I personally doubt that it has to do with timing. I wonder if you have the linkage hooked up properly. Engine would not start and running smooth it the timing was off as far as what your description tells me. Take a look at this manual, sections on linkage and setting the governor. If this does not help then you can look into a timing problem.
http://th.manualwebsite.com/pdf/3-11hp_4-cycle_L_head.pdf
http://th.manualwebsite.com/pdf/3-11hp_4-cycle_L_head.pdf