Turn each of the front two Idle Mixture Screws clockwise and count the number of turns until the Idle Mixture Screw touches its seat. In highway cruising at legal speeds in most of our vehicles, the throttle is mostly closed. A 45mm is a good size to purchase for 280z street applications. Auxiliary Venturi part and installed Figure 42. Make sure that the linkage is not binding.
You want to tune the engine by sound. It is unsurpassed for full-race use, yet its infinite tune ability by way of interchangeable calibrated parts allows it to be tailored to suit any engine and application. When towing or transporting a car with Weber carbs, a dry fuel bowl will allow the float to bounce and jump with every vibration and pothole in the road. The question is, why are you looking for economy instead of power at Wide Open Throttle? The pressurized charge lacks the sudden impulse of the spring and is not sufficient to raise the weight and open the valve 21. The mixture screew is very easy to set whether it be a downdraft, sidedraft or multiple sidedrafts. Putting a bigger bypass hole in the valve allows more fuel to bleed back into the float bowl instead of out of the shooters.
. But how to switch from one circuit to the other circuit slowly and quickly? Faster flow generally improves low-rpm torque but requires a smaller diameter choke, higher rpm performance requires the most air flow and a large diameter choke is usually best. So now your car should idle correctly, the carbs are synched, the timing is set, everything is good. Chart showing main venturi sizes for various engine sizes and peak rpm ranges. Smaller engines need less air and larger engines need more air.
Finally, set your idle mixture. Weber carburetion is a lot more than something that's exciting to look at. Some racers remove and replace the internal throttle return spring with an external spring as the internal spring may break resulting in a stuck-open throttle which may cause a fatal crash. Do not mess with them. You can tailor the carbs to suit your needs, driving style, engine, location, weather and altitude conditions, etc. These will be discussed later. This occurs by 3,000 rpm or so, and you're down the road like a shot.
Air enters the top of the emulsion tube through the air corrector which meters the amount of air to be mixed with the fuel. It sometimes is overlooked when replacing parts. This is difficult to do and usually leads one down the wrong path loss of performance, running too rich, etc. So you see, all those numbers and letters really mean something. This is where you make your power.
Needle Valves come with different orifice diameters to cover a wide range of fuel consumptions. This common problem will be addressed in the tuning section. Warm up the engine completely I drive for 6 miles to get to my test road and make sure the air pressure in the tires is right, and fill the tank. If you cannot get a good adjustment on these screws and you have to open the idle speed screws quite a bit to get the engine to idle there is a good chance you have a Vacuum Leak and you need to fix it. Use a pin vise and a set of drills 61-80 to gradually enlarge the metering hole. In any case, stop enlarging the metering hole if you run across this problem. A thick-walled 30mm Venturi and a thin-walled 36mm Venturi Figure 39.
When active, it feeds fuel downstream from the throttle plate so it is exposed to manifold vacuum just like the idle enrichment screw. If it does then the carbs are too large or you have another problem. 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. In the case of the emulsion tube, there are really only a few which work really well for V8 applications; and if you aren't using one of them it is certainly a big part of the problem. You would have noticed this in the benchmark step when your idle adjustment screw was several turns out from full lean. The only mass to move is what's in that one short runner.
The path to the fuel bowl also has a ball valve in a jet 23 where the fuel returns back to the bowl through side of this jet. Going any further will require modification of the drainback hole to provide a bigger fuel supply. First, you must decide your target behavior. Figure 54 shows the selection of emulsion tubes and typical applications. We are here to help - that's the Top End difference. Legg, Weber Carburettor Manual, Haynes Manuals 1996. Of course if you have a preference for the three progression hole 152, we can order it in for you.
You need to see what your mixture is at your normal cruise speeds. This is not cast in stone and you need to make sure your car does not Detonate at these setting. Through holes drilled in throttle plate if modified as in Fig. It snaps into the top of the emulsion tube. See the Vacuum leak tech page for this info.
Emulsion tube Selection Below is a table showing suggested emulsion tube type, for a given single cylinder capacity. Well I guess I should say something about the Starter Circuit! In most cases, the stock pump jets can be left alone. On a sidedraft with 2 screws do them each a little at a time. It is fast and accurate and will take alot of the guesswork out of it. Go back to the next leaner size, and do another run to confirm the problem and clear the plugs. Adjust the spring-loaded screw in the linkage between the two Webers. In fact the Progression Circuit is designed to be the key supplier of fuel during slow throttle transitions.