Small Engines (Lawn Mowers, etc.): b&s 3.5 runs eratic, valve problems, diaphram

Question
 I have an old snapper self propelled with a B&S 3.5 engine. over the past 3 months, when started at times it would run rough and exhaust would be slightly black. I would throttle down to idle and it would eventually start running good, then I could throttle up and it would run fine. Yesterday after starting it ran rough I tried to idle down, didn't work, died. When I manually hold open the choke with a screwdriver it runs rough but dies when I pull it out. Today I put a new carb kit in(diaphram) and I have the same problem.

Thank You
Todd  

Answer
Hi Todd,

The carb may have dirt or gummed fuel inside.  I would probe every small hole with a fine wire and blow it out with compressed air.  Since the engine is old it may have valve problems or low compression.

Let me know how you make out.
Michael