Tractor Repair: Surging in an 8n, jet adjustment, volt system

Question
I have an 8n (47 model).  It has been updated to a 12 volt system but still has points and condensor.  I just took the head off and cleaned carbon from under the valves, and re-seated them.  I also put in new points, condensor, plugs and distributor cap.  I thouroughly cleaned the carb (about 10 times).  I checked the book and feel confident all is well.  I had the govenor rebuilt a couple of years ago (maybe 100 hours back).  This thing runs great up to half throttle; it pulls good and pulls a 6' disc in 3rd gear handily even at half throttle.  If I push the throttle up over that magic point, it surges terribly. If I pull out the choke to just the right place, it pulls fine again.  
Can you help me?
Frustrated in Arkansas,
Stephen Stilley


Answer
Stephen does the carbie have an adjustable main jet if the answer is yes turn it anti clockwise about half to a full turn.
If this helps the problem eg runs better may be it will need a little more or a little less just try it and see.
The main jet adjustment is a knurrled knob usually in the base of the carbie.
Hope this all helps with your problem Stephen if it fixes it drop me a line and let me know.


       Regards Danny Last