David Brown 30C Series 1, still with a problem

Описание: I have been having issues with the 30C for a long time now and every time I think I got it worked out it bites me on the arse. Engine is running rough so all new spark plugs, condensor, point, coil and wire HT leads made no difference. Looked at fuel supply from the dual tap to a new needle and seat for the Solex FP-30 carby and no difference. Cannot get any adjustments on the air mixture screw and the carby is spotless inside and has all of the correct numbered jets inside which have not been tampered with. Set the fuel height to exactly the correct height. Next was the dizzy itself. I stripped it down and checked that the mechanical advance was working and not binding up at all and cleaned it out again as I did this years ago. Set the static timing using the David Brown "M" for magneto mark on the flywheel which is 7' BTDC and with a test light adjusted the dizzy so that the light went out with the points just breaking as described in the manual.
I did find a problem in here and that is this dizzy is a different model Lucas to what is specified in a manual older than my tractor but same model. I have the DX4A and it lists the DZ4A. They both use the same points, and cap and internals but may have a different advance but I cannot find this out. The next problem I found is the rotor button, it is a loose fit on the shaft and can be moved back n forth which would stuff up any timing. I am looking for the correct rotor button for this dizzy and think I might have one.
Now it is running better than the other day where I had flames shooting up the chimney but the engine does not sound as smooth as it should and still has a flat spot. Had a full engine rebuild about 6 years ago but is a rally tractor so has about 100 hours on it.
I have double checked the valve timing and that is spot on so the next thing to look at is the head off and check the valves themselves and then fit new springs as these are the originals. Hope to find the problem in here somewhere as I am running out of ideas.