Adjusting the Vespa carburetor: Instructions, jetting table and professional tips

Adjusting the carburetor on the Vespa - here's how

VESPA - CARBURETOR

The choice of carburetor size has a considerable influence on the performance and characteristics of the engine. Useful carburetor sizes range from 19 mm to 35 mm. In addition to the experience of other mechanics, there are also formulas or tables for determining the required carburetor size. This is largely determined by engine speed and cubic capacity:

VESPA - ADJUSTING THE CARBURETOR

Many engine failures are caused by poorly tuned engines. Critical factors are compression, squish, ignition and carburetor adjustment. The correct carburetor setup is not as simple as the ignition timing and is not as easy to control. Our jetting tips table will help you to find an initial point of reference.

CARBURE

Modern synthetic oils make it difficult to interpret the spark plug pattern. However, the spark plug can be used as a rough indicator after a full throttle run.

• Grayish/snow-white: Combustion too hot - do not continue to operate the engine! Risk of sticking or a hole in the piston.
• Black/sooty: Indicates too rich a tuning or too high an oil content.
• Fawn: Optimum spark plug pattern.

OPERATING TIPS

So that you don't have to start from scratch when setting up your carburetor, we have put together a small overview of carburetor setups that work well. You should use this as a guide to get the engine running. Then you should make it richer from range to range and then jetting down. No guarantee!

FAQ: Frequently asked questions about carburetor adjustment

How do I set the idle throttle correctly?

The idle throttle is set when the engine is warm. First screw in the idle mixture screw (often on the side) completely and then unscrew it approx. 1.5 turns (basic setting). Then use the idle throttle screw (top, lifts the slider) to set the speed so that the engine runs smoothly and does not stall. Now use the mixture screw to find the point of maximum speed and then correct it back to normal level with the idle speed screw.

What does "four-stroke" mean?

When a 2-stroke engine "four-strokes", it does not ignite at every revolution. The noise sounds stuttering or bubbling (similar to a 4-stroke). This is usually a sign that the engine is running too rich in this area (too much fuel, too little air).

What are the symptoms of a lean setting?

An engine that is too lean (too much air, too little gasoline) gets hot and dangerous! Symptoms are

• Engine revs up at idle speed and is slow to come down again.
• Bright/white spark plug pattern.
• Performance gap when the gas is opened up quickly (Mööööp noise).
• Ringing or rattling at full load.

How do I proceed when jetting off?

Safety first: Always start with a main jet that is too large (rich). The engine will stutter and not turn over. Now gradually reduce the jet size until the engine revs cleanly and runs smoothly at full throttle. This will prevent sticking due to a too lean initial setting.

What role does the air filter play?

A decisive one! A sports air filter or omitting the filter allows significantly more air through. The carburetor must then be sprayed with more air (larger main jet), otherwise there is a risk of engine damage due to leaning. Holes in the air filter box (a popular tuning option in the past) also require the jetting to be adjusted.