Weakness at high RPM and overheating could be a lean or weak air/fuel mixture. Check fuel delivery and main jet circuit.

Engine running different with lights on (Except at idle) could be ignition problems.

A stock 75 DT400 even when running good is not going to give very good performance above 5500 RPM. Look at this dyno chart with a brand new machine and see how rapidly torque and HP drop above 5500 RPM:

Sorry if this was already answered, but do you have a stock carburetor and what jets are in it?
Here is a table with DT400B/C carb jets and settings:

Thanks for the video
Mine seems to runs as the spare 175 MX, the 2nd bike on the video.

Tomorrow I’ll try to set the timing an other way, check annother clip position.

Could it be the ignition coil?

Carb is 160, 40, 25, clip 4th position
Many thank to all of you!

I set the timing a bit less advanced and it’s a bit more regular when accelerating.
I went for a quick run and the strange thing is that on each gear, when reaching 5000 rpm, the powe totally disappear like engine running out of gaz!

As I can’t really check the ignition, I want to be sure my carb is ok.
What should I check first? Floater level? Needle clip?
Thank you

Here is a picture of the spark plug

Here is the carburator

What are the ID numbers on your needle ?
You also listed some jet sizes . One of which was 25 .
What exactly was that , as it doesn't tie in with any of the specs for your model .

Is there a thick brass washer under the main jet? Without it the needle jet can slide up into the venturi and wreak havoc on tuning.

Good catch MarkT!

Part number 6 here : www.partzilla.com/catalog/yamaha/motorcy...0c/carburetor-400b-c

Here is a picture of my DT400 carb with ID # 50061:


I still think it might be lean at full throttle, try the washer and a 170 main jet. One at a time!

Yes main is 160, pilot 40 and the jet under the floating arms is 2,5
Set the clip on the middle position 3rd