You really need the shroud, this will increase the cooling at idle and low speeds. Also make sure, the radiator fins are clear, not clogged with bugs. The reason it is running worse when hotter than normal, is probably the gas is boiling in the carburetor. Also check your initial ignition timing if the problem persists after installing the shroud, too much initial timing can make the engine run hotter. Post your results. Good Luck.