Also, remember that the needle valve has to be adjusted every now and then. If it sputters on transition, it may be the low speed needle. You may have some old, gummed up castor on the needles themselves. A thorough cleaning of the carb and ports would be a good place to start as well. Even when flying every week, the high speed needle especially has to be tinkered with somewhat, due to temp/humidity and density altitude changes. If fuel tank placement wasn't a problem before, then it's likely not a problem now. A new plug would definitely help matters. Sometimes a new plug will solve testy transitions all together. Good luck...