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An important device is the intake-manifold preheating, also known as carburettor preheating. It uses the hot exhaust gases to heat up the intake-manifold and the carburettor. This improves the mixture preparation and prevents the icing of the carburettor during cold and wet weather.

A bimetal spring controls a flap in the exhaust-manifold. When the engine is cold, the flap is open and guides the exhaust gases under the intake-manifold. There they heat up the intake manifold and the carburettor.

When the engine becomes hot, the bimetal spring looses its tension and the flap closes. Now the exhaust gases flow directly into the exhaust tube and don't touch the intake manifold anymore. This prevents a too strong heating of the mixture when the engine is hot.

Move the mouse pointer over the pics to operate the flap!

In reality this device is often out of order. Often the shaft of the flap is seized or the spiral bimetal spring is corroded or missing. Who wants to drive the Kettenkrad below around +10 C learns easily, how fast the carburettor builds up ice and stalls the engine.

If the flap is seized in "open" position (preheating ON), this will heat up the carburettor too much in summer and this will cause lack of power in hot weather.

If the flap is seized in "closed" position (preheating OFF), ice will build up in the carburettor and stall the engine.

Those, who really want to drive their Kettenkrad at speed, should take care, that the carburettor preheating is working properly.

This picture shows the spiral bimetal spring, which controls the exhaust flap. The valve shaft is in the COLD position (flap open, preheating ON).


Andreas Mehlhorn, 2006-2009