Small Engine Troubleshooting Problems? Ask a Mechanic Now.
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It sounds like you have gotten some trash into the jet on the carburetor which is causing a partial blockage and you have to keep the choke on in order to get enough fuel through the carburetor for it to run properly. If you will remove the bowl from the bottom of the carburetor and clean it as well as the bolt/nut that holds the bowl on you should solve the running problem.
No, changing the oil isn't going to help your problem. The carburetor is on the left side of the engine if you are standing behind the machine, it has a heater box or cover over it. You may be able to get to the bowl by just going up under the cover if it is open on the bottom, if not you will have to remove the cover in order to get to it.