean that's a sign that an iar leak down is in order not a fatter jet. Alos use your static compression reading
to help. If the static reading is the same look at seal. If the reding is down it ( hesitation) may be related to loss of compresion, ring wear ect.
I should add after installing a new / rebuilt or cleaned carb one should remove the spark plug and crank to fill up the bowl before starting. Caution here too, not to crank to long as not to lose lube propertys of the oil in the fuel on a piston moving up and down creating excessive heat leading piston scuffing. A squirt of foggin oil here helps when excessive cranking time is required. Agian caution, not to crank to long and heat up the starter
motor ((
Internal Combustion Engine ? )). If you did not do this there may have benn fuel residue in enough volume to start the Engine than no fuel was present and it went lean heating up the tip. After you shut it down there was fuel in the carb and the plug tip is now cool. I should also mention in one of your previous statments you said you where doing a speed run and ran out of gas and then turned to reserve. This will also cause a lean condition. I and lot others run the fuel pet MUTT in reserve position and add fuel as the light comes on so this lean condition does not take place as a precaution. If you have a weed eater you have noticed this before. As the weed eater runs out of fuel it runs its best just before it dies. That a lean condition. Do that enough times in our machines and it will lead to Engine damage. One thing that happens is it get real hot and too much heat will destory parts. An example would be excisive heat on an Engine running at top speed, lets say 70. At that Engine speed ther is a loss of fuel that cools the piston could melt or parts of the dome could burn away or lack of fuel which alos has oil in it to lubracate the the piston skirt it could put part of the piston to the cylinder wall and then as the rings try to clean it of due to the cylinder pressure it will get stuck in the groves of the piston wher the rings set and the rings stick or get snaged ion the ports. Now you have cylinder cumbustion gases passing past the rings and leading to piston siezure or a ring snaged breaking the piston and doing major damage.
In short the carb must be clean metering properly and full of fuel.
edited: added text