Feeler gauge approach for balancing
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.
Disconnect all the linkages from the
carburetors and loosen the connecting shaft between them.
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."
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.
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.
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
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.
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
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.
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.
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