TE/TC 4CS FORK STICTION

I've been noticing recently my 2016 TE300 forks are suffering from really bad stiction. They compress a bit when putting the bike's sag weight on the forks, but when sitting down on the bike, they don't move at all. Also when riding on the tarmac, they are locked solid, and don't move at all unless hitting a pot hole.

This all became extremely apparrent when trying out a mates 2017 FE501 yesterday, the forks were super smooth with very little stiction. Even though this has Explor forks, the base parts are exactly the same as 4CS, ie top tubes, seals, bushes etc.

So first thing was to check fork alignment and installation, then checked to make sure nothing was bent, then removed the forks.
One side had huge amounts of preload on it, needing about 5KG more 'push' to get the fork moving, and both forks were very noisy and graunchy from 1/3 travel onwards.

I then removed the springs, and found both damper rods to be under 'spring loaded' extension, one side a lot more than the other, (which explains the preload). I pushed in the damper rods, and they got stiffer, and started noisily graunching and sticking towards full compression. The rods returned to fully extended under 'spring' return.

I then dropped the fork oil from the bottom chamber and put the base cap back on, now the damper rods moved easily with no 'spring' compresion, and sat fully compressed under gravity, all graunching and sticking now gone.

Next I reassembled the forks with no oil in, now they moved smoothly with no horrible noises.

I'm guessing there is air in the bottom chambers, which should/shouldn't be there, and an uneven amount per leg giving odd preload. As far as I know, all the air should be in the top chamber, and none at all in the bottom chambers?

The forks have had oil changes, one leg has new top and bottom tubes and seals (accident damaged) The dampers have never been apart or revalved, bushes are OEM, and I'm using Putoline 4wt oil.

Anyone shed any light on this please?

sounds like air in the bottom section which they shouldn't have as the rebound rod has a slot machined in it that at full extension bleeds air past the cartridge seal into the top section.
I hope you replaced the little purple cartridge seal whilst you had them apart, they fail very easy especially of you don't wrap the rod thread with teflon tape prior to removing it.

Are you refering to an internal seal inside the damper? As I have not stripped the damper down, just removed it. The seal on the top chamber seperator and the 2 on the base valve are fine.

yeah the little purple seal than runs on the 8mm rebound rod. This fails very easy and should be replaced often.

Problem solved!
Putolene HPX R SAE4 oil is a load of shite in 4CS forks.
Just spent £91.08 on 5L of the proper WP oil, and forks are back to normal. Also serviced my mates XC300 frorks, and they all now feel the same and good.
Seems the Putolene has little lubricity, so causing sticking of the bushes, and juddering of the damper shims and oil reservoir pistons.
Surprised no one has had this before, as Putolene seems to be the easiest after market SAE4 oil out there. Shame Motorex don't do SAE4, as I've used Motorex for years on my other bikes with no problems.