Needle and seat discovery.  This may apply to all old enduros.  I was having a problem with my DT400c  constantly leaking past the needle when riding and when stopped even though when on the bench it seemed to work OK. So from my jetski days I have this small pump that goes up to 6 lbs pressure for setting the pop off pressure on a jet ski diaphram style carb.  I turned the DT400 carb upside down on the bench with the float bowl removed and with the old needle and seat i could not get it to hold any pressure even for a short period of time. This is just using the weight of the brass arm in the float bowl to put pressure on the needle.  When I installed a new from Yamaha needle and seat  without doing anything else it would pump up to 3 lbs before it blew off and it would hold 1.5 lbs.  Now the bike runs as good as any of the 15, 400's I have done all with the same jetting. The bike has  good throttle response compared to before and best of all it doesn't drool if you stop and forget to shut the petcock off.  From this point onward I'm doing new needle/seats on all my rebuilds.  Just a tidbit from an old guy.  Doug
Doug

Great advice! 

Yamaha charges a lot for the needle/seat on some models (I believe the C is one of those).  Genuine Mikuni is available at considerable savings most of the time...  I think there is a thread where Yamahoper ID'd the DT400 needle and seat on the Niche cycle site.