i will side with ronmar on this one and part company with doug
1. the utterpower cd is of significant value to just about anyone on various levels and at various levels of experience
  think what you like of George but he has put alot of effort in collecting the info contained on that cd and for the
  money it is well worth the expense. unless you are very well experienced in all that is lister, changfa, and st head.
  Even then my bet is there is at least one thing on that cd that you will learn, and that one thing is worth the price of
  admission.
2. George's kit is fairly priced for what one has to do to produce and sell one, sure you can build one yourself if you are so
  inclined, if not then there is one available to purchase. i am unaware of any other dealers supplying this and other parts
  to the diy guy.
  i figure why not support those that actively try to support the cause?
  its ok to be a cheap, tightass old bastard,, we can all be that way, but why deny someone from trying to cover expenses?
  if George makes a buck it gets reinvested in development of other stuff folks need or want.
as for you genhead, you are certainly overloading it at 4200 watt on one leg, the harmonic winding probably cannot compensate for
this much load,, with your setup at 3600rpm i would be surprised if it will carry much more than 2500 watts per leg reliably, and that is at sea
level, derate 3% per thousand feet above sea level is a good rule of thumb.
so i guess i would start there, figure out how much below 2500 watts per leg you can realistically pull, and apply that load and see how it does
i would expect ~130 volts no load and as ronmar has said somthing around 110-115 at full load
if you tried to pull 4200 off one leg, then going down to ~100 volts on that leg would be expected and normal operation, albeit out of range
because of overloading,,,
i doubt you have any issues with the rectifier at this time, but they are noted as being somewhat weak so i would get a suitable replacement
either from George or wherever you like.
also derating the rpm of the engine will likely only make the problem worse, you will have less hp available and thus less to turn the genhead.
and yes you will need a meter to set the rpm to the proper hz, or a killawatt unit which will be useful for other stuff later.
alternatively you can plug in an electric clock and compare its timeing of the second hand to that of your wrist watch
at proper speed 60hz they will be in sync,, if the wrist watch is faster your genhead is rotating too slow and visa versa
you can time it over 5 minutes to increase your resolution if you like, in any event i would target as ronmar has stated ~61-62hz no load
and expect it to be ~59 hz at full load or somewhere close, the govenor quality will have a significant impact on how well it regulates the hz.
so for instance set it up to go to 62 seconds on the electric clock for 60 seconds on your wrist watch at no load.
good luck
bob g