Merkblätter zur Restaurierung der überschweren Kräder BMW R75 und Zündapp KS 750.
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H - P Hommes
© 2006 h-p hommes
Water crossings with BMW R75 and Zündapp KS 750 are, in principle, easy to manage.
The only question is how deep is the water that hast o be crossed?
The possible water depth is 350 mm for the BMW R75 and 425 mm the Zündapp KS 750.
Before crossing unknown water crossings the water depth has to be measured.
So take your shoes off or take your rubber boots on and measure the maximum permissible depth for your bike.
Measure the water depth by rolling up your trousers accordingly, and determine the route crossing the water where your trousers do not get wet.
Most people drive through a stream without checking the depth before crossing, but when the stream or river is 20 to 30 meters wide, it may get critical.
It may be possible that there may be a deeper spot or a big stone hidden in the river, and this occurrence should be avoided in advance of hitting them.
For the BMW R75 the passable water depth is the top edge of the valve cover.
For the Zündapp KS 750 it is the bottom edge of the air cleaner
The Zündapp can go through a lot deeper water crossings than the BMW.
This is due to the higher lying carburetor for one thing, and the other reason is that the magneto is protected by a cover.
It is also evident in practice; where a KS 750 crosses the river easily and a BMW gets stuck after only a few meters.
„Getting caught“ in a river is almost never due to the fact that the engine sucks in water.
Mostly the ignition system is not sufficiently protected.
Ignition failures result because of penetration of water in the system and then the engine dies.
Only then water flows through an open valve into the combustion chamber.
The R75s often have problems when the carburetor gets under water. Water gets in the carburetor stops.
When the bike stops and gets stuck in the water it should quickly be brought ashore.
Otherwise, water penetrates everywhere where there are opportunities.
First there is the combustion chamber of the engine.
Staying a longer time in water makes the water penetrate the housing and water can then flow into the oil sump.
Furthermore in the BMW water can flow through the ventilation hole of the transmission and into its housing.
In the Zündapp water can flow through the ventilation hole in the rear wheel drive ( I closed mine just as in the BMW)
How do I get through a river that is 10 cm deeper than my bike can manage?
You should never go on your own. During our trips we proceed as follows:
We take a KS 750, which can cope better crossing a river than a R75 and attach a long rope at the front. Then the co-drivers wade across to the other river side. This I in case of emergency; they can pull the bike using teamwork out of the water. Once we have the first bike on the other side, we use this one as a towing aid for the other bikes to cross the river. Therefore we avoid immobilization time and then all bikes can cross the river without problems.
What to do if a bike spent some time in water?
Check oil level. Is water in the engine housing?
If yes, then change oil. By no means start the engine with 1 or 2 liters of water in the engine.
Oil floats above the water.
The water penetrates the oil filter and through this to all other engine lubrication points.
And this becomes expensive
Also check the oil in the transmission and various rear drives.
Unscrew the spark plugs, throttle on full force and kick down the kick starter vigorously.
Then water can shoot out the spark plug holes.
When hardly any more water comes, cover the spark plug hole with your thumb and kick a few more times vigorously so that under pressure the remaining water can escape.
Screw the spark plugs in again and start the engine.
The water hammer
It is well known fact that water cannot compress.
If there is a larger amount of water sucked into the engine and it is abruptly stopped, then considerable engine damage can result.
The connection rods can bend or the cylinders can be torn off the housing.
This can happen but not necessarily because with some caution this can be avoided.
When I drive slowly into the river and the engine sucks a bit of water, it can stop without damage.
But if a driver takes a reral run, drives with a high revolution speed into the river and the engine sucks water, then there is a great bang and some other ugly noises.
After that, apart from the driver swearing, you can only hear the rippling sound of the river. Almost romantic.
Unfortunately, this silence of the engine can only be cured with a lot of work and the cost of material.
First the damage looks not that bad, but then you notice that the cylinders are approx. 2 – 3 cm protruding to the outside.
The exhaust has also deformed accordingly.
The worst, however, is: Man is still stuck in the river with his bike; woman is sitting in the sidecar having a whinge and noticing that the water is slowly rising in the sidecar.
But the sidecar is not a boat because it does not float.
It is not only the high engine revolution speed that creates the damage.
It is also the thrust of the bike that tries to turn the engine over by the momentum of the wheels.
But this condition is not possible because water in the combustion chamber hampers any further rotation.
Then the engine is disrupted at its weakest point.
Such damage brings very good sales figures for the spare part suppliers but I do not begrudge it to anyone.
So be careful when crossing rivers. Important is getting through it and this is often better to use manpower than treating your bike very hard, and as a result the destruction of valuable material.
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