Small Engines (Lawn Mowers, etc.): Briggs and Stratton Engine Still Oscillating and Dies., briggs and stratton, engine rpms

Question
This is for a Briggs and Stratton 9hp 198707-0141 for Craftsman Riding Mower Model 247.27020.  This small riding mower is going into its third season.  It is one of the Yard Bug style single 27.5" blade riding mower.

The mower will start fine, run for 7-10 minutes and then the idle will begin to oscillate (up and down) and then finally die.  You can see the choke arm ( I think that is what it is) go back and forth with the engine speed.

Here is what I have tried:  Checked and cleaned fuel line and put in new fuel filter, Replaced Battery, Replaced spark plug (proper gap) and cleaned plug wire contact, removed Carb and cleaned with carb cleaner spraying through all jets and cleaned float bowl and intake, Lubricated all springs.  I put it all back together making sure seals were lubricated and set properly.  There were no cracks in them.  All of the above was done VERY thoroughly.

I did notice (before and after everything I did above) that once the mower dies there is a very small bit of white smoke coming out of the carb. Comes out faster if I open up the choke plate.  

Thanks for any help.

Answer
Hello Ryan:

Check the Gas Tank Cap and Ensure it is Venting the Tank Properly. From your Description it Sounds like the Cap Isn't Venting and a Vacuum is Building in the Tank and Restricting the Gas Flow to the Carburetor. The Arm you are Watching Move is the Governor Arm and it Regulates the Engine RPMs in Conjunction with the Throttle Cable and Throttle Spring. How Long Before the Engine Will Start an Run Properly Again? Hope this Helps. I am here if you Require more Assistance. Let me know what Happens, Please. Thanks.

Good Luck

Respectfully

John