Small Engines (Lawn Mowers, etc.): Craftsman Twin-19HP lawn tractor idle issues, briggs stratton, fuel pumps

Question
Mr. Jones,
         First, thank you for your time. I bought a Model 917.258562- Ser. 070397C 008096 Craftsman 19 HP Twin cylinder lawn tractor with a Briggs & Stratton- Model- 42E701, Type-1631 01, Code 9706115A motor. Cleaned it up, changed the gas, the usual stuff with an old unit that's been sitting, etc. Got it running, but it "lopes", or surges up and down at idle, runs a little better with the choke on 1/4. Did open up the little fuel pump at the front of the carb, cleaned it, but still doing the same. What do you say? (Tried adjusting the mixture screw with a little result, but not 100 percent)

Answer
Surging is almost always caused by a dirty carburetor.  Late model carbs, such as yours, are very sensitive to any debris because they are designed to meet strict emmission restrictions.

Anyhow, I'd start by removing, dis-assembling and cleaning the carb with compressed air from an air compressor and spray can carb cleaner.   Blow air through all the carb orfices, especially the really small passages.  Your carb is pretty simple but you should take some pictures before and during the removal and dis-assembly so you know how it goes back together.  As for the fuel pump, they either work or don't work.  I can't recall any fuel pumps that did not deliver enough fuel.
Below is a link for servicing the fuel pump.
http://engines.myfaqcenter.com/Answer.aspx?p_faqid=3352&body_0$tbKeyword=fuel pump

There are lots of videos on You Tube on carb cleaning.

http://www.youtube.com/watch?v=RJ0qPDYwQ_w&feature=related

Let me know if cleaning the carb fixes the mower.
Eric