Hi Guys....
A cat amongst the pigeons for you all... Here is what I found bringing a 1000rpm engine to stability at 400/500/650/750 RPM....
The LENGTH and TENSION of the spring makes a very big difference to the response of the engine - too long soft and it will hunt badly.... Find a spring that almost does the job(Preferably 2 so you have a spare when you stuff the first one up), start the engine with the spring in system and shorten it, one or 2 coils at a time, till the response improves to your satisfaction... There should always be enough tension in the spring to fully open the rack when the engine is at rest, stopped. The spring should not be more than about 1 3/4 to 2 x its rest length when fully extended, ie at full governor closed position... If you find that you cannot get full rack opening, but the machine holds stable under low load, increase the length of the spring to one that has a greater number of coils.... Its a finicky dance, but one that can be done without the changing of governor weights...
Remember, the governor weights do not exert a linear increase in tension on the spring over the entire rev range too if they are allowed to travel too far... they exert a very low pull when the engine is running slowly, then go into a semi linear range as the arm weights are parallel/90 degrees with the cam shaft, then as they extend further, they can eventually go almost "over center" on some governors and go into a "non-linear" force mode for want of a better description... The trick is also to use the "near linear" areas of the governor travel to regulate the engine speed - If you find that hunting is a major problem, possibly shortening the linkage attached to the governor arm a bit will improve things as well... I have also found that when shortening this link, maximum power output, while affected, was not too bad - It was still "rolling coal" and had plenty of juice going to the pots to keep things stoked!!
Hope this helps...
Regds
Ed