Small Engines (Lawn Mowers, etc.): BS 4HP 10A902 Runs rough, black smoke, carburators, carburator

Question
BS 10A902, type 2171-E1 4HP on a push mower.

John, thanks for taking my question. (I saw a similar post, http://experts.about.com/q/Small-Engines-Lawn-1746/B-S-10A902-engine.htm) but wanted more info. Mower ran great all summer. It is six years old. Started up and began to run rough and huff out black smoke a few weks ago. Cleaned air filter, new plug and cleaned carb with carb cleaner. No effect. Took carb and tank off engine, thoroughly cleaned everything. (This is the all plastic, no  adj carb, BS# 498811, on a 10A902 engine.) Found a fair amount of debris and sediment in tank. Cleaned all very well. Ran for two sessions great after that. Fired it up today, and it ran very poorly, and huffed out lots of black smoke till it fouled out the plug and just about quit. After reading the other post, this carb sound like a pain...am I better off just replacing it. The carb looks cheap, only $17.00 online. Should I replace all gaskets and o-rings as well? Any other suggestions? Any reason to replace thr tank as well? My problem sound very similar to the other post. It appears to have adequate and constant spark. The mower is in great shape otherwise and I hate to ditch it over a $17.00 carb if that's all it is!

Thanks in advance for your help, my back grass is getting high! ;-)

-Frank J. Ranelli
Columbus, Ohio
fjranelli@columbus.rr.com

Answer
 Hello Frank:

 The Plastic Carburators are a PAIN. Its Hit and Miss when Rebuilding them. If a Piece of Grit Scratches the Main Jet, the Carburator is Trashed. The Main Jet then Allows too Much Gas and the Engine Runs Rich. It is Hard to Remove the Varnish that the Gas Forms Also. Most Times the Varnish takes 24 hrs to Desolve Properly for Cleaning. With the Plastic Carburators you have to Soak for 3 to 4 hrs and Blow Everything Clean and Then Resoak Again. It Normally takes  4 or 5 Times of Doing this to Properly Clean this Carburator. Replace the Carburator and Associated Gaskets and O-Rings. Hope this Helps. Let me know what Happens, Please. I enjoy knowing the Outcome. Thanks.

 Good Luck

 Respectfully

 John