When our 54-Oval Beetle had this problem, I read on forums that there are multiple (Oval) beetles affected by this problem. Once in 4th gear is driven faster than about 90-100 km/h, the engine stalls and the driving speed falls back to 60 to 70 km/h. This problem does not occur or hardly ever occurs in other gears. There are many possible causes to consider for this problem:
- Ignition problems: contacts, faulty ignition timing or vacuum or centrifugal advance, faulty ignition coil
- Carburator problems: freezing (intake manifold), jet blockage
- Fuel supply: leaking fuel pump diaphragm, line or hose blockage, filter or strainer clogged or worn, defective fuel tap
- Clutch problems: clutch plate worn or slipping clutch
So, truly a trial and error problem. We tried everything, starting with cleaning the carburetor and the jets, checking and adjusting the ignition timing and vacuum and centrifugal advance, et cetera. At the Budel event, we bought a used distributor and replaced the younger non-original one. The problem continued to exist however. We never thought the problem eventually lied in worn and clogged strainers in the fuel tap! The tap was almost completely blocked although I completely cleaned the fuel tank and tap two years ago! I bought a new fuel tap on Ebay and after fitting it, the problem was solved!