Is my Vespa taking in false air?

Frequently, it is read in internet forums that there is uncertainty about the causes why the Vespa engine does not run properly. For this reason, we want to systematically get to the bottom of this and publish this wiki article to find out if the Vespa is drawing false air, and what can be done about it.

Introduction

False air leads to undesired oxygen entering the combustion chamber in an uncontrolled amount. This alters the ratio between gasoline and oxygen and leads to a lean mixture.

Especially with newly acquired old vehicles, it can happen that the scooter does not work as expected. The causes for this can be diverse.

False air symptoms are more pronounced in Smallframe Vespa PK 50 and V50 engines due to the simple carburetor technology. The Largeframe engines of the Vespa PX, Cosa, Sprint etc. have a more complex Si carburetor with a mixing tube, which is why some of the listed symptoms such as "the engine revs up" do not appear so clearly, or their occurrence is not so pronounced. The mixing tube of the SI carburetors can compensate for slight false air to some extent.

Typical symptoms of false air in Vespa PK 50 and V50

Typical signs of false air in Vespa PK 50 and V50 are as follows:

The engine revs up and then goes out again.
The Vespa doesn't start, even if the fuel supply is ensured, the ignition is okay, and there is sufficient compression.
There is no load at idle. The additional oxygen at a constant proportion of gasoline causes a hotter combustion. This causes the engine to rev up.
The recommended values such as jet size and adjustment of the mixture screw on the carburetor do not show the expected effect.
The idle cannot be adjusted properly.
The Vespa only runs halfway with the choke pulled.
The engine speed does not return to idle speed when the clutch is engaged and the throttle is released.
The spark plug is white.
The engine has a piston seizure.
The engine shows loss of power when accelerating.
A knocking can be heard on the cylinder head.

First measures after detection of false air symptoms:

The first measure for symptoms of false air is the maintenance of the carburetor, as this measure provides a solid foundation for further action.

How to properly clean and seal the Vespa carburetor is described in the carburetor cleaning. Proper carburetor cleaning and sealing is a basic requirement. To properly jet the carburetor, we have written the article Vespa carburetor jetting for you.

If the carburetor has been properly cleaned, jetted, and sealed, it is time to seal the intake manifold. The intake manifold is sealed on the engine housing for Vespa V50 and PK using a 2-hole paper gasket for V50 and PK 50 S and a 3-hole gasket for PK 50 XL and PK 50 XL2. For the PK 50 XL and XL2, the carburetor is not sealed on the intake manifold with a felt ring, but with an O-ring. This should not be forgotten to be replaced when sealing the carburetor.

Opinions differ on the use of sealing compound. If the engine has sealing problems at the intake manifold, a high-temperature-resistant silicone sealing compound is suitable. This is very resistant to oils and gasoline and can be useful for this purpose in case of false air problems. However, it should generally be possible to do without sealing compound.

Next, the throttle cable at the carburetor should be checked, which, in case of defects or incorrect assembly, can also lead to symptoms in the engine running that could then mistakenly be interpreted as false air. A defective throttle cable can prevent the throttle slide from returning to its zero position, causing the engine to rev up at idle and when releasing the throttle.

The cable housing (sometimes also called Bowde) of the throttle cable should be laid without kinks to enable frictionless function. The throttle cable should be adjusted with the adjusting screw so that the throttle grip has minimal play. The outer cable should sit firmly in the seat of the adjusting screw when turning the throttle grip and not slip out. If this is not the case, this deficiency should be corrected by using a new outer cable of the correct length.

The adjusting screw for opening the throttle slide at idle is located on top of the throttle slide cover. The one for the throttle cable is not responsible for this and should only be used to adjust the play of the cable (as little as possible, as much as necessary). Further information on carburetor adjustment and setting can be found in the article Carburetor Adjustment.

After sealing the carburetor and the intake manifold and ensuring the functionality of the throttle cable, it is time to replace the oil seal on the magneto side. This is colloquially also called a Simmerring after the engineer Walther Simmer, who was employed at the Freudenberg company at the time. The general component designation (also of other manufacturers) is "oil seal".

If you are doing this for the first time, we recommend the workshop for safe removal of the flywheel. Removing it does not require a new adjustment of the ignition.
The removal requires special tools, such as a flywheel holder and a flywheel puller for Vespa PK and Vespa PX and Vespa Cosa. For the Vespa V50, a flywheel holder and a circlip pliers are required.

When removing the oil seal, special attention should be paid not to damage the oil seal seat.

Note that the Vespa V50 and early PK S models have a crankshaft with a small 19/20 mm taper and therefore require a different oil seal than the Vespa PK 50 XL and XL2, which have a crankshaft with a 20/20 mm taper.

A few drops of two-stroke oil on the inner sealing lip protect the oil seal during the first engine revolutions and can improve the durability of the oil seal.
The oil seal should be carefully inserted with uniform pressure. The oil seal must be inserted in such a way that the inner sealing lip faces inward towards the crankshaft chamber.

Next, we check the area around the cylinder base. To do this, we jack up the Vespa at the rear, remove the rear wheel, the two screws at the top, the bolt at the bottom of the exhaust, the cylinder cover, and the four nuts including circlips at the cylinder base. To facilitate lifting the cylinder, the shock absorber bolt can be removed to tilt the engine downwards and create more space to work. This facilitates work, especially on the V50 and PK, as this designation really does justice to its name in some places. The cables are not affected by this.

After the cylinder has been removed from the piston, clean the area with universal thinner or brake cleaner and apply a new cylinder base gasket. For the cylinder base gasket, high-quality sealing compound such as silicone special sealing compound can be used to ensure better sealing. As already written above, there are different opinions on the use of sealing compound. Piaggio used only paper gaskets without sealing compound.

The spark plug thread and the sealing surfaces on the cylinder head must be in perfect condition. If the cylinder head is warped and leaking, it can be straightened ("planed") by grinding on a flat surface.

If the false air problems persist after these measures, the replacement of the clutch-side oil seal is necessary.

The last section on the topic of false air is short and concise. If the false air symptoms mentioned in the first part of the article persist, opening the engine is necessary. The housing halves, especially in the crankshaft chamber area, must be carefully examined for damage.

Literature Recommendation

The "Das Vespa Schrauberhandbuch" (ISBN 978-3-95843-043-3) illustrates many work steps and explains them in a task-oriented manner. This ensures the success of the engine overhaul.
Although the literature is not intended for Vespa PX engines, SF and LF engines are similar in their basic structure.

Spare Parts

Tip: We recommend gluing the oil seal with an oil- and gasoline-resistant adhesive such as Loctite 641.