Small Engines (Lawn Mowers, etc.): Surging and not accelerating B &S mower., briggs carburetor

Question
Hi P.K. I bought this 2 y.o. 3.5 h.p. mower knowing it needed repairs (not running). The inlet manifold had collapsed, so I replaced it. The mower started straight away, but surged in a rhythmic fashion. If I tried to open the throttle, there was only a modest increase in revs, and the mower twisted on its wheels with the torque on each surge. I replaced the carbie diaphragm - no difference. Where to from here?  Many Thanks.  David

Answer
Does this have the plastic carburetor?  I'm not sure why, but this year I have had to replace about ten of these already.  I don't know if it's the ethanol in the gas or just coincidence, but in the past I've replaced maybe four or five a year.  Maybe I'm just becoming more short-tempered.  At least they're cheap.

Is it blowing any black smoke?