Opening the screws more should make it a bit richer but it should still idle. As such it is important to smooth progression between closed throttle and acceleration and for part throttle driving. When checking the seated position to make only light contact with seat, aggressive seating will damage needle and seat of carburetor, 4. The smaller the main venturi, the more effective this action is, but a smaller venturi will inhibit flow. This carburetor requires excessive initial ignition timing advance to avoid stumble on take off if the ignition timing was retarded, we would have to open the primary throttle plate to obtain the necessary idle; this would expose the progression bleed holes which in turn would require that the idle mixture be leaned to compensate for this richer mixture condition. Repeat this with the other carburetor. You want to tune the engine by sound.
Twisting doesn't help much as these are press-fit and not threaded joins. Main jets are in steps of 5. Emulsion tube selection The emulsion tube and holds the main jet and the air corrector, and is located 13 in beneath the jet inspection cover 4 in. This will deliver all of the available fuel to the engine and not by-pass fuel back to the fuel bowl. More airflow means more horsepower, but mostly at higher engine speeds. Too lean is even worse.
Be sure for initial carburetor set up all air by-pass screws should be in closed position. A lack of spark or weak spark will make it impossible to tune the carbs properly or to their maximum potential. The engine should start to smooth out. It is important to note that Webers are very rarely 'ready jetted' so factor the cost of jets etc. Let's assume that your venturi size is appropriate for your engine. This will give a 'safe' starting point for the main jet size. The smaller the main venturi, the more effective this action is, but a smaller venturi will inhibit flow.
Buying second-hand When buying Webers second-hand ensure that they are a matched pair. This is especially true of old British cars and cars that have point type igntion systems. There are many of you running cars successfully with choke sizes outside this range so don't e-mail me about it. Note whether engine speed increases or decreases. These carbs have fixed venturis so the choke size is not an issue. So what do we do now? Once again, you cannot tune carbs that do not have adequate or proper fuel delivery. If it falls on its face and will not take any throttle or runs better if you back off the then you could have a few different problems depending the type of carb.
The purpose of the main venturi is to increase the vacuum acting on the main jet 15 in in order to draw in and atomize the fuel mixture in the most effective manner. This will be done by adjusting both carbs up or down the same amount and re checking for synchronization. As long as the engine stays running idle speed is not important at this point. Once you have matched both carburetors you will need to set the idle to the desired idle speed setting. The relationship between jet size and fuel mixture. Do this procedure with each mixture screw. The engine will begin to sound like it is about to stop as the ratio of air to fuel rises.
As we mentioned earlier, a slightly rich mixture 12. Turn the mixture screws in until the idle starts to stumble and get rough. These carbs are commonly used in pairs, this makes the synchronization important, be sure to bring the high flowing carb down to the low flow carb. Begin by insuring that for initial carburetor set up all the air by-pass screws should be in closed position. On a sidedraft with 2 screws do them each a little at a time. Generally speaking you need a little more advance with a Weber than with the stock carb.
Proper adjustment is critical for a good idle. The pump squirts additional fuel into the throttle bores when the throttle opens to compensate for increased airflow. Chart showing main venturi sizes for various engine sizes and peak rpm ranges. Increasing venturi size will result in slower but greater airflow. You have worse gas mileage, and the power gain is slight enough to be psychological.
See our Ignition tech page for more details. The float opens and closes a needle valve that allows fuel to enter the bowl. The accelerator pump by-pass valve is located in the bottom of the fuel bowl, one is required per carburetor. No amount of adjustment will give a good idle if the throttle spindles are bent or leaking air or the linkages are loose on the spindles! All carbs should be bench adjusted to same setting. This is a particularly valuable tool for cars that get track use so that you make sure you do not lean out and put a hole in a piston.