Hi everyone! I have been chasing this miss at or around 5000 and up on my 1976 DT400. The bike came to me with a 40 pilot and 160 main jet. the bike had a very interesting sound when holding the throttle at about 3000 rpms, loud, metal to metal sound. I sent the video to some folks in the know and got back various responses with the most experienced replied thinking a ring might be catching in a port. So I dove right in and took the head and jug off, sent it out to be assessed, the cylinder guys said there was a spot that bothered them and it should be corrected, they bored it 1.5mm over and I then put it back together. I cannot say if it ever was revving much above 5000 when I got it home because I was afraid the thing was going to destroy itself if it kept riding it. As an aside I’ve had lots of motorcycles two stroke and four stroke but never one that just fell off like this. After reassembling the bike was when I found and began working on this lack over rev issue. Read forums, figured too lean so I put a 180 main in it to check if it was lacking WOT fuel. before I go any farther I have more info. the carb is clean, yes, the exhaust isn’t plugged. New air filter and the rubber connector to the carb is clean and not obstructed. New intake, the company used mine to make a mold and made one for me. it has a new BR9ES, (bought three of them) I adjusted and double checked the output of the oil pump at idle and at WOT. I oiled all moving parts when I assembled the top end. Bought the bike from an elevation of 3400 ft and am at 900 here. it idles like a dream, it runs perfect up to the 5000 ish range. When I leaned it back down from 180 to 160, then 150 it ran a bit better but with all of the posts on here basically saying that would be way too lean I just can’t believe this is the only 400 that needs to be leaned out even more to make it run right. And of course there’s the sunroof piston gig I don’t want to breach. At the moment I have a 50 pilot with the 150 main. Clip second from top, and as I said it runs excellent up to 5000 then it falls off, either it misses hard when it’s rich, misses less when leaned out but also completely looses power. I’ve ridden lots of MX 250 two strokers in my past and currently have a 2002 CR250 so I know how these two strokes should run. And I also had a WR500 back in the day so I have ridden big bore two strokes. I have never run into this situation. I considered that it may be the  30 year difference in bikes obviously and watched YouTube videos of these bikes pulling strong all the way up to redline so I’m sure something is wrong. I also have a vape 12V system to install but wanted to basically get it running right prior to installing that. So if it happens to be an ignition problem I don’t really want to keep buying parts to “get it running right” before I install the vape. And I’m not cheap, I put a battery in it, painted the whole bike, new seat, fork seals, gauge faces, tires, tubes, vape, redone the top end, etc etc. so I’m not afraid of spending money but I’m at a loss as to what to do at this point. What do you think? 

BTW, the ring catching sound was actually too lean at the pilot, it sounded terrible! That is how I arrived at the 50 pilot in the end. 

Could be ignition, BR9ES is not the correct plug to use with the original resistor cap, you can lose significant spark energy.  Make sure plug gap is 0.020" to 0.024" max.  If it's missing at higher RPM, I'd go closer to 0.020".

If you are comparing to a 2002 CR, you will never be happy.  Specifically, that era DT400 does not like to rev and does not have much power...  peak is in the low 20's.  Stock, the HP drops like a rock at about 5500 rpm...  and keep in mind, the 50 year old tach never was that accurate and may be off quite a bit after nearly half a century. 

In the Tech Library here there should be a road test of the 75 or 76... that engine works best as low RPM. 

Oh I get the whole comparison deal, that's why I went out to youtube and watched videos of other DT400s of the era. 
I was planning on going out to get a BR8ES today and try that. I will gap check. 
This thing doesn't really drop off at the 5500 it doesn't  even rev past it. 
I realize all the other points you've made. "never be happy" LOL! Who me? 

Don't use any 'R' plug with a resistor cap. 

Thanks for clarifying my point Alan! 

First & only thought for lack of performance is "Muffler". I'll admit I only got half way down the story then got interrupted but muffler spark arrestor is the cause of much grief when it gets restricted with carbon. That's the second time I've written that in two days. Gotta be right one time haha.

I wish you were right but alas I’ve been down that road and it flows freely. Definitely not plugged/clogged.

Ok but just to be sure to be sure have you run it without the complete tail muffler plus slide a good fitting piece of pipe in where muffler fits as it builds up where the muffler pipe stops. Push it right in to be sure & look in with a light. Sorry I do go on but been caught out by all that over the last 65 years haha. Was about 12 years old in 1958 when my 125 excelsior wouldn't rev so took the mufflers off & it lit up, but main problem was it had grown carbon in the port itself right against the piston. Had twin exhaust ports. Sorry, Where was i lol.
Sorry couldn't stop myself--well its twostoke at least.
photos.app.goo.gl/ozKCdybkrbmpfe43A
Spark arrestor someone put up on the site. Nice clean one but has angled vane things so can't realy see for sure & gets pluged from within & ya get round balls of carbon bouncing around when trail riding & jump up against the inside of it.
photos.app.goo.gl/UgeRLgKTJm3xEd1x8
One more. My 100 twin headers were chocka blocked & almost totaly plugged further in & around.
photos.app.goo.gl/4pUmwqSJF2aBjCVS7
Happy to be wrong though. Let's get it sorted.

Autolite 4054 is a non resistor spark plug Mark T turned me on to them and they work great. Amazon has them and some auto parts stores 

Got me a bunch of those 4054 plugs too. They work perfectly!