Like Mark T and I have both mentioned, timing! Another thing that really helped with ease of starting (not to mention run better/smoother in my humble opinion), is the 12v conversion, I used Vape but Rex's also sells a kit. I wouldn't even attempt to start my DT400 unless I had boots on, now I will do it in tennis shoes.

All the handbook has to say on the subject of starting is:
"Kick the crank with full strength" it says nothing about using the decompressor to aid starting.
I always press down gently on the kick start lever until I fell the decomp open amd keep slowly pressing until I feel some resistance starting, then I return the kickstart lever to the top of it's stroke and give it a "full strength" kick. If it does not start repeat the process.
Seems to work for me.
But I would check your timing, if over advanced it will make it kick back whenn trying to start.
 

"All the handbook has to say on the subject of starting is:
"Kick the crank with full strength" it says nothing about using the decompressor to aid starting."

Probably telling most of you stuff that you already know , but the decompression valve on the 1974 bikes onwards (ie starting with the DT360A) is linked to the kick start and is automatic. On the earlier bikes it was manual, via a lever on the left side of the handlebars, and there is a warning sticker by the petrol cap which tells you to always engage the decompression valve before trying to start the bike!   There are also instructions in the owner's manual. I'm guessing that bikes with re-sprayed tanks don't have this sticker.......?

Yep, it is linked on the DT400 but nothing specific in the manual that I can see.
 

Reads like your big problem is 'kicking back' & realy shouldn't. Could strobe where its firing in relation to tdc & if around 3mm before tdc then something aint right as its designed to retard for kicking. If its got the early system with the double banger pulser is it possible someone has fitted separate connectors instead of multi plug & got the white/green & white/red swapped over. Pretty unlikely but if it worked it'd fire on advance for kicking. If its got the later single wire pulser then forget that thought & go to the crank key, or go there anyway.

Had hard kickback at my DT 250, even it´s not well-known, doing that. Timing was 100% correct. I found out, spark was there, but too weak. Tried another coil, problem disappeared. But finally switched to VAPE 12V