BOXER & ROTOR
TADEUSZ PAWLAK    i    RYSZARD GLEN
Specjaliści BMW R75, Zündapp KS 600 i 750

THE AIR FILTER BMW R75

Tadeusz Pawlak

Durability and reliability of the engine depends to a large extent on the cleanliness of the air delivered to the supply system. In the BMW R75, air delivered to the two carburetors is cleaned by the wet air cleaner placed on the gear box. Air is tentatively cleaned from thick impurities by the multilayered steel gauze. Fine dust settles down and bonds with the oil at the bottom of the housing. The cleaned air through two steel intake collectors is delivered to the carburetors.

The number in our catalog: 2010

In Africa or on Russian steppes however, this filter hadn’t been sufficient and had to be replaced by a different one. It was decided to try a filter with felt cleaner cartridge. There wasn’t enough place on the gearbox, so it had to be placed on the fuel tank. Such project required changes also in the fuel tank itself. The Kraußmann Company got picked to project the fuel tank, and the filter was produced by the Filterwerken Mann & Hummel Company. The filter cover, helmet-shaped, gave a characteristic shape to BMW R75 itself.

The filter cover is fastened to the fuel tank with a hinge and so is possible to draw back. There is an inscription informing about the tank’s capacity (24 liters) and the size of reserves (3 liters) imprinted on the cover’s top. Under the cover there is a felt cleaner cartridge and the rotary regulator of the air flow volume (it fulfills the task of the starting mechanism – sucking).

Contemporary exploitation of felt air filter.

The most important part of the felt air filter is it’s felt cleaner cartridge. We can’t expect that after 60 years since the production date the cartridge would still be in the proper condition. they usually are worn through and used. Their filtering ability is questionable. In most cases they are totally used up, just like the one in the picture below.

It’s very important for the filter to pick out all of the impurities in the air. But it’s also important to let enough air through. If one of those conditions isn’t fulfilled, the engine won’t work properly. We can easily find out how important is the second condition from the recommended by BMW filter instructions for use. It orders to clean the filter after every 1000 kilometers, or 500 kilometers in very dusted air. In the tropics the filter should be cleaned during every full refueling. Cleaning means taking the cartridge out and brush it off strongly several times. Damaged cartridge should be replaced for a new one. In the store one should check if the new cartridge is made of a good quality felt, which ensures good filtering and proper volume of the air flow. If the felt is too thick, there may ensue considerable deficiency of the engine’s power (especially in the higher revolutions range).

The number in our catalog: 2030

The first version of the felt air filter hadn’t got the rotary regulator of the air flow and there was no “START” and “FAHRT” inscriptions imprinted on the fuel tank. Cleaner cartridge was settled directly on the ending of the intake collector sticking out of the tank. hole in the cartridge is exactly the diameter of the collector’s ending. Collector’s ending had to be rebuilt, so it’s size and the size of the outer diameter of the air flow regulator was matching. In the first version of the fuel tank, collector’s ending was too big.

The regulator makes start of the cold engine easier. After switching the lever to a “START” position, all of the holes are closed and carburetors are able to produce the “richer” mixture. Regulator was equipped with special valve (in it’s upper side) which opens after the sucking starts, so the air can be freely delivered to the carburetors. The valve’s work can be noticed on slow revolutions (from under the lid we can hear a gentle sound of “clapping”). While buying a new regulator one should check if the valve’s spring has sufficient tightening strength. The valve should be opening with specific worth of the produced negative pressure.

The number in our catalog: 2029

The quality of the connection between the filter and carburetors is also important for the proper engine’s work. Felt filter deliver air with two collectors made of suitably bent tubes. One should check if the inner diameter of the collectors is proper and if their inner surface is smooth. Often self-taught producers make collectors from rolled up and welded sheet. Then the weld inside causes unfavorable whirling of the air. One should be aware of that buying a new collector.

Next issue: Collector’s rubber linking can’t be neither worn away nor leaky. You can’t expect that the original ones will be in good condition. New ones, if not made from a good quality rubber, will soon be used up. Rubber linking is a subject to constant unfavorable action of the petrol’s fumes inside, and solar radiation outside.

The clamping rings of the rubber linking shouldn’t cut into the rubber after clamping.

The number in our catalog: 2503-1

 

 

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