The bike is WEAK on low end and top end power.  Haven't figured it out yet.   Compression is 130+ though.

Any ideas out there, would love to hear them.   I have: 
adj spark 2.5 to 3.2., curr aligned to stator tab/flwheel. 
2 carbs tested.  Both same result. 
several plugs tested. 
leak tested at 1psi in about 5 min; so, there is a leak somewhere, none visible I could find. 
spark plug/ignition at least appears in good shape.
points 0.013"
condenser 0.22uF
new coil, emgo. 
head torqued 25ftlb
new crank seal, mag side (didn't make lot difference) changed, clutch side no leaks.  doesn't "smoke" heavily
needle and jet changed getting rid of blubbering at low throttle.  OEM mikuni parts, same numbers.  O-2 and 5D1
sprayed around engine w/carb cleaner while running, no response.
rehoned cy linder, add rings.  compression 130+


I am welcome to all ideas before I pull this engine apart. Thank you. (before I forget will a 75 or 6 DT400 fit the mounts on DT1?)

I got nothing, sounds like you touched all the bases.

I don't know for sure on the engine swap BUT I am 77.6% sure it will not fit as is. I have 73 MXs and I know those engines will not go into my RT1s/DT1s so I am fairly certain the 75/76s won't fit without modifications. I am happy to admit if I am wrong.

Check to make sure the pipe isnt plugged up. My 72 LTMX is and talk about a dog! i gotta burn that baby out.

Good one Enduro Nut. I have never had a problem with the pipe; guess I have never ridden enough in my 70 years!!!

Great idea. I'll swap out a known good pipe from the DT3 and see what happens.

Long story, but I'm already on pipe 2. However, pipe 2 is a DT2/3 and I can't really see the center of the pipe, so its very possible its clogged up. Only thing it ran similar on first pipe, but I'm going to try cleaning or viewing it anyway.