OK, so I got everything out and measured the oil.
Forks extended 17 3/4 to the oil.
Compressed 9 3/4 to the oil. Per instructions from Sonic Springs (150mm = 5.9 inches compressed) that's where I'm getting that compressed number.
I'm confused, I would've thought not having enough oil would've made it a softer ride???
ok, I'm only going thru this one more time.
you have a pre '94 fork, the valving components differ from post '94, and this is why there are 2 mothods based in difference of the valving components.
I feel the info you have been given is incorrect for YOUR fork. Sonic sells very few kits to pre 94, and when they do, I think they are giving out false and incorrect info...
ok.?
level in a post '94, taken with spring removed, and compressed fully, is 171 mm per oem spec, or close to 6-3/4"
you see what I'm getting at?
?
so forget the compressed fork readings, and stick with the oem method, you only replaced springs.... not the internal valve components the take care of damping and rebound rates...
now, as to "why" its stiffer, unless you fully understand the damper, and the effects on hydraulics, it makes no sense to you, but I'll say that in this case, there is air being trapped in the cartridge, and it is cavitating, and "hammering" the internal valving, the correct oil level keeps the complete valve section full, and will work smoothly and consistantly, add an air pocket within the same valve, and it will not work the same, once the air passes the control orifice and the oil hits it, it will slam. Thus the harsh feel.
Air in a closed valving system does not like to be compressed, ain't the same as the air just sitting in the void above the oil in a fork tube... it all has to do with the valving...the mixture also has a tendancy to encapsulate air into the oil, because it is being forced to mix in the valve, this will cause foaming and aerated oil, which is even more inconsistant to control flow rate.
so again, extend the forks, and make the level correct for the valve to work.
play with the oil viscosity all you want, this changes rebound rate and smoothness of function, but just use the correct oil level.....
I will also say this, Sonic does NOT diferentiate or sell springs on their site based on specifics of model year, for the Concours....
their instructions may say to make the spacer a different length tho, it sits on top of the spring...
all of this has absolutly no bearing on the required oil level completely functioning the valving cartridge of the fork.
I'm done with this now, best of luck, I tried to give you intelligent and concise answers to the problem.
I can not explain this more clearly than I have already.