Small Engines (Lawn Mowers, etc.): John Deere engine hard to start, or starts then stops, briggs stratton engine, john deere engine

Question
Hi, I have a three-year old John Deere Walkbehind lawn mower JS20 that uses Briggs Stratton engine (http://degagnequipment.com/node/52). I think the newer model now is JS25.

One week ago when I was mowing the lawn, I found that I could not restart the engine after I stopped it to bag the grass. I have replaced the spark plug, replaced the air filter, and checked the belt, the blade, (they look pretty good) etc. over the past week. Nothing helped. The engine usally ran for half a minute and then died off.

Then I read about dirty carburetor on the internet and I bought a bottle of Gumout small engine carb/choke and a bottle of fuel stabilizer today. I sprayed the Gumout into the hole in the attached picture

[IMG]http://i28.tinypic.com/2me1gra.jpg[/IMG]

The engine started and ran fine for about 1 hour after the treatment. Then the engine started to get weaker and weaker and eventually stopped again (the fuel tank was near empty). I went to buy some more gas and added it to the tank. Now I could not start the engine, or if the engine starts, it will either vibrate hard or die off quickly.

I do not know if this is the cause or not. But if I manually spin the blade underneath the mower, the blade spins smoothly for about 270 degrees, but it gets very very tight to turn for some certin angle. This is true for every 360 degree spin. Sometims it gives scratching-like noise. I have detached the engine from the mower and did not find anything that's blogging the turning. The blade hub looks fine too. Is it possible that the axle itself is bent? It looks fine to me though.

Thank you very much!

Answer
I believe you have a bent crankshaft by the description of symptoms your giving me.  

Remove the blade and the blade adapter

Remove the Spark Plug wire to prevent accidental starting

With the help of an assistant, tip the mower up so the air filter is facing up.  Have them slowly pull the start rope.  

Watch the crankshaft turn, I think you will see it does not turn smoothly.  It will be obvious that its bent.

If your mechanically inclined you can replace the crankshaft, but it requires total disassembly of the engine.  If your not mechanically inclined I would seriously consider replacing the complete unit.  The Labor cost to repair this will far exceed the value of the unit I believe.