My 1976 DT400 is bogging down when you reach about 5k. When in neutral it will cleanly rev way past 6.5k. I decided to raise the clip on the needle one position and this did show some improvement so I raised it to the top position, this again has helped but not cured the issue.
Can anyone help me please. It has a new Mikuni Needle and Needle Jet (6F9), Main Jet 160 and Pilot Jet 55.
Mant thanks

I'm stuck in exhaust mode for anything related to lack of top end performance. I'm probably sounding like a cracked record that keeps going back over the same thing haha.

I seem to see more and more of these issues. Its always a 400, and its always about 5krpm. Ive posted the exact same thread multiple times. Unfortunately, I never fixed the issue. Still runs like crap. Out of curiosity, how is your gas consumption? Is it excessive?

I have actually got a carb from a DT250 which runs pretty much ok on this bike so that rules out exhaust etc issues.

Without knowing more, i only half agree with carb fixing it. I say that because i'm guessing a 250 has leaner settings & smaller main jet which will try to get the best out of a restricted exhaust. I mean--if its restricted bad enough you'll need to go down in jet sizes to get it to rev. Ok it might rev but have less power with it.. Sort of a similar situation to kids minibikes that have a washer in the exhaust header which limits the revs to 4 stroking rich to about half throttle. Can lean off the main & get more revs but not much more power, just more revs. Then the kid grows up so out with the washer & up with the main jet & Whoooha whataya done to my bike, its so fast lol. Ok long story & likely not relevant, but there ya go--& hope you've solved it. Did the needle & needle jet wear possibility ever get looked at in the 400 carb.

Hi there, sounds pretty much like the issue I had with my 400. My Ignition system (likely my pickup) was broken/worn out. I replaced it with a kit from Rex.

To test before replacing, I got myself a timing light and looked at the timing, it was okay at idle but didn't advance at all when reving.