I have a 1976 black and white Yamaha DT400B which I have bought recently in the UK. It is a UK bike with matching engine and frame numbers starting with 513-.
A previous owner has spent a fortune restoring the bike and it is just about mint. It starts well and runs very well when under load. I haven't ridden 2-strokes much since a DT250MX in the late seventies and KH250 in the eighties but I can't remember them being as erratic as this DT400B. When I close the throttle for an approaching bend it is almost like the engine continues to detonate and is very jerky. When you increase the throttle even very slightly (under load) the ride is smooth again until you decelerate again. When ticking over the rev counter needle flicks fairly violently.

My Yamaha manual says the carb should be marked 500 61 but the one on the bike is marked 501 00. I don't know if that would cause the problem. I noticed that the manifold between the cylinder and carb was perished so I have coated it with a spray glue and flexible paint as a fairly good temporary fix but this has made no difference.
It's a shame because the bike is excellent and I've wanted one since 1976 but the jerkyness on the overun makes it very difficult to ride apart from when under load.

Any suggestions regarding a possible cause will be much appreciated. 

I'll start the ball rolling with my ideas be they good or not. Sounds like it's a notch rich on the needle if it smooths out with a whisker more throttle to make it pull a bit more-- 'till it catches up i guess'--, so try that, drop needle a notch--clip up!!. But-- firing on a closed throttle is sort of a different second issue & could smooth out a bit if the idle was leaned off as much as you dare. Could stop it completely by backing off the stop screw so the slide hits the bottom but of course it won't idle when stationary. Slide cutaway could change it too talking the first problem, like going from a 2.5 to a 3 for example. But there's a danger in leaning off those big motors too much as in when you back off from an extended full throttle run the like plenty of fuel on the run down to avoid seizing. So no winners really. If you're not thrashing the daylights out of it & just cruising the lanes & back country trails you should be right to lean it up a bit. My main experience with this type of problem is with the earlier 360's up to "73 & they were bad enough with firing on a closed throttle into a corner.

 Just another thought. Could strobe the ignition just to be sure its in the ball park & not over advanced or might even like being backed off a fraction on the backing plate. Whatever you do make sure that flywheel is Very Tight as destroy crank keys & the taper. All mine are damaged-- but not by me i might add lol. I worked for the local Yamy dealer & saw lots like that. Here's a pic of one of mine just as an example so hold it solidly for a positive feel.

I have no idea if our problems are related or not but what you describe sounds very much like the symptons with my freshly rebuilt 74 DT360. Easy to start, idles just fine sitting in the driveway, pulls great through all gears, but let off the throttle and it trys to buck me off. Cruseing is an on and off the throttle kind of thing. Very annoying on an otherwise wonderful bike.

 I owned and drove this same bike in my  younger days and then it sat for 30 some years until  I rebuilt the engine over the winter. I do not remember it having this problem back in the day and honestly don't think it did. I've been asking  questions and trying a couple small things but haven't come up with anything that helps much. Haven't given up though and  I'll be watching this thread very closely, hoping you find a solution which may or may not cross over to my 360. Fingers crossed.

Curse of the big-bore Yamaha Enduro.  Those symptoms have been reported from the early 360's through the 400's. 

Not all of them do it and some do it worse that others so there is hope.  I'm not sure anyone has ever figured it out for sure but several have improved it dramatically. 

Thanks for the info and suggestions. Just to re-cap. My 1976 DT400B starts and runs very well when engine is slightly under load all the way to full throttle. The problem is that when you close the throttle say for a bend or pull the clutch in the rev counter needle and revs bounce between 2,000 and 3,000 revs and then over the next 10 - 15 seconds gradually decrease to tick over fairly smoothly at 1,400. The effect is like you are blipping the throttle but the throttle is closed.
The cable moves freely and engine ticks over fairly smoothly once it's settled down.
According to my Yamaha manual the carb should be marked 500 61 but the carb fitted is 501 00, do members think this would cause the "hanging revs" problem. All of the jets / needle / slide cutaway etc are correct for my bike EXCEPT the pilot jet which a 50 and should apparently be a 40. Any thoughts on that ?
I will order a 40 pilot jet to find out.
To cause the "hanging revs" do members think the throttle slide will be hanging or could it be down / closed and the high revs caused by something else.

Any suggestions will be very much appreciated.  

Slow to return to idle isn’t from too large a pilot jet. I have a 52.5 in my 76 DT400C. Yours may actually indicate a lean condition. I would turn your air screw in and richen it up that way. Forget the 1 1/2 out and turn it in until it gets good and blubbery. That will help your issue. I think my air screw is only 1/2 turn out with the 52.5 pilot. A smaller pilot will make your issue worse.

Pedalcrazy, Thanks for the info, I'll try the air screw and keep you posted.

Hi I have exactly the same issues with my DT400b 1975, when I got the bike it was like it and basically it is not nice to ride. on throttle closing mine was hanging between 3 and 4k rpm, the only was to get it down was to pull the clutch and wait. when pulling its great but when it is not under load its bucking, kangarooing etc so you have to keep adding power.

Also I had starting issues around 10 kicks to get it going, then if I stalled it at a junction I had to get off and go to the side of the rd for 10 mins of kicking, with my 11 stone frame I run out of steam by the fourth kick.

Anyway I have not cured the problem completely but on the way to it.
I stripped and cleaned the carb, no change stripped again, moved the needle from position 4 to position 3, replaced the main jet from a 170 to a 1 60, replaced the choke plunger to a none bodged up plunger. set the float height from 17mm to 23mm
result slightly better.
I then decided to take the plung and fitted a rexes speed shop electronic ignition on the bike, the result is a start within 2 kicks cold, same hot but a lot of kick back when hot. bike runs great and the hanging has dropped about 50% from before but is still present, just not as bad. I have also now developed a slight rough run at 1/4 throttle gurgling of the motor. So next things to try are

pilot jet has a 50 or a 60 in it, cannot remember now, book says it should be a 40 so was thinking of changing this, I forgot what was stamped on the needle, I don't think it was what's in the book so was thinking of pulling carb, getting the correct needle and seeing if I can get the centre tube out to see if anything stamped on this in case mine has a dt250 carb on it.

how do you remove this bit?

GaryDean

You haven't cleaned the carb completely if you never removed the needle jet.  With main jet and washer removed, needle jet (aka main nozzle, emulsion tube) pushes out from bottom to top.

Hanging revs is a classic lean / air leak symptom so keep that in mind...  continuing to lean out carb could result in disaster...  especially main which has zero effect at closed or nearly closed throttle... a good full throttle run might put a sunroof in your piston.