Take the cover off your throttle housing.
Look inside and you'll see the cable and how the thumb lever actuates it.
The cable should slide all the way back without hanging up, even momentarily.
Take the air filter/intake tube/whatever else you have on the carb off so you can see the slide.
Move the throttle back and forth to see if the slide gets hung up anywhere, it should snap right back as soon as you let off the throttle.
Crank it up with all of these things removed and see if you can duplicate the problem while watching the thumb throttle.
If it starts acting up and the thumb throttle cable looks fine, shut it down and take a look at the slide.
If the slide is all the way down, and the thumb throttle isn't hung up, then you've got other problems that you need to address...but the first thing is to make sure it wasn't the throttle linkage giving you problems.

My 250 has similar problems when the throttle cable wound up laying on the cylinder head...it melted the teflon cable guide inside the housing and caused the cable to have tons of drag inside there.
The thumb throttle felt fine, but when I took the cover off I noticed the little arm was moving back and forth but the cable wasn't moving...it was stuck at full throttle and the arm was sliding over the cable.
An old cable, kinked, damaged, dirty, or routed in too tight of a radius will cause the same problems.

Now, do you have a kill switch?
If you do, is it normally functional?
Does it work when the engine starts revving up like crazy?
If you have a kill switch and it works normally, but won't kill the engine when it starts revving out, you've probably got an air leak somewhere.
if you have a kill switch and it kills the engine when it starts revving out, you might still have a problem with your throttle linkage.
A runaway engine caused by an air leak is usually propagated by detonation/pre-ignition from the lean condition associated with air leaks, so a kill switch that shuts the ignition off won't stop it.
However, if the ignition does kill the engine, that means the engine is functioning normally, no lean condition causing pre-ignition or detonation, so the carb must be metering fuel with a raised slide.


As for why it won't start now...you might find that the slide is stuck open, and you might want to tighten those clamps back up.
Loosening the clamps won't help anything now, if they were too tight and the rubber is dry rotted, you have no choice but to tighten the crap out of them.
Grk was telling you to take the carb off and flex the boot...when you flex or bend the rubber, any cracks that weren't visible before will show up.
If there are no cracks then you have nothing to worry about.
But, if you actually have an air leak, things are going to get a little more complicated....