Can I use 2.5 wt oil?

You can use 2.5 weight , no problem. However, Kayaba 01 fluid is lighter than 5 wt. I do not think you will be able to tell the difference between the stock fluid and an aftermarket 2.5 wt, I can't anyway. They will perform close to the same. The fork springs will make the biggest difference. You are on the right track there.

Servus dirtbikedad,

There are several good threads about oil viscosity on this forum (search for ATF) and the gerneral consensus is the following: oil viscosity mostly influences the (ultra) low speed range of the damping, i.e. the range where the clickers work. This is not the range where your harshness occurs; at least not from the damping side! IMHO many people make the mistake to run light weight oil or open the clickers fully to archive a plush ride, but this is a fake. All you archive is that the fork runs through its stroke without much damping. This might feel plush at slow speeds, but as soon as the pace picks up the effective damping is reduced to a fraction of your travel thus causing it to feel harsh or even bottom.

For the Kayaba 01 oil, I seem to recall that is a multi-viscosity oil meant to work at a broad range of temperatures, which it does not! Anyway I don't know for sure, but I would recommend a brand name 5wt oil. On my 98 WR400 changing the oil to Öhlins 5wt already helped a lot.

Same for the springs. I ended up using 0.44 in my WR, but 0.42 seems too soft for your weight and this bike.

This is just a personal opinion, but I consider it the wrong way going softer. Soft is not plush - you want a controlled ride. I would change the oil in the forks to a 5wt and keep the stock springs. Set your sag and go back to the stock clicker settings; try not going softer, but stiffer till you really get a harsh ride. This will help you telling harsh damping from harshness caused by too little damping. If you want to ride and not wrench on your bike, take the easy way and get a revalve - sometimes this even works :p

Michael

One thing that might help is to reduce the oil level in the forks, for some reason unknown to me, yamahas seem to react to changes in fork level dramaticly, and become harsher as the fork oil level go up. Just a thought.

captain hookleg its funny you should mention harshness from oil heights on YZ's.A friend had a 01 YZ 125 and if we run the oil height higher than 115 i seem to remember it was harsh yet on the 01 Cr i have my oil height way up at 90 and no problems.I know the fork is different(bumper vs bladder and cartridge sizes )but even so the YZ forks seem easy to upset.

Dirtbikedad,
drehwurm has ya headed in the right direction. I would keep the stk. fork springs and add about 5 mm preload to them.( If softer .44 at the most.) I believe they are soft enough that you're feeling the harshness as they ride down in the stroke.Where are your forks in the clamps? Add a little preload, set your oil height 95-100 (5-7 wt.) Put your forks 10-12 mm up in the clamps. Run 20-25 mm free sag on back. Mess with your clickers a little and see how it works.


doc