Selection of carburetion is extremely critical on the air cooled engine. To ensure long life the fuel/air volume must match the engine demands throughout the RPM range to assist in proper cooling. The fuel curve (flow vs. RPM and vacuum rate) must be extremely close or damage that cannot be detected by seat of the pants methods will occur. The vacuum signal the engine sends to the carb will be different on every engine dependent upon things such as piston speed, valve size, port size and velocity, carb size and venturi, exhaust size and design, and on, and on. The European new car manufacturers often have to change the carbs during production runs of the engine as the ports of the head and intake manifold change during production. The 44 IDF carb and most Dellortos are for the 1600 Fiat Abarth engines and the Weber alone has 39 different versions of the same carb for the same engine.
As the vacuum rate changes in production through tooling wearing out they recircuit and rejet the carb to perform properly providing the correct amount of fuel and air. When this is needed they will re-number the carb and it will now fit a group of engines from such and such serial number to another such and such serial number. When needed they will then make changes again to cover the next group of engines. Without redrilling the circuitry and re-jetting the carbs to match the signal sent from the engine's vacuum rate the carb is unable to deliver what the engine really requires.
Many engines may seem to run OK, yet may not be correctly distributing the needed fuel/air ratio for the horsepower and cooling. Few people realize that an extremely large part of the VW air cooled engine's cooling is derived from the proper carburetion. That is why the engine should never be leaned out to optimize mileage, it is simply false economy. It may provide up front savings on gas but is costing far more later to replace the prematurely worn or damaged parts. Instead of getting 80,000 to 100,000 or more miles of life out of the engine, problems show up in as little as 10,000 to 40,000 miles. We spend a great deal of time on the phone with many of these people telling us that their car runs perfect with their 40/44/48 IDF Fiat Webers, 40/45/48 Dellortos, or their Holley Weber Pinto 2300cc progressive carb used in both dual carb as well as single center mount applications.
Then in the next breath they tell us the reason they called was because the engine is overheating, has blow-by in only 5000 miles, etc., has already failed in some way, or required a premature rebuild. These failures are often not related to the improper pounds of fuel and air per HP, however, in most cases that is what causes these failures. I, personally, find that as a poor return on any investment.
For maximum life it is necessary to use carbs with the progression circuit and fuel flow curve to match the VW engine. No Weber or Dellorto carb I have ever seen was made with the proper circuitry for the needs of a VW engine. I only found two Webers that did not already have way too large of passages for the VW's needs. We are the only company able to offer properly circuited carburetion for the VW air cooled engine and modify our carbs to work with specific engine sizes, head combinations and matching components.
This gives you smooth all around performance and correct fuel/air ratio through the complete RPM range. This is one reason we limit the types of carbs we recommend and sell, because all of the other cheaper carbs that already have circuitry holes, which are too large, cannot be reduced to the VW engine's requirements.
Head temperature is the air cooled engine's worst enemy. It causes more damage than all other failures combined and is seldom identified as the problem. This is one reason the carburetion circuitry must precisely match the requirements of the VW air cooled engine. The pounds of fuel/air per hour per horsepower distributed to the engine is responsible for up to 50% of the cooling efficiency.
My only exception to this rule has been in race applications. These engines operate over a much narrower RPM range and can be adequately tuned to work in that range only and are commonly apart for other repairs long before the damage can be done by the carburetion. Off-road or road racing applications should use the 40/44/48 IDF or such Dellorto carbs due to their superior float design for rough terrain or hard cornering.