The BMW…..again.




There are advantages to running an old bike that was in production for over 25 years, especially ones as popular as the BMW Airheads.  For example, parts supply is good, and many parts were updated or improved over the years.  Often these can be retro fitted to older models either directly, or with a little ingenuity.  As an early example of the breed, my 1974 R75 thus has scope for improvement in a number of areas.

Take starting.  On cold mornings, pressing the starter button would produce only a lazy response from the engine.  The reason for this lies in the technology of the time as well as in the bikes design.  The original starter motor in my bike (made by Bosch), used electro magnets.  This meant that to get the bike to go the starter motors coils had to charge up whilst still leaving enough current in the system to produce a decent spark at the plugs.  The compromise here is that to leave enough juice in the battery for the spark, the starter motor is not very powerful for the size of the engine.  If cold mornings are added to this mix, perhaps also with a battery that is getting a little tired, then winter starting may well become a problem.  There is a solution, but in turn, there are further problems to be addressed to make it work.

In the mid 1980’s BMW redesigned the Airheads, giving them, amongst other things, a lighter flywheel to make the engines rev more freely.   These later engines also got a modern starter motor (made by Valeo, and largely the same as those fitted to a load of cars from Peugeot, Citroen, Vauxhall etc.).  It has powerful permanent magnets giving it a much greater ability to turn over the engine, while also leaving more current available to produce a spark in the ignition circuit.  The lighter flywheels on these later bikes have fewer gear teeth around their circumference, and the Bendix in the starter motor that meshes with these (the mechanism that retracts to disengage the starter motor once the engine starts), has fewer teeth too.  This means that later starter motors are not a direct fit to early engines.  Damn!  Being made of aluminium rather than cast iron like the Bosch ones means that they are a whopping 6 pounds lighter too.  Double damn!

However, on an internet search I found a company in the States that sells a Bendix to fit a later starter to my bike.  I bought one, closely followed by a suitable starter off Ebay.  There is one more potential problem in fitting a Valeo starter.  On a car these starters are generally fitted low down on the front of the engine.  Here they are cooled by air as the car moves.  On a BMW Airhead, they are on top of the engine, and under an aluminium cover.  There is little or no cooling.  On some early fitments of Valeo starters the extra heat caused the glue holding the magnets into the starters to fail causing the starters to jam.  Fortunately the one I bought was a later model, with the magnets positively mounter to the motor casing.  To be certain that the new starter was in good condition, I had it checked over and new brushes fitted by a local auto electrician.  He used to work at the Shorts aircraft factory, so he should know what he is doing.  The bike now started instantly in all conditions and all was well……………for a few years.

There are some disadvantages to running an older bike, not least of which is that things do go wrong on occasion for no apparent reason.  Picture the scene the last time I was out on the BM.  I had stopped for petrol and after paying came back to the bike to continue my journey.  I thumb the starter, and the engine turns, but somewhat reluctantly.  More worrying on a bike that was sitting beside petrol pumps and which had just had its tank filled was the dense cloud of smoke that appeared from under the petrol tank closely followed by a strong burning smell!  At this point common sense took precedence over valour and I beat a hasty retreat, expecting to see an eruption of flames that would be the funeral pyre for the old BMW.  After 30 seconds or so without conflagration, and a few strange looks from my fellow motorists who were continuing life as if nothing had happened, I returned to the bike and quickly turned the ignition off.

I wheeled it sheepishly away from the pump to a safer place, where an inspection of the wiring revealed nothing visibly wrong.  The ignition lights still came on, and things like indicators, main beam and the horn all still worked.  The last wiring change to the bike had been the fitting of heated grips.  They were not in use at the time, but as a precaution, I removed the fuse from this circuit.  I did not dare to try out the starter motor again, so roped in a few passers-by to help me bump start the bike.  It started, so home suddenly became an urgent destination.
The old Bosch starter, partly reinstalled.

Damaged wiring on the Valeo starter.


At least I now knew where the problem was.  Still, I dumped the R75 in my garage, and since I have other forms of two-wheeled transport, I happily ignored the recalcitrant old beast for a few weeks.  When I eventually relented and removed the Valeo starter from the bike I found that the insulation on the power wire from the solenoid to the starter motor brushes had broken down (see the picture).  These carry a serious amperage, so I suppose I should be thankful that I did not burn down an entire petrol station.  Still, since the brushes (and this attached wire) had been replaced relatively recently, I have no explanation as to why this should happen.  Now that the BMW is in semi-retirement, it is never likely to need the sort of cold starting power that the Valeo starter was fitted for, so the simple fix has been to replace it with the original Bosch one.  The BMW lives again.  It’s all good character building stuff, I suppose. ;-(

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