I am thinking the exhaust is coked up. might try removing the silencer then start it up and let it warm up a bit then try revving it, if it revs more normally you had found your problem. i would burn out the silencer and remove the pipe from the bike and burn it out too.

if you baby two strokes too much it really cokes up pipes and silencers and it kills not only revs but power too.

keep in mind though a stock 75-76 DT400 makes it's best power at 5000 to 5500 rpm if you had not ridden one before.

one other thing if you are using the stock air box you have probably replaced the air filter and oiled it like a new dirt dike take the filter out and try it. the airbox and an oily filter are too restrictive and the other thing I noticed if you take out the tool tray and the cover that covers the top of the airbox you get some free throttle response and power also
Doug

Thanks for you message!
I never get any kick back! Very easy to start, few sec with choke on then the stable at 1500rpm.
The sparks is clear.
I tried without the muffler even without the exhaust but no change. To be sure, I’ve done a huge fire to clean it and have it repainted.
So as now, the bike runs, but the acceleration is not “linear” with misfire and detonation, and it’s hard to pass over 5000rpm.
Also I find the engine is heating much more than my other DT’s.

Any risk to take it for a “run” and see?

If you have a dial indicator, I would mark the flywheel and case at the timing point... should be 2.9mm BTDC on a 400...

Then use an automotive timing light to see where the marks you made at 2.9mm BTDC align with the engine running.... you might find the timing is retarded quite a bit.

CDI's do go bad too... Terry had one on a 175 that would start easily but just not rev at all. Sounded really rich and popped and sputtered. He sent it to me and I tested with a known good CDI box and coil. No change. His CDI box and coil ran fine with my stator. Stator resistance readings were all within spec, it just wouldn't run right.

Consider sending your ignition (stator, flywheel, coil and CDI) to DEET who can fully test and determine if your issue is ignition related or not. If it is, he may be able to repair. If it’s not, you have either a timing issue, a carb issue, or a plugged exhaust. That would be my plan of attack. Eliminate the ignition as an issue, then the rest should be fairly easy.

Don't suppose you have the white/red tracer & white/green tracer crossed over. Probably can't with original wiring double plug [if that's what they have].

Problem I learned from experience with CDI from a member here and then from helping Terry with his... ignition can "bench test" just fine. Good spark. But bike won't run right. I'm going to have to look up the videos from Terry's bike.

Which is one reason I like points... get them sparking and they run great... if not, you can replace the condenser. Electronics and trigger coils in the CDI are not as easy to figure out and no way to fix them... Bad CDI can create all sorts of weird issues with running but spark good and start easy.

Hi everyone,
Have time this Sunday to spend on the DT.
Not better when removed the air filter, and it’s not oiled.
So the bike is still struggling to reach 5/6k rpm and cant reach 7000.
One thing surprising me the most is the overheating of the engine and the exhaust really high compare to my dtmx 400?
Also one other strange thing, the enginr runs differently with lightings on.

Overheating like that is usually a timing issue, I'd say...

Here's the video of three CDI ignitions... All had good spark and engine started easily.

Terry's stator wouldn't rev over idle. My MX175 stator would rev up but has no power and doesn't run right. DT175 stator runs great on the same bike.

In this case, I swapped CDI boxes (all take the same black box) and ignition coils and it made no difference.

All would have passed a "bench test" as they produce good spark... one kick starts... just don't run right.

Unfortunately, even Yamaha CDI troubleshooting... past a few basic resistance tests... was "replace with known good".