Honda-SOHC

 Hi from Cambridgeshire – I am new to the sohc forum.
I started riding again a few years ago after a break of 25 ish years , and the number of bikes in the garage has  grown to 4   unfortunately not all on the road. Presently there is:
1958 – Francis Barnett Cruiser – mid restoration – and has been in that condition for a few years  slowly getting there.
1967 BSA Barracuda – on the road
1976 2x Honda 400 Four’s  ………
 One now fully restored and on the road after a total rebuild .
 The other one was the donor bike for the rebuild [ since there was a lot of parts missing] ,which I am now collecting replacement parts for a rebuild .   

So apologies having said hello  I now would like to ask for some help …………
As part of the rebuild, I rebuilt the engine in 2 stages:
1 replaced the missing parts, carbs –sump oil pick up etc etc ,  had the engine running  did a couple of hundred miles  but the engine was rattley and the cam chain adjuster was not working …. But the engine ran ok  …..
2 … so out came the engine  total strip down to allow the fitting of  new cam chain ,      [at some time the chain had been allowed to be slack and had riveted the pivot point on the adjuster- apparently a well-known 400 four problem] new rings, Seals , gaskets etc.  Adjusted the clearances and set up the timing static and with a strobe.  Balanced the carbs, then keeping the revs down started to run in the engine.
………..But ………..
Below 2000rpm if you open the throttle the engine has no power and dies back for a couple of seconds then will pick up ….. is ok above 2000 rpm  …. Really frustrating trying to pull away at a junction.
Checked all around the carbs for air leaks etc   ….. rechecked the timing ….  checked I was using the right marks  Even checked that TDC is TDC all ok…  then advanced the timing approx. 5 to 10 degrees and the bike runs 95% better ………. What Have I done wrong?? I was very careful about setting up the cam chain alignments / timing but maybe its wrong???
Anyone seen this before?