Cam overlap can cause a drop in vacuum, and obviously, the vacuum advance on a stock carb and distributor won't work together properly. We have introduced the most advanced distributor the VW market has ever seen to solve this problem.
The 010 Distributor
Bosch came to the rescue with the 010 Distributor for early hot-rodders. This distributor eliminated the vacuum advance system, and used ONLY engine rpms to dictate the ignition timing. For high performance engines of the time, this was the perfect solution, since these engines were usually run at idle or full throttle!
The 009 Distributor
In 1971 (in the USA) the VW engines were shipped with a 'smog' distributor, which had a vacuum retard in addition to the vacuum advance. The engines were also changed to the dual port configuration, along with a change in carburetors from the 30 PICT series to the 34 PICT series, which had the additional port for the vacuum retard. These new carbs were also LEAN in their operation, since they had to conform to the new tailpipe standards. The 009 distributor was introduced, and was a very inexpensive 'replacement' unit for VW engines, and it was almost a duplicate of the earlier 010 distributor.
These 009 distributors were (and still are) sold by the ton. Early VW engines (pre-71') had no problem, but the smog engines when equipped with the 009/010 distributors had a pronounced and annoying 'flat spot'. A 'flat spot' is a hesitation just off idle, and can range from being almost unnoticeable to getting broad-sided or rear-ended by approaching cars! (Many 009/34 equipped owners mistake this hesitation for POWER. They don't notice the hesitation, but they DO notice the kick in the back of the seat once the engine catches and it starts accelerating! They mistake this for 'more power' since there is such a difference between the stumble and actually operating properly).
The vacuum advance distributors do not have this hesitation since they advance the timing when the throttle is opened as part of their operation. Obviously, the 009/010 is only rpm based, and this vacuum advance doesn't happen. When a 009/010 is combined with the lean SMOG operation of the 34 series carb, the flat spot is the result. Current 'solutions' are all modifications to the carburetor, which richen up the fuel delivery in various forms, whether it's the idle circuit, the main jet, and/or the accelerator pump circuit.
The error with these 'fixes' is that they are curing a symptom, not THE PROBLEM. The problem is the lack of additional advance just off idle, not lean operation. The stock distributor/34 carbs didn't have a hesitation.
The SVDA Distributor
Enter the SVDA distributor. A production VW distributor with the 009 advance curve (close enough), and a vacuum advance unit! Believe it or not, there is an additional advantage to the vacuum advance (on engines that have the proper vacuum port; 34 PICT series carbs), and that is the gain of 4 mpg improvement over the 009/010 distributor! Same performance, plus 4 mpg improved mileage, and NO FLAT SPOT. The drive-ability of the SVDA/34 carb has to be experienced!
30 Series carbs can NOT use the SVDA distributor since the vacuum signal is not correct and won't pull the advance in on the small canned SVDA (you notice the early distributors use larger vacuum cans than later distributors because the vacuum signal is different).