Small Engines (Lawn Mowers, etc.): Lawn Mower, sears craftsmen, costly problem

Question
Mower is humming away for the most part.  I noticed that is seems to be running a bit rough.  Maybe it needs a tune-up?  It seems as if it wants to stall but it won't.  What do you recommend?  I thought that maybe the roughness would smooth out after letting it run a bit but it didn't.  It's not so so bad but I know that it could be much better.  

Thanks,
Tim
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Followup To
Question -
Hi Mark,
I recently received a second hand lawn mower from a friend ( Sears Craftsmen 4.0 Mulcher )  Upon fill it with gas I noticed that the fuel started leaking out of the air filter.  Can you tell me what's going on?  Does it sound like a simple fix and does it sound like a costly problem?

Thanks,
Tim
Answer -
Hey Tim,
The carb of the mower probably has extensive clogging and should be cleaned out. What you shoutd do is remove the carb and soak it in gasoline for about 1 hour. After that run pipe cleaner or spiked wire through the carb holes. Pour more gas though the carb and re-install. Let me know how it works out.

Answer
Hey Tim,
Nice to hear from you again, what you should do to give the engine its normal performance again is change the spark plug, change the filter, and empty the oil and add new.
To Change oil:
1) Run Mower for 5-7 minutes.(Warm/Hot oil drains better).
2)After Stage one is complete tip the mower on its side and let all the old oil drain out(Wait About 5 minutes).
3)Once the oil has drained add new fresh 5W-30 motor oil.(CAUTION: DO NOT OVER FILL)
4) Check the oil level and add or take out until oil level is good.
5) Start the mower up and see how it runs.
Hope this is more good information for you. Let me know how it is after the tune up.