The Great Big Lawnmower Thread

Don't let the engine 'runaway' at high revs, it will be damaged very quickly, if not already. Hopefully It cut out due to excess fuel consumption because of the high revs, but it's usually because of overheating or seizing. Find what's keeping the throttle wide open, it might be sticking on the carb. Spray with wd40 and wiggle everything in sight.

Yes, its external. No need to strip the carb, but look in past the air intake to see if something like a twig is jamming the throttle valve open. The first valve is an auto choke, which is normally closed until you start up. The throttle valve is normally open under light spring tension, there should be second spring which pulls against the governor lever. When the engine starts that lever is operated by a centrifuge in the engine which pulls the throttle butterfly valve closed, reducing the engine speed to running speed equilibrium. When cutting, the load against the blade and the wheel drive slows the engine, whereupon the centrifuge lever slacks off sufficiently to open the throttle and up the power while maintaining near constant speed. Find the little throttle lever on the carb and see if its easily turned about 90°, start and watch to see the governor lever pull against the throttle to close it. The governor spring may have popped off

Should twist easily. It's probably jammed in the cavity, or the shaft is stuck Plenty of wd or penetrating oil inside, poke and twist. If the shaft is sticking, you can pop out the flap, its held by two screws, then withdraw the shaft. Clean and oil.