Awesome work, I found the same on my 77 dt400 airbox, super restrictive. I did a different port for mine, it shoots straight into the boost port from the case

been riding this bike for the past 6 months did a few hundred miles now I want more power! I'm guessing it's around 30 horsepower right now.
I can tell the difference between 90% throttle and 100% throttle which tells me a 38 mm carb is definitely not too big and this engine may still be intake restricted.
was inspired by turbodan to try out a KX250 v-force Reed valvewidened the reed valve duct just 2 mm or so to get the reed valve to fit, and I also enlarged the boysen ports a bit while I was in there
 
I used motoseal to sandwich the kx reed valve between the sc500 intake and the cylinder 
the KX250 duct lines up perfectly with the sc500 intake manifold
 
couldn't help but decide to widen the exhaust port to 54 mm in the blowdown area
 

the stock airbox was not cutting it with the 38 mm carburetor so I decided to do some modifications to try to make it work
I welded on a 2-3/8" pipe 
you can see how much smaller the original rubber hoses very restrictive
 
I also noticed my air filter element was getting sucked off the cage and that the triangle inlet is very small area, I removed the foam layer from inside the cage, and I widened the triangle in the inside the box
 
 all together with 60 mm silicone hose duct from the 38 mm carb  

I like the blue tape on the cylinder walls to prevent porting tool marks .

I really like the idea of opening up the stock airbox, I had not considered that as an option.  I've got the pod filter clamped right on to the carb but I would really like having some better protection of the filter element.

The reeds have got to be the biggest restriction in this motor.  Boyesen makes the 606 reed for these DT's, if I remember correctly, which also fits a late 80's CR125.  We've got quite a bit more displacement here, should have quite a bit more reed area.  Those KX reed blocks can handle 50+ HP with no issue and at 70mm wide I don't think you can stuff a much bigger block in without risking grinding through the casting.

Last I checked I was around 56mm on the exhaust port width.  Maybe a hair over.  The good book says you don't really want to go much further than that as you will risk diminishing ring life and possible ring snagging and catastrophic failure.  I'm running the Wiseco piston with modern rings so I feel pretty good about it.  With the old Dykes rings these things came with, 68% of bore diameter is likely as far as one would want to go.

Did all these "stage 2" mods and didnt touch the carb. Went for a ride today and first impression was that it felt stronger in the midrange 5000rpm but its now lean at cruise and slow to return to idle, and it idles lower than before, I was at a 155 main and 55 pilot, now im thinking about 60 pilot and raising the needle, and I think I can get away with the same main jet possibly, need a new plug to do chop. Being leaner after a mod is usually a good thing, hopefully it will make more power after jetting changes.

You might actually need to go leaner on the jetting, at least on the main.  The stock reed block attenuates the intake signal.

I'm running a 135 main and a 48 pilot.  Very lean needle too, I think I'm back on the N1AL.

I ended up just turning IN the air screw 1/2 turn, to 1 turn out, and DROPPED the needle 1 position to lean the needle. Now it's back to running great. It will cruise at ~25mph and does 85mph gearing limited, my pipe induces a hard rev limit at around 6300rpm, but if I think about it I prefer the torque over the outright RPM. Still on 155 main, 55 pilot (keihin jet sizes)

Here's a video of how its running, I get on it at about 3 minutes, and 5 minutes. My stock DT360 will only do 60mph up the hill at 5minutes after mods will happily do 90+ up the steep hill. Does 0-60mph in 6 seconds or less so far


 

sounds awesome!
did you change the sprocket ratios?
do you think a rex cdi would noticeably change performance over a stock CDI for properly operating stock DT360A?  I don't like to fix what's not broken and she always starts on 3 kicks max

Thanks! sprockets are 44/16
As for the CDI, I notice the difference, but its not about a big Horsepower increase, The hotter spark lets you run slightly more timing to pick up some power, but mostly its about how easily it starts with a slow kick, its harder to stall, idles lower without stalling, doesnt foul plugs, and generally runs just a bit smoother, less 4 stroking because hot spark can ignite the poorly scavenged mixture. It's not a cure-all, but I had 3 different DT360s all have failing ignition systems, I cant trust a stock DT360 to not possibly leave me stranded in the woods. It runs clean and reliable now and the 12v aint bad either.
I avoided it and wanted to really give the stock CDi system a chance, I didnt want to spend all the money on the Rex or Vape system for not a sure performance gain, but overall I'm glad I did!