Off of my 75 DT400. Running a B9ES NGK with UNI foam wrapped over original filter cage. 165 main 40 pilot 6F9 needle with clip on 4 down from top. This thing starts and idles but breaks up all the way to full throttle. Since I have a foam filter, I tried a 180 main. Not good. Tried moving the clip down another notch and runs worse so moved it back to the 4th position and 165 main. Still breaking up but not as bad. Here's the plug at 3/4 throttle and killing the engine.  

OK so can't add jpeg at this time. 

Uni filter is known to be more restrictive than stock. Some have modified it to be single layer like stock. Or get a stock foam filter element from Yamaha (many are still available)

Also sensitive to oil used... most thick sticky foam filter oils make these old bikes run crappy. I like Twin Air "Liquid Power" foam filter oil... have used it since the 1970's.

I suck at oiling filters with normal oil... I usually way over-oil. Twin Air oil is super easy to get right.

Old racer filter oiling trick was to drench foam filter with old premix fuel. When gas evaporated, enough oil was left in foam. I don't recommend this... just mentioning it to illustrate that not too much oil is needed.

(Twin Air filter oil is very thin out of the can and uses the "premix gas" principle... you soak foam and squeeze out excess. Overnight the solvents evaporate leaving the foam perfectly oiled with the special Twin Air oil)

Be good when video audio can be used, but [in my opinion] if its breaking up from the word Go all the way to full revs i couldn't see the filter doing that. If filter was So restrictive that it affected it at first touch of the throttle then it'd be maxing out at about 1/4 throttle, maybe exaggerating a bit--but.
Also--i'm stuck on exhaust restriction--but once again if so bad that its affected at say 1/4 throttle then it would barely get much higher, so doubt its that either. I'm no use sorry.

I cant see DT400 in partzilla in "75 but just thinking 'if it runs' the same carb as shown on DT360A then does it have the small Oring on the jet setter that seals the bowl hence the main jet fitting. I would thing they went away from that in "75.

RT325-Yeah, partzilla does not show 75 but 76 is the same. The main is screwed into the emulsion tube (newer style).

Mark- I used the bulk uni green foam about 1/2" thick and wrapped the "cage" that was there. This obviously is more restrictive like you're saying. I figured the opposite but what do I know.
I may just get the Yamaha one and not have the guess work.

Thanks

Because there are no pictures currently. My first stop was a 165 main jet on a 400 CC BIKE. Without knowing the altitude I would be running a larger main jet especially if you’re running pre mix and oil injection. 200 is not too big. I also run Yellow UNI air filters and I grease to top and bottom with red grease and use light WD - 40 perhaps not the best but light oil capturing small particles.

i haven’t looked on Partzilla what jets are suggested but a HODAKA 100 or 125 might run a 165 main..

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DT400 stock is 160 main and 40 pilot. The filter is UNI foam over the OG cage. I had 180 and ran worse, then moved the clip to the bottom (full rich) and was really terrible. Went to the 165 with stock clip setting at 4th down and still not good but better than the larger main. I may have oiled the filter media too thick. used gas mixed with 2 stroke oil and kind of brushed on then let it drain for a day. I can tell you it's not saturated though.

I'm not at my pc, the electrode on the plug is black and the porcelain is looking normal to me.

Also right now just fuel in the tank and using injector only.

How does it run with the needle clip in the 3rd position?

Try fitting a smaller main jet and putting the needle in the middle groove. Its very easy to tell if the main jet is too big, by switching the fuel off, riding bike on the main jet, and checking if it runs noticeably better, just before the fuel runs out.