Help with timing the engine
 

To make a long story short my CDI box has been replaced with a new one (reconditioned by an old and experienced Bosch engineer).

With the new CDI installed the engine revs up smoother especially in 1000-3000 rpms range. Also some little hesitations above 5000 RPMs has vanished.

The problem is that my timing is now set at 0 degree (TDC) instead of 5° ATDC degrees as per usual.
At 6000 RPMs, with vacuum diconnected, timing still drops in 35° BTDC (hard to say but it should be 33,34°).

If I set the timing at 5° ATDC the engine rev-up slower and I experience lot of misses while idling, general rough engine performance (pops trugh the carbs on accel and pops thru the exahust on decel), hesitations in the 2000-3500 and particulary beetween 5000-6500 RPMs.


Strange story, I know, but all those problems disappear with timing at TDC.

Also has anyone an explanation for this behaviour? What to check ?
Can I damage something if running with this timing?

Car is a 1973 2.4 T with Zenith and Dizzy Bosch distributor with new points, cap, rotor and fresh Beru cables, Spark plugs have 300 KMs (Bosch WR5DC).

Carburators has been restored one year ago and recently checked. Distributor seems to be ok, no play, and good vacuum advance mechanism, dwell is at 38.7° (it should be 38° +/- 3 degree)

you need to make sure the weights in the dist are not stuck or rusted. IE, they move freely.
if you feel the need, you could send it off and have it rebuilt.
don't know if you have any vacuum pots but make sure they are connected properly and connected to the correct port.

if that does not fix it then set timing for 35 and don't worry about idle

The vacuum is a retard and should only work at idle as the butterfly is closed and the throttle body is making vacuum below it.
If you have vacuum at high speed then the vacuum tap is wrong as its above the butterfly.
Bruce

The user manual says that te timing ha to be checked at idle with the vacuum hose connected and at 6500 RPMs with the vacuum hose disconnected.

Can you please explain ?
I can't understand how my vacuum tap can be wrong considering it came from the base of the carburetor and has never been modified.