Hi all I’m pulling my hair out with this I bought this bike last year as a non runner but it had been restored and unfortunately the guy that did it passed away and the family sold it on that basis . Now I got it running but it refused to rev anything over 4/4.5 k  so I investigated and found that the main bearings were worn the compression was about  140 psi and the ring gaps over 1 mm.  
So at the moment I have fitted new main bearings rebore new wiseco piston all new seals oil pump rebuilt etc. it now starts great and picks up but still won’t rev over 4.5 k . 
I have checked the timing with a dial gauge and strobed it and it’s on the marks at tickover but does advance as you rev it . The carb I’ve tried mains from 140 to 180 and currently running a 90 pilot jet  the plug is a chocolate brown colour and no pinking and very little surging on the overrun , I’m struggling now to think of what next ?  Can the cdi be faulty?  Or anything else I’ve missed ?  
please help I just want to enjoy it…….  

Have you checked the condition of the pipe? It may be carboned up. 

This is the rear silencer nothing like I’ve seen before  not sure if it’s standard ? The bike originally was German registered.  ( can’t add pics for some reaso ). 

 from near Salisbury, UK.

Just for elimination purposes (and assuming that carb is clean with correct Mikuni (ie not from a repair kit) slide, needle and jets), you could briefly run the engine without the silencer and then without the air filter.

Pulsers on the early cdi enduros were known to go bad, but not sure if it wasn't sorted by the time the MX came out. CMSNL have a brand new pulser in stock, but search fleabay etc for a cdi box.

Regarding photos, you need to use a third party host such as Imgur. However, to protect from spammers and bots, a new member has to make a dozen or so posts before they are allowed to attach images.

Remove the complete muffler assembly off the main chamber & give it a rev. Be sure there's not a ring of restrictive carbon where muffler pipe ends where it fits in the chamber. Poke something the size of that pipe just to be sure it's clear right inside.

Thanks for the replies the carb jets I bought from Allen perfornace. Genuine mikuni stuff.  I’ll take the rear silencer off and try that. I have already tried the air filter  ( which is new ). That made no difference . Is it best to soak it ?  

If it won't rev with the air filter removed it'll only be worse with it in. If it's a sponge filter just squeeze oil through but squeeze it out good so not dripping overnight.  Muffler will "or should" have a restrictive spark arrestor deep down in there of you pull the dnd bit out & look in. That's if it's an original muffler. Can you load a video on youtube & link here. Did I see you're new so restricted with pics. Not sure why that is but ya not the first one to say that. Welcome from NZ by the way.

They don't advance as you rev it...  you didn't mention the year model but certainly the late 70's models did not advance.  Advanced timing kills high RPM power on a two stroke...  more modern two strokes with computerized CDI boxes retard the timing at higher RPM.  

Timing might be slightly retarded at a slow idle as timing is retarded at low RPM for easier starting, but marks should align by about 2,000 RPM max and stay there.  In any case. should never advance beyond spec!  

And are you sure it's advancing?  MOST timing lights are designed for low RPM use...  it takes time for the circuit to react and for the bulb to light...  this very slight delay from spark to bulb flash will make fixed timing look like it is retarding slightly as RPM increases.  Some timing lights, notably the "dial back" type, have a enough circuit delay to make the "apparent" retarding very obvious....  but the delay in the circuitry is what causes this illusion.

Final thoughts (beyond the great suggestions already given by others) is to confirm the accuracy of the Tachometer.  They gum up at read low sometimes.  In any case, the 400 is not a high revving (or high HP) engine.  Power peaks around 5,500 and under highway load they won't rev to redline.... happiest "short shifting" and using the gobs of torque between 2,000 and 5,000 RPM.

 

On the German log book I think it said 1978  stupidly I didn’t copy it Before I sent it to the dvla  ! It definitely advances but I’m not sure what what revs it stops. I need to try the exhaust. It’ll rev fine not under load but just doesn’t want to go when it’s loaded if I put the 170 main in which is standard it’s miles too rich. It’s a lot happier with the 140. I’ve tried various plugs and currently running aa iridium  bx8 I’m not new to two strokes, I already have a Suzuki t500 on expansions and ported which I have built up and that really flies  but this thing has me baffled. 

Check for fuel starvation pull the fuel petcock off the tank check for blockage in the tank and the fuel tap bowl.
My 175mx had the same symptoms and it turned out to be that