Hi all, I’ve replaced the clutch cable and adjusted it per the manual (engine end and lever end).
However, there seems to be some clutch slip.
I’ve taken the clutch apart, hoping to find the springs/plates out of Specification. However, all is within tolerance.
The springs are 36mm (should be between 35.5 and 36.5)
The friction plates are 3mm (should be 2.7 min, 3.0 nominal).
What else can I do? I’ve bought ‘heavy duty’ springs ("15% stronger than original parts"). Will they help?
I’ve heard that ‘roughing up’ the friction plates and/or the steel plates will help? Is that right? If so, what ‘grade’ of grit paper for each?
The bike has done 5,000 miles and I’m using oil for wet clutches.
Many thanks….
 

Heavy duty springs might help but also might make your clutch hard to pull in. I have never had to use heavy duty springs in these Enduro's. Make sure your metal plates are flat, not warped at all. I think there is specs for this in the service manuals. Also be sure there clean. Even though your old friction plates measure good there old and can be hard and brittle. Best to replace all friction plates and clutch springs with OEM parts. Replace metal plates if not perfectly straight. Also make sure you have some end play in the clutch adjustment before doing your final cable adjustment at the perch lever. 

I am fortunate that the DT400 that I just got has a great clutch. Had you changed the discs since you have had it? If not, I would suggest that if all looks normal to check the steels for warpage. The old Yamahas had the rubbers between the discs and those are best to eliminate, but this gen do not have the rubbers.
Did it just start slipping? You may want to switch trans lube as some can cause slippage even if they advertise for wet clutches.

I had a Yamaha R5 that the clutch was acting up and I ended up changing the friction and steels. After that no more issues..

Hi - Thanks for all the info/tips. I've also just noticed that my replacement friction plates state 'heavy duty' on the box as well (EBC CK2240)...

Firstly, carefully inspect the plates for any sign of overheating (they have a strange smell if they have got hot).

If OK put them on a flat surface (a mirror will do) to check if they are warped. If not warped, then get the steel plates blast cleaned, with a coarse medium, lightly face the friction plates with 240 wet and dry on a flat surface, and then reassemble.

That might stop the clutch slip but its a fair amount of work, and cannot be relied on to work. Personally I would just fit new friction plates, and steels as well if they are distorted. If the clutch still has the anti rattle rubbers fitted and they have gone hard, that might cause clutch slip.

Thanks. There is no strange smell or smell of burning. I've checked the metal plates. They seem perfectly flat. They are now perfectly clean. I'll fit the new friction plates and new springs (Heavy duty! - I've already bought them) and see how I get on.
One last bit of help please... I've read that it is easy to overtighten the six pressure plate retaining screws. Its not entirely clear to me from the manual what the correct torque setting is. Can anyone confirm please?

Do NOT EVER blast clean the steel plates.  They will warp and be rendered scrap.

I tighten the pressure plate screws to goodntight ft/lbs by hand. Make sure the screwdriver fits well as they are JIS Phillips (Japanese Industrial Standard) which your US #2 Phillips may not fit perfectly. I'm assuming your 400 clutch is similar to the 175, but larger?

On your steels you can use a gray scotch brite and use regular oil for wet clutches not synthetic and give that a try. Just hand tighten your spring screws 

Blast cleaning plain steel clutch plates, helps to retain oil, and is something that has been carried out on competition bikes with high power outputs, but marginal clutches over many years.

Modern machines have dimpled steel plates, and the dimples fulfil exactly the same function as the blast cleaning on plain steel plates.