I had the same problem on my 1990 Evinrude 9.9 (two stroke), and can confirm the richness theory. I now painstakingly measure out 50:1 as per owner's manual and haven't had any starting problems.
Do you use one of those marked-out jars that is graduated for different ratios? The graduations are labeled according to the amount of gas you're adding; you fill to the appropriate graduation with oil, add some gas, swish around, and transfer to the tank. No more converting fiftieths of a gallon to ounces or whatever
