Ok on the latch.. I tested this 50 pilot and the bike runs a lot better at the low throttle position. I also have the idle set at 1100 rpm (the electronic ignition helps there), seems to help with the 4 stroking when letting off the throttle.

I can actually apply 1/4 throttle and get to 35mph and hold it there. No hiccuping.

Ed

I'll have to give the 50 pilot a try, also lower the idle speed to 1100.

I tried a lot of things with carb to reduce the chug chug. In the end I put a #55 pilot jet (#50 is stock) with a #190 main jet and maybe I lifted needle up a slot.  Runs amazing, meaning powerful and the chugging is very tolerable, even on a long curve 

I've saved a few things I've read here from you guys in a word document so when the time comes for me to really deal with the surge I will have a few things to try. I'm always interested in hearing more so if anyone has other ideas please speak up, that surge is so annoying!

Hmmm, My DT400 had a 40 pilot and the Yamaha manual states a 40 pilot as stock. I know there are discrepancies in some of the literature. 
Mine would break up like crazy on any main jet above 150.

I forgot to say the bike I am referencing is my ‘74 dt360a, which was had the  worse chugging symptoms. ‘78 400 was way smoother. I should have wrote it down but I think the stock pilot jet was #80 maybe because much larger carb?

 

Try closing the Air screw to say just a half turn out, ride it and see ,,then post back result

That was one of the first things I tried, didn't help but thank you for the suggestion!

Hey guys, decided after the gym this morning I needed to take the 400 out again to see if I wasn't just being paranoid, below is a video I took after doing some laps around the hood.  I think this is excessive rattling but then maybe I'm just being paranoid, my DT2 is whisper quiet compared to the DT400...  The second video sounds even louder even though I took them seconds apart, sorry about the sideways video...

DT400 engine rattle

Rattle #2
www.youtube.com/shorts/XMWUmNnW4Ck
 

Ok"   sounds a bit rattly ,  i like the way your revs drop,[ no hanging]  it will run a long time like this, ,,just ride it ,