Hi all - does anyone know what could cause my clutch to be heavy/stiff? I've fitted a new cable because I thought it could be that, but it's just the same. Or maybe they are all like this? I know heavy and stiff are relative terms, but it just feels heavier and stiffer than I feel it should be...Thanks....

Your description of being heavy or stiff is hard to assess, depending on what you are comparing it to. My DT400B has the same clutch feel as all of my other DTs from 1975. However, if I were to compare it to my 2017 Yamaha SCR950 it is a day and night difference. The newer bike is much smoother and the pull is consistent throughout.

Sorry I can't help troubleshoot your concern. But I bet you will get some good suggestions soon. Just wanted to give you my input on the 400 being no worse than my others.

Thanks Schu - yes, it's not measurable, and yes, maybe I'm just too used to modern bikes (which I've given up on as they are just too fast). I'm looking forward to going out to bike meets again where I'll hopefully bump into an owner of a similar bike to compare...
 

Hi Jammyyammy, I'm just thinking through the components that could cause your clutch lever to 'feel' heavy. Perhaps the obvious one is have you or a PO fitted heavy duty clutch springs? Do you know the condition of the clutch actuator arm bearing? I'm ignoring the cable as a problem since you say you've already tried two out, and hopefully routed them correctly with no kinks. Does the handlebar lever operate smoothly without the cable in situ, as it is possible to overtighten the lever pivot nut and cause the clutch handlebar assembly to nip the lever.
Lastly could you have possibly wound the clutch actuator arm adjustment screw in too far? If so this would cause the adjustment screw end pawl to rub excessively in it's locating slot hard up against the clutch actuator arms internal shaft. If so hold the adjustment screw steady with a screwdriver, undo the 17mm locking nut and back the adjustment screw off by 1.5 turns anti-clockwise. You can now wind the adjustment screw back in clockwise but only screw it back in up to a MAXIMUM of ONE turn (One turn gives the full range of available adjustment) that suits your clutch setting, and retighten the locking nut. Remember to hold that adjustment screw still while you are retightening the locking nut.
Hope this helps.  Andy.

Hi Andy L - thanks for taking the time to post. The info is really useful. I'm trying to contact the previous owner of the bike in the states. It's been a while and there has been no response. By the state if the forks/bars/front wheel/tank etc (which the importer failed to notice/tell me about - that's another story), he may never respond... But, if he does, I'll ask him about any clutch work/upgrades.
The lever on its own is free. The cable on its own is free. Will have to check out all the other good stuff when time allows. Again, many thanks for posting the info.

Loosen of the adjustment screw to the side of the engine casing, I'm just in the process of building my engine and fitted it last week. When in too far it does bind. 

www.partzilla.com/catalog/yamaha/motorcycle/1975/mx400b/clutch

Been a long time but looking at the pic, if you let the cable fully loose & adjust the arm 'down' as far as the loose cable will allow, then the arm will move back & pull on the rod with better leverage--to a point. & also everything "ANDY L" said above about not screwing in too deep but you still need half a turn of useable adjustment to lift or lower the arm but if you lift it right up then keep turning clockwise & it goes tight then you've gone too far, so back to the high point & back down anti clock & 'that' half turn up & down is where your at for max depth of the guiding pin on the adjuster. Bit of a weird design but works good if done right. That very fat pushrod has to be fitted before the motor is assembled too or--as my workmate found out at work--it has to come apart again as can't go in as normal from the right side. I know my link is for the MX400 but think they're all the same design. Unlikely to have had heavier springs fitted but could always dismantle to check but not sure if they're color coded for tension if aftermarket springs. New 'lubed' cable & run in as free flowing route as you can up to the lever & adustmentset right. Can't do more than that.