OK I have 2 of them Im working on right now a 76 and a 77 and Ill try these suggestions THX
Doug

My 76 DT400 is doing the same thing, so did sealing up the intake fix your issue? Mine has a lot of cracks in it and was thinking of sealing the cracks with silicone...

Does sound lean [too clean] just off idle which might carry on higher up if it's acting up cruising on the needle. Maybe up the pilot jet which might richen it a bit in the progression period between idle & slide cutaway/needle jet but lift the needle a notch too [lower the clip]. Check float level isn't starving it--things like that.

New member today...wanted to share what I have found to greatly reduce the surging and bucking on the DT400 B & C. I completely rebuilt a barn find 76 DT400 not running for 25 years. Went through EVERYTHING! When I fired it up with 170 main jet, 50 pilot jet and needle clip on middle slot and mixture screw out 1 1\2 turns it bucked and surged on deceleration. I hadn't read any blogs yet but I wasn't happy. Felt like it might break the trans, or chain of flywheel key. So many hours and parts later I am pretty happy and enjoy the bike. I could share a ton of info on all I did to try and correct this but I'll cut to the chase. One suggestion on this site was to change the slide from the standard 3.0 to 2.5. This was my last resort but it kind made sense to cure the problem. I believe the bucking and surging is because of a lean condition. I bought a 2.5 slide from Summit Racing and installed it today. The slide made 1/4 to 3/4 throttle a little too rich so raised the clip one notch. Fixed that. Bucking and surging much less noticable. I ran the mixture screw in as far as I could and still have an idle. This richened the idle mixture and lessons the surging more. I also agree to keep idle as low as possible...1400 rpm is within spec and will keep running. I'm rambling on but sooo excited about how it runs. Ended at 150 main jet, 52.5 pilot, needle on 2nd notch from top, high flow and filter, b9es plug .022 gap, timing at 3.0 mm btdc. Hope this helps.....I am glad you all helped me!!

Oh yeah...forgot...the bike is on 3rd oversize Yamaha piston, all parts are Yamaha, high flow air filter is only aftermarket part. Located in Iowa for sea level comparison. Mixture screw is out only 1\2 turn from seated but runs smooth from idle to 6500 rpm (afraid to redline it...only 250 miles on complete rebuild!) Again rich idle lessons the bucking and surging. Use the needle to fix the midrange rich and keep the idle rich. I hate autocorrect ?. I read a 1975 review of the DT400 and even they commented on "lurching" so I think we're trying to fix a factory flaw!!!!!

Why do so many DT400 B and C bikes reportedly have the bucking and surging? Mine included. I built it back stock except 3rd oversize Yamaha piston. I read a 1975 cycle review of the DT400 and even they commented on "lurching". I think we're trying to fix a factory flaw!!!! I overcame about 95% of the bucking and surging today!!!! Ordered a 2.5 slide from Summit Racing and installed it today. Made the bucking and surging less but made midrange a little rich. Raised the clip one spot to lower the needle (leaner mixture 1\4 to 3\4 throttle). Fixed that. Next, turned the mixture screw in as far as I could and still maintain idle. Lessons bucking and surging even more! Finally keep idle as low as possible 1300 to 1400 rpms. Now I have eliminated 95% of the dreaded bucking and surging!!! Happy to supply more setup info too if interested!!

"Bucking and surging" at light throttle cruise has been a complaint about the big bore Enduros going back to at least the RT2. Some do it worse than others.

I agree with bigdog. When rebuilding my 76 DT400 the little hole was plugged with carbon. The more I dug at removing the carbon the more amazed I was at how plugged it was. I had to have the jug bored and the machine shop blasted all the carbon out. Also, verify your decomp valve is hooked up and adjusted correctly. Pull the rubber back from the valve and be sure it moves with the kick starter. The cable should tighten and pull on the valve as soon as you move the kick starter. Otherwise could be too slack of cable, worn cam on the kick starter, broken cable or broken end on the cable. I've had several DT400 enduros and never even the slightest kick back. It will "sneeze" through the hole we're talking about if it doesn't start.

I installed my TrailTech temp gauge today under spark plug (get the 14mm version) and took my DT400C out for a cruise. Bike has 600 miles on complete engine rebuild with fresh bore. Air temp was 85F and full sun. After 45 minutes of riding county roads I had along uphill ride at 55 MPH and saw temp get to 379F. after a little puttering around and some idling it went down to 330F and I was home. A great $45 investment for piece of mind to see what's going on with the engine.

Do you by chance have the link for this 2.5 slide?
It seems the original slide used is much longer than a typical slide.

thank you
Tom

Hey Tom....Yes the 2.5 slide I got at Summit Racing was short like yours. I had an extra 3.0 slide so I cut the top off of it and used it as a spacer to sit on to top of the 2.5. VERY IMPORTANT....you must make a spacer like I described! Do not run the 2.5 slide without it. The short slide will let the needle come up past the needle holder and hang the throttle at wide open. The spacer just needs to sit on top as you fit the cable and spring back onto the 2.5. It can't go anywhere once you slip it back into the carb. Make sure and measure the original 3.0 height so you cut the spacer to make the 2.5 identical. Cut it a little long if necessary and grind it a little to dial it in. You may have to raise the needle clip (lower the needle) now too as the bike will run richer. Good luck!