Balancing the carbs on a 308

You wrote "vacuum", but did you mean "Kg/hr airflow"?

 

The best reference for servicing and balancing Weber cars is the softcover book, Ferrari Guide to Performance by Allen Bishop. There is a specific section on the Weber DCN. Since you are having problems getting your carbs in balance, I recommend following the steps in this book. Before attempting to balance the carbs, you need to check all the float levels and the ignition system.

 

I suggest before adjusting the carbs is ensure you have no vacuum leaks. Spray the base of the carbs (where they meet the gaskets) with carb clean while the engine is idling. If the idle changes speed based solely on the liquid, then you have a vacuum leak. If the idle does not change due to liquid, you are good.

308 carbs are some of the hardest to balance correctly. Not only are you trying to adjust the A/F ratio, you're balancing idle air circuit and all carbs together.

Be sure to sync each bank individually before attempting to sync both banks together. Meaning the two carbs for the 1-4 bank should be relatively the same A/F ratio (turns on screw) and same vacuum (airflow).

The following tool can help greatly to balance the carb (within a bank and bank-ot-bank):
https://www.amazon.com/Latest-Rage-5385730856-Carburetor-Airflow/dp/B00CMC57R0/ref=asc_df_B00CMC57R0/?tag=hyprod-20&linkCode=df0&hvadid=309898128610&hvpos=&hvnetw=g&hvrand=8096127774999153145&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9061142&hvtargid=pla-568420224882&psc=1

 

Make sure you don’t have the linkage rod end caps screwed down tight, there should be play in them. Screwing them down creates binding

 

+1 on the leaks. Those carbs can leak through the butterfly bearings. Many people swap them for sealed bearings. Spray carb clearner directly at them. Also make sure your ignition is correct. You may have different ignition control for front/rear banks. That can cause all sorts of problems if one is advancing and the other is not.

 

If the car ran fairly well before your balancing attempt, I suspect there is possibly a mis-step in your balancing procedure. If the idle is very uneven, you may be to lean on your idle mixtures. I always begin with disconnecting the linkage between front and back carburetors, bypass screws gently closed, idle mixture screws initially at 4 turns from closed. I then adjusted between front two carbs then back two carbs to achieve 3.5 to 4.0kg/hr. All four should be very close to the same kg/hr at idle of 1100rpm. Bypass screw adjust to even out between barrels and cross linkage screw linkage between same bank carbs. Next step is to reconnect linkage between front and back and synchronize.

 

I have a 348 and attempting to adjust the vacum on both banks there was a different and I could not get it into range even with the air screw all the way down. Turns out PO tried to adjust idle with the butterfly stop AND one butterfly stop was a wee bit different leaving one more open that the other. Outside leaks and what not check that could be a stop flap too open or too closed there should be a gap that is measurable. Those should be the same as well as other things first I imagine. Just a thought. Good luck!