>>New Info<<
I was at MMP in Tooele, UT doing an Advanced Rider Training class put on by Utah Sportbike Association (great class by the way) the past weekend. I spent about 30 minutes Saturday evening talking to Lenny Albin, owner of Nitron USA Suspension (he worked 10 yrs at RaceTech, then 5 yrs at Ohlins) and designed the RaceTech ST1300 damping valves).
His advice - if you run straight rate springs, the OEM oil level of 62 mm is way too high. His recommendation was 130-135 mm (set with no springs in, fork tubes and damper rods fully collapsed). Based on this, the first thing to try would be check and properly set the oil level (actually the air space) to 130-135 mm and see where that gets you.
He did not mention specifically about OEM or aftermarket valving - so maybe keep 10 wt since the OEM damping is considered too low to begin with, going to 5 wt would make it even more underdamped - just seems logical). 5 wt is the correct viscosity for using the RaceTech valving.
He also said that on the ST1300 for typical combination use, he would target a front sag (measured by the RaceTech method - see their website) in the range 40-44 mm, thinking 35 mm range is too stiff for typical road AND track conditions.
This guy knows his stuff - he's been doing MC suspension engineering for 20+ yrs (He's also a very nice guy - he was swamped with work, and still took 30 minutes to talk to me about the ST suspension). I'd pretty much use his guidance as a real good starting point, and it will probably be plenty fine right there.
He also mentioned to consider the Nitron rear shock options (5 different versions available). Very high quality, made in UK, and his pricing runs a bit less than the competing aftermarket units for the ST. I'll be talking with him about that this winter.
My 2? - YMMV of course.
Randy
< end New Info >
There seem to be many unknowns concerning your current set-up. I'm 185# and the racetech chart calls for 0.95 kgf/mm springs (what I ordered).
The OEM springs are progressive wound, so that 0.86 value has little direct significance for comparison. At your street clothes weight, I doubt you will want to be under 0.90 kg/mm single rate springs. The OEM springs take up more volume than the RT springs in the fork. If oil was put in by volume (assuming ALL the old was removed), the level will be a bit below the OEM spec. Checking/adjusting oil level height is what you need, since you are actually setting air volume in the forks with this adjustment. Also note - if you are using PVC pipe/tube for spacers instead of the thin aluminum ones, that will use up more air space. Using the same oil level with heavier plastic tubes will have the effect of running higher oil level (less "oil level dimension" which is actually an assessment of air space).
The OEM SS-8 oil is estimated at 10 wt nominal. By most accounts I have read, the ST front is underdamped with the OEM valving and SS-8 oil. Going to 5 wt would make that worse. I think Racetech recommends heavier oil with their springs and OEM damping valves to Add damping, and I believe a larger oil level dimension (lower physical level, more air space, that would yield a more uniform spring rate over a larger range of fork travel).
Racetech recommends their 5 wt oil WHEN USING THEIR VALVING SYSTEM, which is way different than the OEM.
Knowing the rider sag would also be helpful to help assess. The oil level, if very high, could actually affect the sag some, and create ride harshness (like too high spring rate) at even moderate fork compression. The higher the oil, the more progressive the effective spring rate (spring force + air pressure force) becomes. If your sag with those .90 springs, preload set in the 15 mm range, not excessive stiction (worn bushings or bent tubes), and sag is less than 36-38 range, my guess would be fork oil level is too high. That is only a guess based on limited info.
What I would do:
Check your front and rear sag using the RaceTech method (available on their website, takes 3 people to perform) before doing anything else.
Use the rear preload adjuster to set the rear sag at about 34 mm at the axle. Note: Preload adjusters tend to leak oil over time, and reduce their adjusting range substantially. My '08 ST with 21k miles on it when I got it last year only had about ? of its normal adjusting range when I got it. The previous owner was 280, had never moved the rear adjuster from its original "Standard "setting, and was running with zero added preload to the rear. He was probably 80+% of the way to bottoming just sitting on the bike. He had never adjusted the rebound damping, and it was to soft for me at 90# less than him. I guess a good thing he did almost exclusively interstate highway riding (smooth roads and not much cornering).
It is fairly simple to bleed and refill the adjuster if needed. See other posts about doing that if necessary.
See if your front compression sag and rebound sag are more than ~15-20 mm different (excessive stiction - bad oil, worn bushings, scuffing on outside of fork tube, bent fork tube, misaligned fork tube, etc.), and what the average sag value is. The target is about 36 mm. OEM is significantly more - in the 40s for typical American riders 180+ in weight.
Do the checks multiple times to make sure you have good sag numbers. Write them all down ( rebound sag, compression sag, and average sag for both front and rear).
If the front sag is way wrong, and the oil level is found to be correct, consider changing your preload (new spacer tubes, adding spacer washers - can be bought from Racetech, or cutting spacers (if sag is not enough) to get you at least closer to the right range. Note: Changing preload length does not have a one-to-one relationship to sag, since there is progressive air spring effect in addition to the linear physical spring response. As you add preload, the progressive air spring effect is lessened, since you have more weight on the forks before you ever start compressing the trapped air. The lower the oil is in the fork tubes (more air space, larger "oil level" dimension), the progressive air spring effect is also lessened. The standard starting preload recommended by Racetech is 15 mm (about 5/8") top of fork tube to the underside of the cap outside of the threads (where cap contacts top of fork tube). If your sag is off, and your preload is not around that 15 mm amount, that might be a good starting point to shoot for.
>>>> Before doing anything else, try and do a critical ride assessment on differing surfaces and different speeds, threshold braking, various levels of cornering lean, etc. Make notes about your impressions of handling in all those different cases. You will use this as a point of comparison for your testing after you rework the front end.
Pull the front wheel and pull the forks per service manual procedure. Remove the caps from the tubes and damper rods. Rather than dump spacers, springs, and oil into a tray per service manual...
Remove the spacers. Try and let oil on the spacers (if any) drain back into the fork tubes.
You can probably fish the springs out with a piece of wire with a hook on the end. Pull the springs up above the oil level and let as much oil as possible drain back into the fork tubes (so you can assess the oil level you are starting with).
If they are racetech springs, they will be marked with the rate near one end. Confirm they are 0.90 rate springs as you think they are.
Push the fork tubes into the sliders until they bottom. With fork held vertically, check your oil levels. You can make a "dipstick" from a piece of wire. Mark with masking tape about 150-160 mm from the end. Put wire down fork tube until tape edge is even with top of tube. Withdraw and measure the distance of dry wire between tape and where the wire has oil on it. That is your present oil level. If you were able to not drag out too much oil on the springs, the levels should be with a mm or two of the same in both forks. Record this number and compare to OEM and or Racetech recommended level.
If the oil level is much too high (see Racetech recommendation for those springs and your weight) (or too low), at minimum dump out the old oil and replace with new. I would start with the weight and level recommended by Racetech for OEM valving and their springs. Since you are on a budget, the Honda Pro oil, or another recognized brand, should be fine. RaceTech makes a supposedly super slick, full synthetic oil that is supposed to maintain viscosity longer than conventional oils, and provide significantly lower fork friction and stiction. It is, however, $60 for 2 quarts needed for the ST forks. Maybe overkill for typical street rider...
Depending on your mileage, it would probably be a good idea to disassemble the forks and thoroughly clean the all parts per service manual prior to re-assembly. If budget allows, and the bushings have some miles on them or show wear through the teflon coating, all four are less than $40 (from RockMountainATV.com in UT), put in new bushings. I would always suggest new fork seals - ~$20 for pair. Replace dust seals (if they are dried, worn, or cracked especially). Manual says replace the copper seal washers that hold the cartridge to the bottom of the slider (assuming you took them out). Other say reuse once is generally fine, though.
Reassemble using the PROPER assembly and re-tightening order on all aspects of fork clamps and front wheel and axle reinstall. Failure to do so can cause binding in the forks leading to harsh ride characteristic, and front wheel alignment bad due to improper wheel axle mounting and tightening can lead to a number of handling problems.
Repeat your critical ride assessment (as done before starting) on differing surfaces and different speeds, threshold braking, various levels of cornering lean, etc. Make notes about your impressions of handling in all those different cases. You will use this as a point of comparison for your testing after you rework the front end.
There are a lot of subtleties in suspension assessment, and what your suspension use and feel goals are.
Hope that helps. The guys at RaceTech were very good about helping me choose components for my particular circumstance and suspension performance goals. I'm sure they'll take a few minutes to talk to you about it.
Good luck in getting to a good solution for you.