How to make the correct carburation for a Vespa engine

Make the right carburation for our Vespa may be mindblowing for every Vespista, but not for the expert ones; with this article we try to understand how to resolve it.

What is the carburetor?

As we know, the carburetor, it’s a crucial part for every engine (except for the direct ignition ones); this component manage the entry of fuel inside the engine; infact it create a mixture of fuel and air that goes throught the ducts studied and made “ad hoc” for the “Venturi effect”, that make the engine work correctly.

They exist in two types and many sizes: “single body” and “double body”; Vespas have the “single body” ones. For example my Vespa PX mount a Dell’Orto 20.20 carburetor; the first number it’s the size (in millimeters) of the entry from the air filter, the second one it’s the size (also in millimeters) of the exit to the intake mainfold.

The carburation – the first operations

Just that mix of air and fuel mentioned above, it’s created by a correct carburation with the right amount of air and the right amount of petrol that come thought the carburetor’s pipes.

First of all, we have to check if the carburetor (in all of its parts) and the air filter are perfectly cleaned, so we must check if the petrol pipe (from the tank to the carburetor) is in a good condition and finally we should check if all the air intake on the bodywork aren’t blocked.

You may need the “overhaul kit” with the spare parts for the carburetor (gaskets, pin, float and jets).

The key momento of carburation

Now we can operate on the regulations that are on the carburetor, which are basically screws.

Idle speed adjusting screw: this screw works on the “guillotine” keeping it open just enough to get a correct RPM on idle engine speed. We can find this screw near che accellerator cable; by unscrewing it we’ll increase the opening of the “guillotine” at idle, by screwing it we’ll get the opposite effect.

Carburation adjusting screw: this screw works on the mixture of petrol and air, we can find it near the ignition pipe or near the air filter; by unscrewing it we’ll make a “poor” carburation, by screwing it we’ll make a “rich” carburation.

I remind you that the “poor” carburation has a large amount of air, the “rich” carburation has a large amount of fuel.

For setting up in the right way the carburation, it’s very important to warm up the engine, so by unscrew the idle adjustment screw you have to try to keep the engine slightly revved. After that we have to screw completely the carburation adjustment screw and wait that the engine goes off.

With the engine off we’ll unscrew the same screw a turn ad an half; fire up the engine and with the idle adjustment screw we have to try to find the correct idle engine speed.

Final step

At job done we must to take the Vespa and go for a ride on street. After a bunch of kilometers we have to remove the spark plug fo check the electrode’s color:

  • hazelnut color means that we have the perfect carburation;
  • white colour means that we have a “poor” carburation and we have to unscrew the carburation adjustment screw by an half for time;
  • black colour means that we have a “rich” carburation and we have to screw the carburation adjustment screw by an half for time.

We are at the end of this short “how to…”; I remind you that in any case this whole procedure will only be successful if the engine is in good condition. If it had mixture leaks or air aspirations from the gaskets, finding the right carburetion would be impossible.

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