I surmise you have a clogged jet and are simply overwhelming the float system when on its side.
Running like this may give you far greater problems.

Post the Sears 917.xxxxxxx number or engine brand, model, type/spec so we know what you have.
Different carb types may need different trouble shooting procedures.

There is no 'tip-over' switch. The engine/blade control bail handle is a 'dead man' switch that cuts off the engine and brakes it for a fast stop if you let go of the bail. You know, like if you are cutting a slope that's too steep, trip and fall, and the mower lands on your face.

I have seen this a lot of times with float type carburetors. The seat for the needle becomes loose and the bowl starves for fuel.
When you change the angle of the engine it gets the gas it needs and runs, but not for long..

Model 247.370340

Roland please supply precise info ! This thread is 4 yrs old Bud .

Easy. From the serial it is an MTD Must Tip Down...

Mtd doesn,t tell us much ! What the He'll are the symtom,s , mower appears to be a Craftsman with a Briggs Engine .

Sorry. I am experiencing the same as what is mentioned above and then I added my model number to the discussion. My lawn mower ran out of gas and I filled it back up. After I filled it back up, its starts right back up but quickly shuts down. I changed the air filter, the spark plug, and emptied the liquid (gas remnants) from the cup under the carburetor. After all of this, I tried starting it and had the same experience. By chance, when I tipped it on its side, it started and actually stayed started for about 15-20 seconds and then would shut off again. It is a Craftsman with a Briggs Stratton Engine. I am not sure what else to do at this point. Clean the carburetor? Replace? Does this have something to do with the float/floating pin?

Often just a clogged air filter causing the mixture to richen to the point of double-stroking. Tipping on either side reduces fuel flow to rebalance the mixture and it will then (under no load) run at normal governed speed. Check air filter first.

Not the air filter... I have the same issue. Took off the air filter and there was no change. Runs while tilted only. I just put on a new carborator (=$20) but the lawn mower hadn’t been run for years. So I’m going to take it off to see if I can check the float issue and put it on again.

Oh... and the air filter is brand new as well.

Ran into this same problem with a honda motor on an edger. Did the whole rundown. Air filter, plugs, carb kit, new float and needle, cleaned jets, even eventually replaced carburetor, just to discover the oil was low when I had given up and planned to junk the motor and was draining fluid. Topped off the oil, and guess what? it ran without having to tilt it to the side. 🤷🏻‍♂️

Low oil cutoff safety switch are quite common on long run engines such as generator sets . Not so common on lawn mowers however . I have ran many mowers on quite high off camber inclines and never had an oil starvation issue , 2 strokes were preferred for such type cutting back in the day for obvious reasons .

My engine would only run when tilted forward. I added oil to my generac pressure washer and finally it ran while sitting flat. The low oil was preventing the motor from running when sitting flat.

Put some injector cleaner in the gas. Then pick up the carburetor side of the mower and drop it at least twice. It should start and run now and the cleaner just might solve it temporarily. In the long term, you have to put some Sta-Bil 360 Marine or Startron in every can of fuel you buy for your yard equipment and continue doing that forever.

You have water in the carburetor bowl. Loosen the bowl nut underneath and drain a little out for immediate fix. Longer term, keep your gas tank full between cuts. Dry gas additive helps. High environment temperature swings cause water to condense in a cold empty gas tank when the weather warms up. Water is heavier than gas and drops down in your carburetor bowl. Tilting the lawnmower moves water away from the float valve so it runs when tilted but leveling it out puts the water in the middle of the bowl at the needle valve and stalls the engine. Draining the bowl a bit from the bottom lets the water out.