Should be able to adjust the air screw which is under the carb to get rid of the stumble. Its a bit hard to get to. You need a short flat blade screwdriver. Warm it up and then screw in a bit then out and keep going a little bit each way and let it idle and then give it a rev. The idea is to get it running as smooth as possible and then screw it in just a tiny bit from when it seems to be the best smoothest idle. If it still bogs you might have to adjust the clip position on the needle. Moving the clip up will lean it moving it down will richen it. I think richer will sort outthe problem. Remember to make sure the aicleaner is clean, that ther is no air leaks the plug is good and the fuel is clean and no water in it.