First make certain the carbs are properly installed on the soft mounts. Any air leaks at the soft mounts will thwart any attempts to balance & tune the carbs.
Remove the progression hole covers and look in. At the bottom of the hole, you’ll see several small holes drilled through the floor into the carb’s throat. Move the throttle lever, and you’ll see the edge of the butterflies sweep past the holes.
When the throttle is closed, ideally all four butterflies will be in the same position relative to the first hole in that throat. If the butterfly positiions (balance) is off between the two carbs, then the two butterflies in each indvidual carb might align similarly with their progression holes, but the two butterflies in one carb will not match the two butterflies in the other carb. That’s a throttle linkage adjustment issue, and relatively simple. Turn the screw on the linkage adjuster as required to ‘eyeball’ match the butterfly pairs carb-to-carb.
However, if the two butterflies on a single carb don’t align equally with their respective progression holes, then the throttle shaft is twisted… and that’s bad. It usually happens when you use two wrenches to tighten the nuts on the front and rear ends of the shaft… putting all the tightening torque load through the shaft. NEVER, ever do that!
If the shaft is twisted, the ‘book’ fix is to replace the shaft. However, at that point you have little to lose by trying to untwist the shaft by carefully (!) applying a similar but reverse torque. If that turns out to be the case with your carbs, we can get into more details later.
Adjust the idle speed as slow as possible consistent with smooth running… no missing or spitting.
With all the air bleed screws fully closed and seated, measure the airflow in all four throats. If the two carbs are grossly different, adjust the linkage coupler screw to bring themas close together as possible.
Adjust the Idle Mixture screws for peak airflow… or peak manifold vacuum using a manometer, or peak rpm using a diagnostic tach (not the car’s tach), or loudest intake hiss by ear. All those characteristics follow the state of tune/ balance, so pick one. I just strongly recommend that you NOT choose ‘by ear’.
With each adjustment that improves the ‘tune’ or ‘balance’, the idle speed is likely to increase. So with each adjustment, re-adjust the idle speed as slow as possible consistent with smooth running. You will be constantly re-adjusting the idle rpm down.
With all the air bleed screws fully closed and seated, measure the airflow (or manifold vacuum) in all four throats. If the two carbs are different, adjust the linkage coupler screw to bring them as close together as possible. Re-adjust the idle speed.
Then, in each individual carb, if one throat is “significantly” stronger than the other, re-visit ‘twisted shaft’ above. If there’s a minor mismatch between the two throats in a single carb, then open the air bleed screw on the stronger throat only as much as is required to weaken that throat enough to match the weaker throat… no more. Ideally, less than half a turn will be required, but 3/4 turn is reluctantly acceptable. If one full turn or more is required, then you should be taking another look at ‘twisted shaft’, above.
When you’re done adjusting balance, only one air bleed screw per carb should be cracked open. The other should be fully seated.
Re-adjust the Idle Mixture screws for peak airflow (manifold vacuum/ rpm/ hiss).
Re-adjust the idle rpm to as slow as possible consistent with smooth running.
Each adjustment affects other adjustments. So repeat the adjustments over and over until the last round results in no further change… continuously re-adjusting the idle speed as slow as possible as you go.
When you’re done, adjust the idle speed to the normal running rpm.
Yes, it’s putzy work, and it requires patience. But it’s not rocket science, and not difficult. Just do good work.
The Synchrometer is a very good airflow meter, if that’s the way you choose to go. But a 4-tube manometer makes the task much easier, and the CarbTune Pro is the best 4-tube manometer. The UniSyn is best used as wall art. The rubber tube used to listen to the ‘hiss’ by ear is best used to hang said wall art.
Most Webers don’t have the vacuum ports that allow a manometer to be hooked up, but the DCOE-151 and -152 do have them. Take advantage of them.
Regards,
Tim Engel