Weber Tuning and Adjusting - Weber Carburetor Tuning & Adjusting
1.Warm up engine. (Driving about 20 minutes is best)
2.Disconnect a drop link so both carbs are isolated.
3.Place Synchrometer on #1 left rear. (Drivers side closest to bumper)
4.If using a Uni-Syn, calibrate it by adjusting so ball is half way up tube.
5.Place Synchrometer on one of carb throats on other bank of carbs.
6.Adjust idle speed screws (at end of each carb) to make both banks draw equally.
7.Set the Air Correction Screws (the one with the lock nut) to their initial setting which is screwed in until they just touch the seat lightly. If the engine stumbles at this point, turn the screw causing the stumble outward (usually 1-2 turns) slightly and turn the others on that carb equally.
8.Adjust the air correction screws so all throats are equal to the highest throat measured in #7 above.
9.If all carbs cannot be adjusted to the highest level, adjust the highest level down a little so the others can be adjusted up to it.
10.Start with #1 and adjust the mixture adjusting screw (with spring, cup washer and o-ring) by turning them in until the engine speed starts to stumble, then back out 1/8 turn. A note of caution if the washers are not the proper Cup washers, the o-rings may be damaged and will be unable to seal properly causing popping and back-firing.
11.Repeat for each other throats; then check and re-set the idle adjusting screw to 1000-1100rpm.
12.Make sure both carbs are still drawing air equally using the Synchrometer after re-setting the idle speed.
13.Use the Synchrometer to measure the air flow through each throat make a note of the highest throat value so all 6 can be set to that highest level.
14.Check and re-set the idle back to 1000-1100rpm using the Synchrometer to confirm both carbs are still equal.
NOTE: If a Uni-Syn or Synchrometer is not available, a length of rubber hose can be used. One end of the hose is placed in the top of the carb and the other end of the hose is placed in the ear - listen to the sucking sound and make them all carbs have a equal sound. However, CAUTION, if the engine is backfiring, your ear or hearing can be damaged using this method! Do not use the hose if there is any backfiring!
15. Remember to do things slowly and give the engine and carbs time to stabilize when making adjustments. This is especially true when using the UniSyn because it slows the air flow to the throat it is being used on. Also, don't make more than one change at a time - multiple changes will not allow you to know which change made the improvement or worse, which change made the engine run badly.
Now, adjust the linkage.
1.It is mandatory that both carbs open equally and close fully. They must open completely so the butterflies are both fully vertical.
2.If either measurement is not correct, the ball joints must be lengthened or shortened by releasing the locknut and turning the threaded shaft in (to shorten) or out to lengthen.
3.There are several adjustment points along the accelerator linkage.
4.Important - Make sure the accelerator pedal rests upon the stop under the pedal when the pedal is fully depressed do not allow the carbs to limit the accelerator pedal travel the carbs are not strong enough for this & will become damaged.
Remember, there are several ways to get the same result some people prefer different methods of Carb Tuning. This one works for me. For some variations take a look at the adjusting info found at: