1973 W115 timing and starting

I have been struggling with a W115's timing for some time. After having the carb set, the tappets and timing set by a professional qualified mechanic she would start first and every time at the turn of the key, sound and run beautifully.
Then after running her for 2 or 3 days she won't start, I reset the timing to get her started but she then runs badly pinging and knocking.
I have been through this about three or four times... Any advise would be very very welcome, have any of you had this problem? I really am not sure where to turn next as it seems the mechanics (specialised classic car and merc mechanics) are having trouble with this.
Thanks in advance for any replies.
D