I recently had my DT400 top end ported and head work done by Eric Gorr racing. I have no idea where to start with the jetting. I broke it in idling with short blips of throttle for 4-5 heat cycles then dode it around the yard conservatively 2-3 times.
The bike is a 1977 DT250 to start with, I was running the 250 engine with the 400 intake boot and carb with 32.5 pilot and 195 main jet and it ran ok, kind of lacking at mid rpms. I just put the 400 engine in it over the weekend. 
The bike has a FMF gold series gnarly pipe from a 98 YZ 250 that i modded to fit.

I'm new at tuning/jetting things. Stock main jet for the 400 is 40 pilot and 170 main. I started at 40 pilot and 180 main. Took it for a 6-8 min ride and it heat seized. Got it home and cylinder temp was 265. 

Does anyone have the patience and know how to help me out with jetting this beast? 

My first thought is possible crank seal leak because it seemed excessively lean. I have crank seals on standby. How do I determine if my engine needs crank seals?

I'll add that I have never had the 400 engine running and the bike I only rode 6-8 times on short rides so neither of them have a baseline. The project is a collection of parts that I have had around for 10+ years and I'm just getting around to making a dependably running bike. But it's now registered and insured so locked in on the project. 

You started lean from stock.  Stock pilot is 80 and stock main is 190 in 1978.  

BUT, you also need to know what needle jet and slide needle you are running.  (What carb number do you have?  1M200?)

What are your doing for the air filter?

1m2 carb I need to check the needle and slide numbers. 
Last night I replaced the mag side crank seal after it blew out while pressure testing the engine. I also found that the decompressor was allowing air out when the cable was attached, disconnected and no more leak there. 
I'll be re-pressure testing the engine this evening. I'll get numbers on the carb slide and needle while I'm at it. 
I could use a complete carb if anyine here has in for sale. 

5 psi max pressure.

6DH7 needle 2.5 cutout on the slide. I'll have to pull the carb apart further to get a needle jet number if that's important. The carb wasn't complete when I got the dt400 that the motor came from. I'm doing my best to get the carb back to stock parts. Today I realized that my yt125 has the clip that the throttle cable attached to down inside the slide so that's another step forward. 

Thanks for the help MarkT!

Filter is a k&n p/n H040C4

Thanks for the help MarkT!

That's not the right slide needle.  Off the top of my head, that needle is leaner than the stock 6F9  

And I don't want to sound like a jerk but asking if the size of the jet that affects operation from off-idle to full throttle (most effective from 1/4 to 3/4 throttle... unless needle jet is too small, then 1/4 to full throttle) is important... seems so obvious that the answer is YES.

Not sure what that air filter number is...  are you using the stock air box or a "clamp on" K&N?

 

No jerk at all. I really do appreciate all of the help MarkT. 
There did seem to e a flat spot in the midrange throttle. I have a few needles I'm trying to locate around here.

Stock air box with a k&n filter. 

Today I did a leak down test on the warmed up motor using a new harbor freight leak down tester. It doesn't go as low as 5 psi I did 7psi. Engine pressure gauge was maybe 1 psi higher than the regulator gauge but I think they are just not that accurate. But it looks like I found the leak. The decompressor cable was holding the valve open just enough to cause a leak. After the test I believe I have fixed the air leak just by disconnecting the decompressor cable. 

The test was done with the carb and rewd valve on place as well as the exhaust pipe. So I think I have a pretty good sealing engine, pipe and reed valve.

I stopped at the local Yamaha dealer and they say my slide needle is discontinued. I stopped at Duncan Racing after that hoping they had a needle and they say they won't deal with Mikuni carbs at all. If I want a carb from them it will be Kehin. 
A short test ride after the leak down test says the settings are still way off. 

Off subject question, how can I confirm ignition timing? I am using the 250 stator on the 400 engine and 'm a little concerned with and forgot to mention about this engine swap project. Duncan Racing says they don't want to take on the task of setting the timing for me and they don't trust anyone else to do the work so they had no suggestion of where to take it to have the timing set. 

Thanks again MarkT for your help.