Outdoor Power Equipment and Small Engines - murray rough idle

I have a murray 20" quantum 5.0hp 2n1 push mulcher mower.

It has a rough idle and sounds like it is "surging" idle up and down while sitting there running.
what is wrong and what can i do to smooth it out.

Post the engine model, type and code usually found on the engine shroud just above the spark plug so we know what you have and it will get fixed. Have a good one. Geo

it's a briggs and stratton, 3.5, there was no number above the sparkplug but this number was above the muffler on the shroud on the side, 123k02-0230-e1 0312559

If the air filter is dirty, clean it. If it still runs poorly, remove the carburetor and replace the diaphragm and gasket between the tank and carb.

Sounds like the same problem I have with a 5+ year old Murray mower. It probably needs a carburetor diaphragm (B.& S. part # 495770). Mine needs this replaced every other year or so. It takes me less than ½ an hour to replace and costs $3-$5.
Later, Ed in Philly
P.S. Here is a link to check out. I have one of the recalled models, but it works fine as long as I feed it diaphragms!
http://www.cpsc.gov/cpscpub/prerel/prhtml04/04103.html

The numbers WWC posted are for an engine with a bowl type carb, however I have never seen a 3.5 with a bowl(in recent years anyway) so somewhere, something ain't right.
So if it is a diaphram carb(the carb sets directly on the fuel tank).
You need to install a new diaphram kit, it is Briggs part # 795083 or old # 495770 also Lowe's has a kit #5083 which has the same parts.

Here we go;
Remove air cleaner, there are 2 bolts that hold things together, one on the front of the tank 3/8" and one into the block 1/2", remove these bolts, now "slowly" slide the carb/tank off the intake tube and breather tube, now tilt the tank in to release the throttle linkage and waalaa the carb/tank is removed. Remove the 5 screws from the carb/tank remove carb(don't loose the spring) now spray all holes, cracks and crevases in both the carb and tank surface with brake parts cleaner, remove the main screen(looks like a thimble) with a small screwdriver pry out the main jet(carefully) and clean it, reinstall the jet, it can be difficult some times to get it to snap back in place(I use the rounded end of a screwdriver handle). Remove and clean the fuel pickup stem. Clean any junk/rust from the tank. Install the diaphram on the tank then the gasket(no goo or sealer) now carefully replace the carb(the spring will try to misalign the diaphram), tighten all screws a little at a time so as not to crimp the diaphram until they are all tight. Install the carb/tank in reverse order and you are done. If I missed something one of the real mechanics will correct me. Have a good one. Geo

Thanks I appreciate the step by step instructions, i'll give it a try and let you know how it goes.