Small Engines (Lawn Mowers, etc.): B&S 10A902 engine surging, carburator, carberator

Question
4 H.P. B&S engine that is surging up and than goes down almost to stopping. Has been running great until this last time I started it. Carburetor is all plastic screwed directly onto the gas tank, no float or adjustments. Gas tank has been emptied and cleaned out. Carburetor pickup tube appears to look clean, small tube filter looks clean. The gasket between the gas tank and carb has a very slight fold in it. Could this be causing the surging? There just does not seem to be much to the totally plastic carburetor. The spark plug looks black as if running too rich. What tests can I perform to prove or disprove the carberator is the problem? Is there a particular area of the carburetor I should pay close attention to if I need to clean it? Does not appear to be a weak govenor spring issue. BTW, what does the small pen like spring on the bottom of the carburetor that sits under the gasket, do?  

Answer
 Hello Jim:

 The Carburator you Described is a Pain. Most People Replace it when it Starts giving Trouble and from your Description the Carburator is the Problem. You can Clean and Rebuild this Carburator. It May or May Not Work Fine After the Clean and Rebuild. Soak the Carburator in Cleaning Solution for 2 Hours and then Blow All the Passages Out with Compressed Air. Resoak the Carb Again Before you Rebuild it. The Varnish that Gas forms Inside the Carburator is Hard to Get to Loosen and Remove from the Carburator. The Small Pen like Spring of which you Speak works the Pluse Pump of the Carburator. Vaccum Pulls the Diaphram and the Spring Pushes the Diaphram and Pumps Gas Out of the Tank and Through the Caburator to the Carburator Jet. Hope this Helps. Let me know what Happens, Please. I really Enjoy knowing the Outcome. Thanks.

 Good Luck

 Respectfully

 John