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Mower starts first pull with a little choke. Runs 10 to 15 minutes through all conditions then sputters out and will not start again until it has completely cooled down. New plug properly gapped this season. Plug wire attached to plug. Vent hole in gas cap checked for clear. Exhaust port under deck clear. Cooling fins clean. Gas tank drained and new gas and oil filled this season.

Any ideas out there? Thanks in advance for you help.

I would guess it is bad crankshaft seals. Or a ignition coil/module that fails when it gets hot.

When it stops check for spark. No spark, replace the coil.

Like stated, if the muffler and lower crankshaft are damp the seal is leaking. If it's dry take the coil off and wrap the plug wire in foil then toss it in the oven at 200degres for a hour. Let it cool then coat it in clear coat to seal it back up. I've done it on a e and a f series and seems to work for me.

I don't know if I can be of help, but I had the same problem. What I did was I just cleaned everything I could, and I also turned the idle up. Now it will run until I shut it off instead of 20 min. I would take off and clean what I could before buying parts, it worked for me

just bought the same model for 10.00 and it is doing the same thing.Crank seals would be my guess as the motor on mine has a knock and excessive free play on the crank.grab the blade and check for free play.Of course disconnect the spark plug first but you knew that.If it has excessive free play look for another f motor unless you want to rebuild it.,

Lex3000 said:

Mower starts first pull with a little choke. Runs 10 to 15 minutes through all conditions then sputters out and will not start again until it has completely cooled down. New plug properly gapped this season. Plug wire attached to plug. Vent hole in gas cap checked for clear. Exhaust port under deck clear. Cooling fins clean. Gas tank drained and new gas and oil filled this season.

Any ideas out there? Thanks in advance for you help.

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Hello Lex3000
Like other's have said here, it's time for a new CDI coil pack. I don't feel that it is your Crankshaft seal's because if they were bad it would not want to start Cold. Once and engine warm's up Part's swell as the Crankshaft steel, so I don't feel that your engine dying issue is Crank seal related. I have had the exact same thing happen to me on several Lawn-Boy's quiting once they get hot, and most time's they will re-start once they cool off. The CDI coil Pack material breaks down over time and it then allow's the 2-cycle gas mix to enter internally which when heated up causes a direct short which Kills your engine. If you were to look closely at your CDI Igniton pack I am sure you would see some very small Fracture's in it's Plastic outer covering. I hope this helps, Hammerdown

after closer inspection of my 10400 I discovered exactly what you you said small cracks in the coil.my crank seals are good but the crank bearings are shot.luckily for me I have another f motor on hand to drop in.