By Gordon Lewis
Carburetor tuning is not difficult, or beyond the range of the average person. Just take your time, use your head, and be careful. The basic system described here applies to all multi-carb engines. You can successfully balance your carbs only if all the other tune-up steps and engine components are correct, i.e. the timing, point dwell, valve lash, spark plug gap, etc. must be exact.
1) Attach a tachometer to measure engine RPM. Use the low range as you will be looking for 25 to 50 RPM differences here.
2) Find the idle mixture screws (removing the air cleaner helps). These puppies are at the middle base of the carburetor and face you if you stand on the passenger side. If you have trouble, look in the Service Manual, page 01-3/24, find screw number 20. Most of these idle screws we will be talking about (screw numbers 20, 9, 19) are gnarled. You might want to take a minute and read the next page of the service manual on idle adjustment. It is the abridged version of what comes next here.
2a) If you are starting from scratch, say after a carb rebuild, lightly snug the idle mixture screws on each carb all the way in and back out 2 turns. They turn just like a faucet; CLOCK-WISE is in (lean) and COUNTER CLOCK-WISE is out (rich). You can also try this idle mixture screw setting as a starting point if your car is unable to idle without help from the throttle linkage (see next step).
3) Make sure that the throttle linkage is free and adjusted. Find the idle stop screw, number 9 in the service manual on page 01-3/24. It is on the linkage on the front carb. It may still have a spring around it. If it is pressing on the linkage, make it quit. If your car stops running go find the screws in next step 4 and make the car idle with those. Then come back and make sure that this screw does not press on the linkage.
4) Find the two idle adjustment screws (number 19 in the manual on page 01-3/24). They are near the top of the carb, left hand side and each one has a spring around it. Back them off as much as possible but keep the engine running. The three screws in steps 3 and 4 press against the linkage. When you are finished you should be able to get a low idle without these screws pressing against the linkage. If you can't, back the idle adjustment screws off the linkage as you balance the carbs. You should be able to back the idle adjustment screws off completely and still get a slow 600 to 800 RPM idle. Again, the idle stop screw must be free of the linkage.
5) OK, now for the fun part. Make sure that the engine is at full operating temperature. Note the engine RPM. Start with the back carb. Mentally, divide the idle mixture screw face up like a clock. Start with the screw slot at the nearest 12 o'clock or 3 o'clock position. Don't worry about 1/8 turns, this is not rocket science. Turn the idle mixture screw in (clockwise) by 1/2 turn increments until the engine RPM drops. When the RPM drops from turning the screw in, the mixture is too lean. Turn the screw back out (counter clockwise) 1/4 turn. Blip the throttle a bit and let things settle down. If you get a RPM drop immediately when you screw in, then back the screw out in 1/2 turn increments. Too far out drops the RPM from being too rich. Look for the idle mixture screw setting that gives you the highest idle. When you are satisfied that you have the highest idle and the idle adjustment screw (# 19) in step 4 is off the linkage, you can go to the front carb. Do the same thing all over again with the front carb.
6) Once the idle mixture is set you should have a low idle 600 to 800 RPM without the idle adjustment screws being used. You will want to bump up the idle a bit with these two screws to the 750 to 900 range. Start with the rear carb and turn in the idle adjustment screw (#19) until you get a small (25 RPM) rise in RPM. Then do the same with the front carb. After you get your target idle RPM with an even amount of screw turning on each carb, give the throttle a few blips to see if things settle down to what you expect. Then go to the other side of the car and find the spark plug wires for number 3 and number 4 cylinders. Number three cylinder runs off the front carb and four runs off the back carb. Pull number three spark plug wire off and note the RPM drop. Unless you enjoy electricity, lots of volts and low amps, you will want to use an insulated tool and ground the wire. Replace the #3 wire and try number four cylinder wire. The drop should be about (within 25 RPM) equal. If it isn't then go back to the other side of the car and fiddle with the idle adjustment screws until you get it. LEAVE THE IDLE MIXTURE SCREWS ALONE. Finally go back to the idle stop screw mentioned in step number three. Screw it in until it lightly touches the linkage stop without increasing the RPM.
7) Congratulate yourself if you are successful. If not, then you have two options. First is a carb rebuild. Or you can push the car out into the street and call the M-B roadside assistance number.