Chances are, your mower has a type of governor on the throttle mechanism. Hidden beneath a metal guard, there is a small vane, that sits in the airstream of the fan, mounted directly on the engine crankshaft. If the engine speeds up too far, the vane pulls the throttle closed, in order to slow it down. There is a small spring, usually visible, that pulls the throttle open. So, the vane and the spring work against each other. When the engine is not turning, there is no airstream from the fan, and the throttle is wide open. When you prime the engine, using the rubber primer bulb, you put some extra gas into the carburetor. The engine will start and run rich, for a few seconds, until the slug of primer gas is burned up. Now, the engine has to keep running, with gasoline delivered by the carburetor. If the carburetor is clean, and free of gum or varnish, the engine will run smoothly. If the carburetor is gummed up with varnish, deposited over time, the engine will run lean, start to stall out, which causes the spring on the throttle arm to pull the throttle open. Now the engine zooms, but not for long. It starts to stall again, and the cycle repeats. Until you clean the carburetor. The easiest way is to add gum-out or something similar to the gas tank, and then run the engine.

Usually this doesn't happen suddenly. Since you were mowing at the time, you might have pushed the mower under a bush and caught the governor spring on a twig and bent the metal tab. I've seen it many times in the shop.

Below is a picture of a typical spring and tab. Move the tab a bit either in or out and see if it cures the problem.

Otherwise, some dirt or varnish has gotten into the carb, as perfectly described in the previous post.

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A governer problem will not cause "hunting". You have garbage in the carb causing a lean surge. Clean the carb, leave the linkage alone.

Yep, something caught in one of the jets causing lean surge.

Dissasemble carb and spray clean it with gumout and compressed air.

Also, do not use ethanol fuel in lawnmower engines.

If you have no choice on the ethanol-laced fuel, at least use Sta-Bil Marine Concentrate for Ethanol Fuels found at Wal Mart in 32 oz dispenser bottles. It's blue and super-concentrated - 1 oz treats 10 gallons, so you would only use about 1/4 oz for a 2 gallon can. Put it in your gas can, not directly in the mower. This way, your fuel will be protected for at least a year regardless of what you use it in. This stabilizer is designed to prevent the typical corrosion problems associated with the use of ethanol fuels along with the ususal benefits such as cleaning the fuel system. If you really want to go the extra mile and like to use small engines for 20 years, also add 1 oz of Marvel Mystery Oil to each gallon of fuel, including your 2-cycle mix.

Thanks for your suggestions! I need to mow the lawn this weekend and I'll let you know how it goes!

I lucked out with my newest freebie mower. I started it second pull, then it sat out for a month. It didnt even pop when I tried this week, I drained the fuel, then I used a few oz. of BG12 a low power cleaner in the tank. Then I removed the air flter and shot some carb cleaner. It fired and died. I kept it for a few minutes,squirting CCas the engine died until it didnt die. Then I added a cup of gas and some MMO. I ran it a bit and parked inside the shed. Yesterday it started on the second pull and i mowed the near part of the front yard. It mows OK, BBC works , so does 3 spd trans All it needed was a wheel adjuster bolt

Fabulous - all of your suggestions have solved my problem. Cleaned the carb and put some gumout in the gas. It is like new again. Because of tons of rain resulting in no chance of mowing my back yard it become much higher than it had ever been - a little like a field! I left the mower on mulch and it went through it like nothing. By the way - thanks Baymee for sending in the photo for me!