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I'm helping a neighbor with his mower, he's not mechanically inclined.

He tried starting it up four or five times. Each time, a "puff of white smoke" came out, it ran for several seconds, then died. I went over to look at it, flipped the hood and saw dark liquid coming from the exhaust. I checked the oil and it was considerably overfilled, but didn't look right and smelled like gasoline.

I'm guessing the carburetor failed somehow, allowing the gas to flow into the cylinder and down to the oil sump. In any case, I disconnected the fuel line and drained all the oil. Checked again two days later and a little more "oil/gas drained out". I then put in the proper amount of 30W oil and checked the oil level. Then I removed and cleaned the spark plug, and while I had the spark plug out I told him to crank the engine several times in order to blow out any residual gasoline that might be left in the cylinder. Then I reconnected the fuel line and told him to crank the engine. It fired right up and ran great for several seconds, but then started blowing blue smoke, ie burning oil. After 5-10 seconds I told him to shut it off....

Any ideas regarding what the problem might be and how to fix it? Anything a do it yourselfer might be able to handle?

Really appreciate any help, thank you very much.

Your very first assumption was correct.
The float valve in the carb has failed.
So you will need to fix this before you can do anything else.
When the carb is working properly then you need to drain all of the oil out of the engine.
Refill with cheap supermarket oil do one mow then change the oil.
If you bought a 2 qt oil bottle, change it after the second mow & the filter if fitted.
Fuel diluted oil will cause excessive wear in the engine very quickly.
Go to http://outdoorpowerinfo.com/ , in the repair section you will find very detailed instructions covering carb cleaning .
FInd yours and follow the instructions to the letter.
Before you refit the carb, hook it up to a fuel tank and work the float with your finger to check it cuts off the fuel before the float runs out of movement.
Before you remove the carb, take photos of all the connections at both ends so you can hook it back up when you have finished.

Thanks for the reply.

So is there a possibility that, aside from the carb problem, the mower might still be ok? I figured that since the exhaust was blowing blue smoke there was also another problem, such as oil burning. Could a failed carb float valve result in blue exhaust?

That would be great if it turned out to be just the carb.

thanks

Oil diluted by fuel will slide right past the rings and cause the engine to smoke.
If the cylinder had more than 50cc ( whatever ounces that is ) in it when you cranked the engine then it may have also blown the head gasket which is a cheap & easy fix.
But we start with the carb.
If there is a lot of debris in the bottom of the float bowl ( hint put a catch can under it before you remove it ) then you will need to go back down the line & find the source of the debris.

When you get the mower to start , after it has been turned off you take out the dip stick .
If smoke comes out of the tube then the gasket is gone.

If you are unsure about any step, take photos & upload them here, lots of techs on line to give you assesments .