lawmower problem

I'm sure it's easy but a search through this group yielded little (so far).
My mower sputters and spits, almost always at lower than normal RPMs. RPMs vary constantly, and sometimes it comes very close to quitting. It has, on a few times. This is all while on a level sidewalk, not mowing or otherwise under any strain. It doesn't change when I start mowing the grass.
I've cleaned the air filter and replaced the (very black) spark plug. I have not taken this mower in to be tuned up in the 3 years I've had it. Oh, it's a Briggs and Stratton 4.5 hp engine on a push mower (not self-propelled); garden variety Lowe's purchase.
Any ideas on what next to check, change, adjust, or replace?
Thanks.
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Greg wrote:

The black spark plug is a clue. Black means that it is getting too much fuel (rich). Have you adjusted the needle in the carb (a screw and not the idle screw), or maybe if fell out. Are you really sure the air filter is clean, a dirty filter is often the source of an over rich fuel supply. If you find the carb needle, while it is running screw it in and the engine speed will increase, at it's highest speed the fuel is too lean and just a little more and it will start bucking so you back it out a bit to the fastest speed and then back it out a bit more because at a really lean setting the engine will run hot.
One other possibility is there is something wrong with the choke mechanism. A third possibility is that something is sticking in the mechanism that maintains the speed (usually two wires and a spring connected to an air paddle) or that this mechanism has been taken apart and put back together incorrectly.
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Greg wrote:

Sounds like running too rich. Tried with air cleaner removed? Sure choke is off?
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http://experts.about.com/q/Small-Engines-Lawn-1746/index.htm

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Nice site. Thank you for the reference.
--
Jim McLaughlin

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I'd check the float and the gas tank for moisture etc. Try running it with the gas cap off the tank also if it isn't allowed to get some air in the tank it will do that.

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Clean the carb. Remove the carb and find someone with a parts cleaning machine, it would save you from having to disassemble the carb and put it back.
One time my lawn mower wouldn't start after sitting around for years with gasoline in it, I took out the carb, dunked it into an ultrasonic cleaning machine filled with water and some kitchen grease cleaner. Minutes later it looked very clean and after putting it back, the mower started up with one pull. I also figured out a way to keep the carb clean:
everytime I finish using it, I shut off the fuel supply and let the engine run until there is no gas in the carb. Then I put the mower on choke and pull the start cord a few times to clear the carb. It's been several years since I last cleaned it and I can still start it with one pull 90% of the time. Not bad for a 15 year old john deere.
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