How To Fine Tune Webers/Dellortos

1. Balance adjusting screw 2. Throttle lever 3. Idle speed adjusting screw M. Idle mixture adjusting screw

For starters, you are going to need one special tool (of course), i.e., some sort of carb balance tester such as a UNISYN. The best one I have seen is the unit sold by Alfa Ricambi, which is cheap and easy to use with no calibration needed. If you have a stock airbox, be sure to get the 90-degree adapter. The only other tool you will need is a small screwdriver. A good portable tachometer would be nice but is not required. Here we go:

1. Warm up the engine.
2. Remove the air filter(s) on the carbs.
3. Attach the balance tester to each throat on each carb. Note the readings from each throat. A difference between carbs can be adjusted, but a difference within one carb may indicate a mechanical problem. Check the following:
   • Air leak at carb/manifold or manifold to head. Check by spraying the suspected area with some combustible liquid such as Berryman’s B-12, enamel reducer, etc.—NOT GASOLINE. Be careful and have a fire extinguisher handy. A leak will cause an increase in idle speed.
   • If cylinder #1 is lower than #2, the air bleeder to #1 will cause a small difference. Clamp the hose to test, unclamp for tuning.
   • If #4 is lower than #3, the vacuum hose or brake booster is leaking. Clamp hose to test. Air leaks must be corrected. They cause the engine to run lean and can damage pisions/valves. Mechanism problems such as uneven intake valve clearances, burnt valves or bad rings can cause variations between chambers. Check with a compression gauge or leak-down tester and correct as needed—it’s hard to tune junk. Small variations (5 percent or so) are OK.
4. If all is well, use the spring-loaded screw (#1 in Fig.) on the center linkage to balance the flow between each carb. Get the air flow as even as possible.
5. Rev the engine by popping the butterflies open. A hesitation may indicate:
   • Too low idle speed — screw-in main idle speed adjusting screw (#3 in Fig.)
   • Fouled plugs or poor coil or wires
   • Distributor points/condensor bad
   • Distributor too retarded
   • Accelerator jets not working
   • Idle circuit not working
   • Carbs are still not balanced
   • Venturis are too large or cams are too small
   • Float levels are too low
   • You’re out of gas
   • Intake cam timing is way off
   
   A slight hesitation is OK. Some European Alfas have it and you can learn to drive the car and easily cope with the problem. Large hesitation should be corrected.
6. Now that the carbs are balanced and there is little or no hesitation, pull off the spark plug wires one at a time and note the rpm drop on each cylinder. If the idle jets are set properly, the drop should be within 25 to 50rpm. Adjust each idle mixture screw (spring-loaded at the back of the carb, 'M' in Fig.) until the drop is even. If you get lost, screw in all four jets and unscrew one to two turns then start over, if one won’t adjust, you must clean out the idler circuit. Now go back to Step E. If it revs OK, you’re done—go drive the car and check the results. [CAUTION: Most electronic ignitions may be damaged by disconnecting a spark plug wire, so don’t adjust idle mixture screws this way unless you are sure your ignition won’t be harmed. RHH.]

Notes:

1. Carbureted cars will not idle as smoothly as a well-tuned fuel-injected one, but you can get out more power and it should be more reliable.
2. Idle roughness can be caused by uneven valve clearance or cams. The more overlap on the cams, the rougher it will idle—you can compensate by raising the idle speed.
3. If engine speed hangs up and then drops, the idle mixture screws (M in Fig.) may be too lean (screwed in too far). Back off one-half turn. It’s also possible the carb butterflies may be set on the progression holes. Reset idle screws and start over.
4. Idle too rich. Screw in each idle mixture screw until the idle starts to increase, then unscrew one-quarter to one-half turn and rebalance.
5. Car bogs badly on takeoff. Venturis are too large for the cams, or the cams are really hot—or both.
6. Uneven idle, uneven midrange performance. Check that both carbs are tied together and have a support rod to the motor mount. If the carbs can move around, you will get misalignment of the throttle shafts and sporadic performance.
7. The idle mixture screws on Dellortos are in the rear set of tubes and may be covered with grease. Adjust and recover (keeps air and water out).

Rejetting

If all else is OK and the cams are the ones you want, change the jets as follows:

Problem                 Solutions
Car bogs from takeoff	Use smaller venturis 
Too rich at idle        Lean out mixture screws or use smaller jets 
Too rich at midrange    Reduce main jets 
Too rich at top end     Enlarge air correctors 
Surging under steady state driving: 
   At 1/2 throttle      Increase main jets 
   At slow speeds       Increase idle jets