Piaggio,Vespa 2 Stroke idle problems, air leaks.

Piaggio,Vespa 2 Stroke idle problems, air leaks.

If your 2 stroke engine has a very fast idle or erratic idle it could be due to an air leak.
An air leak in your crankcase will cause a lean fuel mixture which in turn can lead to a fast or erratic idle speed.

It is imperative that these symtoms are investigated as the result of running an engine lean can be disarstrous.

The Theory of Air-leaks

Unlike 4 stroke engines, the two stroke engine must have a completely air tight lower end.

The normal vacuum in the 2 stroke engine's lower end will draw a mixture of fuel and air from the carburetor though the intake manifold. Once this fuel air mixture has been sealed off by the reed valve or a piston skirt, the resulting crankcase pressure moves the mixture through the transfer ports upward into the cylinder. A small "air leak" at a crank seal or an intake gasket will cause the usually perfect fuel/air mixture in the crankcase to become slightly lean...sometimes very lean. This lean mixture can quickly result in a seized piston. Air leaks are, by far, the number one cause of engine piston seizures.

If you do not have the tools to perform a pressure test (leakdown test) then you could try spraying WD40 around the carburettor inlet manifold and crankcase joint. If there is an air leak the WD40 will bubble and you may hear a change in engine speed. The crankshaft oil seals are difficult to check using this method and are a potential cause of air leaks.

The Pressure Test


The step by step is :

A) Seal off the intake manifold with a plug or rubber gasket

B) Seal off the exhaust port with an automotive expanding rubber “freeze plug


  1. Reply

    Hello. I have just found petrol in my engine oil. And engine oil in the barrel. So this would indicate a crank seal failure ? Gavin

    1. Reply

      Does seem likely that is the cause

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