Knez's Home

This is a collection of pictures and information regarding the Weber carburetor supplied as part of the MSS induction kit.

Here is a list of adjustements I made to get my car running correctly. Note, I am at about 5000 feet of elevation.

Useful background information copied from other Internet sources:

Idle jet

There are 2 idle jets per carb, one per throat. Each jet acts totally independent of the other.
Located under the screw in the photo is one of the idle jets. It is nothing more than a brass tube with a calibrated hole in it. These jets are available with larger or smaller holes depending on the amount of fuel you wish to introduce to the engine.
Contrary to its name the idle jet supplies fuel to the engine up to about 3000 rpm so it is a vital part of the tuning equation. This will keep coming up so be clear on this point

Idle mixture adjustment screw

Again there are two of these which act independently on each throat of the carburetor.
These screws can be adjusted down all the way to fully closed where no fuel gets to the engine during idle.
The adjustment of these screws are to be used ONLY to adjust the mixture at idle! Mechanics often turn these to correct low speed running problems and this is totally wrong. The idle screws are for idle ONLY.

Air bypass screws

One per throat
Allows for a compensation between throats of a single carburetor to equalize the flow between the two barrels.
When properly adjusted, one of the air bypass screws is closed while the other is open a measured amount. We will discuss this setting later but if both are open the carb is not set correctly. In some instances both may be closed but for both to be open is not correct.

Idle speed screw

Because the each carb has only one throttle shaft that operates both throttle butterflies there is only one speed screw per carb.
Think of the throttle butterflies as doors that allow the air/fuel mixture to enter the engine and the idle speed screw is a stop that controls how much the door closes. Because the engine needs a certain amount of air/fuel mixture in order to idle, the speed screw allows for a fine adjustment of how close to closed the butterflies are and is essential in setting the idle speed of the engine.
The idle speed screw is one of the most tinkered with screws on the carb and is often opened far too much. Generally speaking if the screw is in more than 1.5 turns from the moment it contacts the throttle then it is turned too far.

Summary

Idle jets control the engine up to about 3000 RPM
Idle mixture adjustment screws are to control idle only
When properly set only one (or neither) of the air bypass screws are open; not both.
Idle speed screw is used to set idle speed only and should be kept between about 1-1.5 turns from the first point of contact.

Pictures