Maybe i just want to start an argument after cocking up something i was machining in the lathe today.
You say it runs better at 2.5, ok that's all good, no argument & can't beat the seat of the pants tuning by road testing.
But you say it was hard to kick over & now @ 2.5 it's not. Hard 'kick' to me means lots of compression in my book.
CDI Mags retard for starting so shouldn't kick back "in theory".
But ok i guess if it 'did' give some sort of kickback at 2.9 & now doesn't at 2.5 then you must be doing something right & if at 2.9 it auto retards back to near tdc for kicking, then at 2.5 it must be nearer still to tdc at kick speed so less chance for a back fire kickback.
How do you set those?, i seem to recal a long line to go by then a short line either side on the stator plate.
Or am i off the 'stator'planet & ya all running Vape setups.
Good if all ya 400's are running well though.
Ahh, i feel better after that. Ok Blast away at Me.

Nope RT No need to blast away!!! I SWEAR this thing kicked like a mule. I'm 235 and there were times I couldn't get it over TDC!
The yellow bike kicked much easier. New piston, bores on both. Changed the timing and I'm tellin ya, it kicks easier!!! And another thing that confirms is I've always been able to open the throttle on a two stroke and kick it once and running. the orange bike was different. two kicks even warm. It was just odd. I agree on the compression thing but maybe becasue I'm 235 it didn't launch me like it might someone smaller. It never Early fired and actually kicked back but it's def different and it runs much better revving up to 7 easliy like it should. No SHIT!

Vape flywheel with a line on it and stator with the same. .5 mm isn't much! But in the scope of something going to TDC It's a lot!
 

Gotta ask lol but is ya decomp working on both bikes as softens the comp noticeably to kick through.

I'm going to check that on the orange one. I'm thinking it's still harder to kick after messing with it today. Put about 30 miles on it today and warm it kicks right over and it's running. On cold start this AM it still to grunt to get it up and running. I was a bit overzealous I guess! It does run better though!

The decomp hole into the cylinder gets carbonned up solid in extreme cases & also the hole from decomp down to the exhaust port outlet gets the same. Just thought i'd mention that because turning over by hand with ignition off of course you'll feel a big difference of compression. It should lift the decomp by memory as you start the lever moving down from the very top so almost in the first inch or so it should start & probably releases near the bottom of the kick stroke but bit of a guess withthout researching. My older 360 type is operated from the handlebar, well it would be if i had a working one haha.

I Cleaned and reseated both decompression units and put in new o rings and cleared the passages. I did adjust the decomp in the orange bike today but since I always speak too soon on here I'm waiting until tomorrow AT LEAST before I pass judgement!  
Warning, I MAY pass gas but I'm not going to pass judgement, yet. 

Maybe there was a tuning expert amongst previous owners, who removed some mm material from the cylinder head, to increase compression.  This was the case on both of my bikes. I found out later,  as restoration was completed 

On my DT400 (76), I also have the Vape and timed it dead on at 2.9, it kicks over so easy I wear comfy walking shoes to kick it and have no problem. I have also had it at 2.5 and didn't notice any difference.