Once the engine flooding condition is corrected, it should run up fine. In-line fuel valves (if they are shut off when not using the equipment) is cheap insurance to prevent the condition from occurring again. If it continues to seem rich, you may need to buy some carb parts.

***"OVERNIGHT the gas drained out of the tank and into the engine. I left the throttle on full by mistake"***
And you can rest assured that WAS NOT a cause of the gas leakage.
You do need to add and use an inline fuel shutoff (per tomplum's advice).

Thanks tomplum and mownie. A shutoff valve sounds great. Since I changed the oil and there was still significant gas contamination, should I disassemble the engine and wipe it down inside? I have a bad feeling that the gas will not go away so easily. Perhaps I should have a look at the valves anyway. I've been in a hurry to cut that tall acre of grass between deluges, but I guess I have to wait.

1) stop the flow of gas/ excess gas the floods the engine. 2) Install a shut off valve to ensure it doesn't flood during storage 3) Change the oil and de-flood the cylinder the final time. 4)If you still have fuel contamination, see # 1! :)

Maybe this is best: Clamp the fuel line for now, remove the spark plug and air cleaner, rotate the engine with the key off or ignition grounded as to where a spark won't ignite the fuel, drain the oil- and walk away. Of course, do this in a safe area where you are not near pilot lights etc. Close it up and add the oil and running the engine will take care of the rest. This is provided the carb issue is fixed.
I'm afraid disassembling the engine will create a very long thread...

***"disassemble the engine and wipe it down inside"***
Absolutely NOT!
But before you run this engine again you should install the inline fuel cutoff valve, and change the oil and filter.
The trace amount of gas left after you change this oil and filter will harmlessly evaporate in 3 minutes or less when the engine warms up.
And no need to have a look at the valves either. This is a flat head engine with the valves in the block, not in the head.
There is no kind of "owner implemented" valve checks or adjustments that can be done. Valve adjustment to this kind of engine is akin to major surgery..............but virtually never needed either. Maybe 10 or 15 years down the road, but not now.
These engines seldom die from hard use, but they will die from running out of oil, running too long between oil changes (every 50 hours of use, change the oil and filter), or if mice build a nest inside the cooling shroud sheet metal ducts and overheat the engine.
So...inline valve, new oil and filter......then work.

Thanks guys. You can be sure I'd rather not disassemble the engine. There was a fairly high degree of sputtering and smoking after first oil change, so it was discouraging. I'll put the shutoff valve and new oil in and try again. The plug was good, btw, just wet and a bit blackened.

And because a significant amount of oil/gas has been transported into the muffler and exhaust pipe, you can still expect to see diminishing amounts of smoke for about 5 to 15 minutes it will take to completely burn the crud off in normal use.
After that, your exhaust plume should be pretty clear, save for the possible momentary puff on cold start ups (but that has nothing to do with your recent events). :^)

OK. Back to my original flooding incident. Did the excess gas in the cylinder then work it's way into the oil in crankcase? And did the gas-oil mixture then work back into the cylinder when running, causing white smoke and excess pressure? Or did the increased fluid level over-fill the crankcase, increasing crankcase pressure? The oil was a bit low, just to the upper edge of the add mark, before the gas got in. About 1 to 1-1/2 quarts of gas leaked in, I'm guessing. I talked to a guy today who thinks I possibly have blow by or cracked cylinder. I just inherited this tractor with property rental. I don't know how it was treated before, but the oil was low and somewhat dirty when I first checked it.

Here is the typical flow path of gas from a carburetor wherein the needle valve is unable to stop the flow of fuel when the engine is not running.
Fuel leaks past needle valve, filling and overflowing carb.
Overflowing fuel from carb runs into the intake manifold and enters a cylinder IF... an intake valve is open while the engine is at rest (stored).
Fuel entering the cylinder at this time will fill the cylinder and the very thin gasoline will seep slowly past the piston rings and fall into the crankcase.
If the intake valve is closed during this time (single cylinder engines), the gasoline might rise high enough in the intake manifold that it overflows the carb and spills out into the open area where the machine is stored.

Once the crankcase lube oil is contaminated with highly volatile gasoline, there will be smoke in the exhaust plume as the engine begins to warm and the gasoline begins to evaporate.
The evaporating gasoline WILL increase crankcase pressure and this will cause a positive outflow of the gasoline vapors through the crankcase breather assembly. The breather vents the crankcase into the intake manifold, so the volume of gasoline vapors escaping the crankcase will be carrying a lot of oil mist (normal) through the oil separator mesh of the breather. So much oil in fact, that some oil will make it past the separator and into the intake manifold to be burned in the cylinder. This accounts for most of the smoke in a fuel drowning event.
Gasoline overflow into the intake can also result in a hydrolock condition in the cylinder as the piston rises and slams into the liquid fuel.
None of this would be the headache it is if the outdoor power industry would follow the lead of the motorcycle industry and put vacuum activated fuel petcocks on the gas tanks. That would provide an automatic shut off when the engine was not running and an automatic fuel on when running or cranking.
I'm honestly surprised that the meddlesome EPA has not mandated such a device be standard equipment (maybe the EPA, along with most of the population does not read about the problems aired in this forum.
You just get this fuel leakage problem under control and then if there is a "cracked cylinder" or other mechanical defect, I'll be very surprised. So far things only point to a fuel drowned engine.

Thanks again mownie. Your flow sequence sounds very spot-on. But my engine is a single cylinder. I ran it for about a minute before it stalled, then gas leaked overnight. Just to be clear, the guy I mentioned earlier said cracked piston, not cylinder as I mistakenly said.

Something I failed to mention in the scenario where the gasoline begins to evaporate and vent out the crankcase breather into the intake.
The venting gases/vapors can be so fuel rich that they can smother the engine down as in "too rich to run". Engine can not run on fuel alone.........he must also have air (and in the correct proportion). Sometimes the oil in the venting gases wets the spark plug and thus kills spark too.

OK, I changed oil for second time and fired it up, and it seems to be working. I got past the smoking stage, burning off remaining oil/gas in exhaust. I'm glad I did 2 oil changes since this probably flushed most of the gas from crankcase. Thanks mownie and tomplum.

bsnewbie - if you DO have fuel leaking into yer oil and you don't get that fixed, won't be long before you'll be back here lamenting about a blown or locked up engine.
If nothing else, install an manual inline fuel shutoff valve and use it each time you run the engine. They're inexpensive and relatively easy to install - usually in the fuel line just ahead of the fuel filter.

Yep. Previously I found a sticking fuel float. I addressed that and the leaking fuel stopped. I am going to re-check my tank level tonight. So far Tractor Supply sounds like they have a shutoff valve in-store. Know any others? My fuel line needs replaced anyway.

***"Previously I found a sticking fuel float. I addressed that and the leaking fuel stopped."**
OK, that's good! But don't be obstinate like some people who have posted such rebuttals as "Don't need a shutoff if the float works right.)
In a perfect world that argument might be the end of the story and the sun shines down and all is good etc.
But it ain't a perfect world and "I have gas in my oil" is one of the most frequent complaints on this forum.
Once upon a time...........all OPE engines came from the OEM with a fuel cock in the tank, or mounted remotely and everybody (almost everybody) knew you had to cut the valve on (and off) to operate the equipment correctly.
Price point manufacturing and the quest for "dumb down" features has eliminated the standard fuel cutoff from most machines now. Much to the chagrin of owners everywhere when the undependable float system drowns their engine.
The ONLY way to know for sure is to install an inline valve and learn to use it whenever the engine is not running.
Put the valve between the tank and the fuel filter to make it cleaner/easier to change out the filter when due.

Looks like I located some in-store fuel shutoff valves + new fuel line, so I'm gonna do it.

Thumbs up!

I had the same problem and I put a gas shut off on it. All was working fine. After two mowings I stopped to dump the grass and the engine would not start.

I get a spark but it will not start even with fuel added to the air intake. Occasionally it will ignite but blows back through the air intake. Do I have a bad valve?

What can I try next?

I know this is an old thread but I wanted to give a Big Thanks to Tomplum and Mownie. Got this same flathead engine in a Sear branded Murray I bought 8 years ago. I thought I cracked the block or something. But I've gotten in the habit of searching the web when I encounter a problem with just about anything now. It took me quite awhile, searching multiple ways and words. Found this thread, went out to the mower, drained the oil, whew, full of gas. Bought and installed cutoff valve and refilled oil. Engine is running like a champ.