Small Engines (Lawn Mowers, etc.): Lawn mower engine running poorly, float bowl, needle valve

Question
I've owned my MTD 21" push/rear bag mower with a B&S 4.5h engine for about 6 years and I've changed the oil once. I just keep adding gas and nothing else. It has run excellent until just recently. It starts great, but after about 30 seconds of running it will start to surge a little sporadically. It also bogs down to almost stopping if I get it into the grass to start cutting. Where do I start? It sounds like a fuel thing to me. I have no experience with small engines, but I have rebuilt a 2bbl carb on an old car many times, so you could probably walk me through anything necessary.

Thank you in advance for your time.

Answer
you can try cleaning the carburetor,pull it off and take the float bowl off.Then take the float out as well as the one jet.make sure that the intake needle valve is opening smoothly and clean out the jet.A good practice is to remove any rubber that might be there and let everything else soak in a carb clean bath for a day or so.Hope this helps