Carburettor jetting

This guide deals with the jetting of your carburetor. Examples include the SHB carburetors of the Vespa 50 and Vespa PK 50, as well as the Si carburetor of the PX, Sprint, etc.
A prerequisite for proper jetting is a clean and carefully sealed carburetor.

Main Jet Determination

Important: Tuning rides should only be performed on a closed course!

The main jet can be selected well based on measurements taken from a dynamometer. However, even without a dynamometer, there are indicators that can help you select the main jet accordingly. In general, you should always start with a larger main jet and gradually use smaller jets until you find the perfect size.

An engine running too rich is less dangerous than one running too lean.

You can recognize a significantly too small main jet if the engine accelerates more when you enrich the mixture by pulling the choke lever with the throttle fully open at high RPMs. If pulling the choke lever does not change anything, the main jet size might be correct. If changes in the main jet do not have an effect, you should first clean the carburetor.

If the engine starts to sputter or loses significant power, the main jet is too large.

Additionally, check the spark plug condition. Clean the plug before you start. Find a long stretch of closed road so that you can run full throttle for a while, and accelerate through all gears with the throttle wide open. Then, at full throttle, pull the clutch and hit the kill switch to prevent the engine's operation in the part-throttle range from distorting the spark plug reading.
Let the plug cool down a bit and then examine the spark plug. You will now see the spark plug condition for the full-load range (i.e., the main jet range). If the plug is gray or white, the main jet is too lean. Examining the spark plug makes sense only if the correct spark plug has been selected.
Comparison pictures of spark plugs have proven impractical due to the constantly changing fuel composition over the past few decades, so a comparative illustration in the form of pictures is omitted here.

Pilot Jet Determination

The correct adjustment of the part-load range is important for good throttle response, good behavior when closing the throttle, and stable idle. In larger carburetors like Si or PHB, the combination of pilot jet, atomizer, air corrector jet (HLKD), and needle is responsible. As with the main jet, the pilot jet should be adjusted from rich to lean.

In the SHB carburetor, there are two different types of pilot jets: a normal one and a long type. The long type of pilot jet differs from the normal type by the part highlighted in red in the picture.
If possible, always use pilot jets with the design that was previously installed in your carburetor.

If the idle cannot be properly adjusted when tuning the carburetor, the symptoms can have different origins, such as air leaks. Addressing the symptoms is fundamentally the right way to go. Small corrections can be made via the pilot jet. If the pilot jet is too large or too small, the carburetor will either not respond to adjustments at the mixture screw, or it will respond very late and at the leanest setting. The optimal setting should be near the baseline setting.

For Si (PX, Sprint, etc.):

The pilot jet has two values that form a quotient.
In general: the larger the quotient, the leaner the pilot jet.

The atomizer is always associated with the air corrector jet (HLKD) and should always be considered when making adjustments.
For the atomizer, the rule is that a smaller number (e.g., BE2) mixes the part-load range richer than an atomizer with BE3. As usual, exceptions prove the rule. For instance, the BE4 is the leanest atomizer.