Hi
Yes, too much prop. 18X8, is the recomended size. See this link:
http://www.dle-engines.com/dleg0031.html In the optional category, says you can use a 19X8, but what ever prop you use. I would try to keep the RPM between 7500 and 8000. 6500 is just to low, and your putting quite a load on the engine.
Vince