Carburettor diaphragms on the wrong track
10.11.2011
You normally know SKANDIX as a spare parts specialist for Volvo and Saab. But what does SKANDIX have to do with two-valve BMW boxer riders?
You will find the answer in the following report from BMW Motorräder No. 38. All we say is: A tip for the BING-VERGASER.
The technology of the many two-valve engines that are still around does not get better from standing. You have to replace rubber every now and then. A tip for the BING-VERGASERS
Almost all two-valve boxer riders are familiar with the diaphragms in the domes of Bing equal-pressure or vacuum carburettors. The carburettors of this type function quite reliably and can be operated foolproof, as the slide stroke is not regulated by an impatient throttle hand. The throttle cable merely opens the throttle valve, as a result of which the vacuum in the intake manifold raises and lowers the throttle slide, which is suspended in a rubber diaphragm. This design guarantees smooth power input along with good exhaust and consumption values.
Unfortunately, however, these diaphragms have become wear items like batteries or tyres. Whereas a diaphragm service life of six to eight years could once be expected, such service lives now seem illusory. It is to be feared that this is due to the fuel and that even E5 fuel is more harmful to the rubber parts than the types of petrol commonly used in this country in the DM era.
The apparently shorter service life of about three years was accompanied by a price increase. While the diaphragms were still cheap in the nineties at unit prices of around 15 marks at BMW dealers, the Munich company lists the recommended retail price at 24.16 euros for a large diaphragm and 21.42 euros per smaller counterpart. Independent dealers offer the diaphragms marginally cheaper, but if a pair of diaphragms is needed, they also charge around 40 euros.
The diaphragms need to be replaced as soon as they have become so soft that the slide is no longer guided cleanly, which results in miserable running characteristics. Another phenomenon, which was not so pronounced in the past, is the swelling of the diaphragms, which, once removed from the carburettor, do not want to fit again into the matching recesses and grooves in the carburettor housing and the dome cover. Usually the diaphragms then have to spend a few dark days on the workbench before they can be reassembled. It therefore makes sense to buy a second set of diaphragms so as not to have to endure waiting times.
The decisive tip for those willing to save money came from Atavar in the BMW two-valve forum (www.2-ventiler.de). He has been using suitable substitutes in the 40cc Bing carburettors of his R 100 GS for some time and has had good experience with them. Atavar said that diaphragms for Stromberg carburettors would fit. He had bought his from a boat engine specialist.
The Swedish Stromberg carburettors can also be found on old Volvo car engines, which suggested that we go to SKANDIX AG. There we found ourselves with Bing diaphragms and compared them with parts for the Strombergs. They were able to help us. The diaphragms were in stock and we took two pairs with us. One diaphragm for the 40cc carburettors cost 5.02 Euros, 5.95 Euros each for the 32cc Bings. One pair went into the "R 100 naked", which had begun its life in 1980 as the R 100 RT and is consequently fitted with 40cc carburettors. We installed the other pair in the R 100 RS from 1992, which gets its fuel-air mixture from 32 Bings. Even the installation was not critical, because the diaphragms fit into the carburettors as if they had been made for it and they even have the appropriate elevations with which they are fixed in the correct installation position.
The diaphragms for the 40cc carburettors were somewhat thicker, which seemed to promise slower reactions to throttle commands. In practice, however, nothing was noticeable. Both sets have been used for three tankfuls each so far, so I cannot offer any long-term experience yet and would only like to refer to the unobtrusive function of the "after-market" parts for a thousand kilometres each. But if the diaphragms from the Volvo dealer last as long as the BMW parts, then the customer has already saved money.
Text and photos: Andy Schwietzer
