Small Engines (Lawn Mowers, etc.): B&S Quattro engine speed cycles and stalls, air leak, cyling

Question
I have a with a 10A900 4HP engine. The speed 'cycles' i.e. revs up and down and will not stabilize.  I cleaned the carb and replaced the diaphragm and both governor springs. It didnt seem to help much and now it runs then dies. Leave it and it will start etc.  

Im an old car guy and pretty familiar with rebuilding SUs etc so I am fairly sure I cleaned it out to spec. This carb is so simple it should just work!!

I assumed the cyling was an air leak but am now suspecting my diagnosis. Any ideas and tips very welcome.

Answer
Hi Mark,

An air leak would cause the engine would run up to a lean high speed condition, so as you suspect, I don't think it's that.  Cycling or "hunting" usually is caused by a weak or defective governor spring.  With the engine now running and quitting I also suspect float problems.

Check these things and let me know how you make out.
Michael