After measuring the stock springs on my 43f (I believe they are stock but I really do not know for certain) I plugged the values into a online spring rate calculator.
It should be noted though, AFAIK the 43f came with progressive springs or, to be more specific; dual rate springs. They have 44 total coils, 50% of which are 4.15mm apart. My measurements are below.
Length: 462.5mm
OD: 34.75mm
ID: 26mm
Wire diameter: 4.15mm
Total coils: 44
Close coils: 23
Wide coils: 21
Close coil spacing: 4.15mm
Wide coil spacing: 9.48mm
As we are effectively dealing with 2 springs here we have to calculate the rates separately then calculate the resultant rates. Spring 1, the 'narrow' spring; the soft one.
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Loads & Rates
True Maximum Load, True Fmax : 655.802 N
Maximum Load Considering Solid Height, Solid Height Fmax : 407.051N
Spring constant (or Spring rate), k : 4.458N/mm
Safe Travel
True Maximum Travel, True Travelmax : 147.094 mm
Maximum Travel Considering Solid Height, Solid Height Travelmax : 91.300 mm
Physical Dimensions
Diameter of spring wire, d: 4.150 mm
Outer diameter of spring, Douter : 34.750 mm
Inner diameter of spring, Dinner : 26.450 mm
Mean diameter of spring, Dmean : 30.600 mm
Free length of spring, Lfree : 190.900 mm
Number of active coils, na : 23
Number of total coils, nT : 23
Solid height, Lsolid : 99.600 mm
Type of ends: open ends
Spring index, C : 7.373
Distance between coils, Coilpitch: 8.120 mm
Rise angle of coils: 4.83 
Then the hard one:
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Loads & Rates
True Maximum Load, True Fmax : 655.802 N
Maximum Load Considering Solid Height, Solid Height Fmax : 655.802N
Spring constant (or Spring rate), k : 4.883N/mm
Safe Travel
True Maximum Travel, True Travelmax : 134.303 mm
Maximum Travel Considering Solid Height, Solid Height Travelmax : 134.303 mm
Physical Dimensions
Diameter of spring wire, d: 4.150 mm
Outer diameter of spring, Douter : 34.750 mm
Inner diameter of spring, Dinner : 26.450 mm
Mean diameter of spring, Dmean : 30.600 mm
Free length of spring, Lfree : 271.600 mm
Number of active coils, na : 21
Number of total coils, nT : 21
Solid height, Lsolid : 91.300 mm
Type of ends: open ends
Spring index, C : 7.373
Distance between coils, Coilpitch: 12.736 mm
Rise angle of coils: 7.55 
Then calculate the initial rate which is:
rate=((rate1*rate2)/(rate1+rate2))
=((4.88*4.45)/(4.88+4.45))
=2.3N/mm
So before the 'close wound' coil has become coil bound, the front forks have a rate of 2.3N/mm. I think a kx80 has harder springs than that!
How much travel is subject to this ludicrously low spring rate? That one is a little tougher as it depends on how much pre-load you are running. If I recall correctly the metal pre-load tubes in the 43f fork are 110mm long, however, the springs do not extend to the top of the tubes. So F' knows exactly how much pre-load is on them but we can make some conservative guesses. The distance between the coils on the close wound spring is 4.15mm, the wider wound spring 9.48mm that is ~43%. For the close wound spring to become coil bound the distance between the wider wound coils must reduce by 43%. Thus, at 43% of the springs free length the close wound spring will be coil bound leaving the fork subject to the second rate exclusively which was shown above to be ~4.88N/mm.
The 43f has a 462.5mm spring. It is completely Coil bound at (91.3mm+99.6mm) 190.9mm. Actually it is more like 182mm, the above numbers were lightly fudged (engineering term) to get the calculator to treat all the coils as active coils. These are not open ended springs but closed and ground. So the spring has a potential travel of 279.5mm. The XT600 has fork travel of 254mm. This leaves 0-25.5mm for pre-load adjustments. Finally with the crossover at 43%, the fork becomes stiffer at 109mm compression (including your pre-load value). Assuming you have a ~30% sag, this means only the first ~33mm is subject to the lower rate).
PHEW! That was a lot and this does not even take into account the rising rate of the air chamber spring above the oil! In short, the front end is under sprung and too floppy, which we all knew anyway!
So what can we do?
Well my first suggestion would be chop the closely wound spring off the top, this would cause the initial rate to be 4.88N/mm, a far more reasonable value. Unfortunately this would cause issues with the fork becoming coil bound. If you had a spare pair of springs, you could chop the wide wound sections of both of them and make a new 462mm spring by stacking them with a washer but that requires you to acquire a second set of springs anyway!
I would wager that an xr250r (1997) with 41mm fork spring would work, they are allegedly 35mm OD and 536mm long. Asfar as I can tellthe stock spring was 0.39kg/mm. If you were to chop off 74mm so you could fit it in the tube you would end up with a nice linear rate ~16% stiffer at 0.45kg/mm which is more like what we need.
There are aftermarket springs available from Hagon, Hyperpro and Wilburs with the following rates:
Make/Model Part number Rate
Hyperpro
XT600E (90-94) SP-YA06-SSA009 6-11 N/mm
XT600 (86-90) SP-YA06-SSA027 3.3-4.4 N/mm
XT600(3AJ) 88> SP-YA06-SSA010 5-11 N/mm
Wilburs
XT600 (85-90) 600-0099-00 4.1-5.9N/mm
These are all I have found so far. The rates came from asking the companies listed what the spring rates were. Hagon never got back to me.
*all of this is on the same level as my usual kind of bodgery, while I believe it to be correct, do not blame me if you end up impaling a testicle while trying to chop an xr spring with an angle grinder only to find my calculations were bogus.
good buy (252 USD - Januar 2012)