One thing I do highly recommend is replacing the points dizzy with an electronic one. That was the first thing I changed when I bought my first zed! It helps with many issues. Check also for leaks around the manifold with some WD40 etc...while the engine is idling, spray some around the manifold/head and if you hear the engine speed up a tad, it means its sucking in some of that WD40 and burning it in the engine like a extra splash of fuel. That should tell you there is a leak.