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Balancing Weber Carburetors

Feeler gauge approach for balancing multiple Webers:

Definitions:

  • Idle speed adjusting screw is a spring loaded screw which controls the closed position of the throttle shaft. You will be able to see the screw and its stop. Turning the screw dramatically will rotate the throttle shaft.

  • Idle mixture richness adjusting screw is located very near to (and pointing at) the throttle plate. While it is also spring-loaded, you cannot see the tip of this screw. If you unscrew it all the way, you will find that it has a pointed tip.
     

  1. Disconnect all the linkages from the carburetors and loosen the connecting shaft between them.

  2. Using the thinnest feeler gauge you have, back out each idle adjustment screw on each carburetor, then screw it in so it barely presses the feeler gauge against the idle stop. Slide the feeler gauge in and out a bit to establish its "feel."

  3. Connect the throttle linkages, adjusting them so the "feel" is unchanged. Note the orientation of the slots on the heads of the idle adjustment screws. They may be parallel to each other. If they are not, put a dab of paint on each so you will be able to follow the amount of rotation you give each. From this point on, you must turn each screw exactly the same amount.

  4. Turn each screw so it just touches the idle stop plus about 1/2 turn more. The exact amount is not critical: the same amount for each screw is.

  5. Start the engine and let it warm up. Use the accelerator pedal to keep it running if it will not idle on its own. If the idle is for any reason exceedingly fast, you've probably done something wrong in setting the screws initially (1/2 turn shouldn't be a fast idle).

  6. Once the engine is truly warm, carefully turn each screw the same amount in the same direction until the engine reaches the proper idle speed. Measure your turns in degrees using one end of the screw slot as a pointer. If the speed varies (a loping idle), make the idle average as close to specification as possible.

  7. Shut the engine off. Screw each idle mixture adjustment screw in, noting the number of turns required to seat each one. If all the screws turned in exactly the same amount, then screw them all back out by that number (and fraction) of turns. If the number of turns varies significantly, then turn them all out exactly one full turn from the seated position. Note the orientation of the screw slots and mark them if necessary.

  8. Start the engine again and let it idle until it is fully warm again. Turn each idle mixture screw out 1/2 turn. Ignore the idle speed until all the screws have been turned out, then note whether or not the engine speed has increased or decreased after the adjustment. If there is no change in idle speed, turn the idle speed adjusting screws evenly so the engine idles as slowly as possible, then repeat this step.

  9. If the idle speed has increased, turn the idle richness screw out another 1/2 turn. If the idle speed has decreased, turn it in 1/4 turn. Again, note the new idle speed.

  10. Repeat step 9 until the fastest possible idle speed has been obtained, turning all the screws exactly the same amount. At some point when turning out the screws, the idle speed will begin to drop. This represents the rich limit of the adjustment. Similarly, turning the screw in will cause the idle to increase and then to drop: the point at which the speed begins to drop represents the lean limit. The screw should be set approximately between the two limits, with a slight adjustment toward leanness if you hope to pass a smog check.

This procedure will adjust a Weber carburetor as closely as possible to an ideal setting without the use of a manometer or Uni-Syn. In checking this procedure with a Uni-Syn, I have found it to be as accurate as with the Uni-Syn alone.

The success of the procedure depends on the high uniformity of Weber manufacture and uses the thread pitch of the adjusting screws as a literal micrometer. A (perhaps) little-known fact is that the number of a Weber jet is the result of an actual flow measurement, and does not just represent the size of the drill that made the hole. The jet is flowed, then its number stamped on it.

    -  Pat Braden



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