Finding and repairing BMW motorcycle noises
I have only found this one a half dozen times. On a /2, a very hard-hitting noise will be heard at idle. It goes away at higher rpm. It seems to come from the engine and sounds like someone is hammering on the engine case. It is really awful. Owners think it is a rod noise. It even sounds worse.
I don’t remember the first one of these that appeared, but I will never forget a later one. The customer came in with this awful noise. He was just devastated, as he had been to my competitor and had was told that it would cost $500-$600 (in 1968) to rebuild the lower end with a new crank. He was told that it was rod knock. He could buy a new bike for only $1200. I listened and ran my hands around the engine a bit and asked if he would be happy if I could fix it for $100. He was delighted. I asked if he would be happy if I could do it in about an hour. Now he wasn’t so happy, as the current labor rate was $7 per hour.
Up under the gas tank is the top motor mount. Here is what happens.
The one in the photo had been “spread out” to remove it from the frame. As you can imagine, a bolt is run through the top part to draw it together on the frame tube. The lower hole is bolted to a bracket mounted on the engine. The bolt isn’t really tightened up, as it has rubber spacers to allow for some slight movement. The nut is a castle nut, and a cotter key keeps it on. It usually works well. Not shown, because I don’t have one, is the short sleeve that goes in between the two upper portions, in the “spread out” place. It keeps them from getting drawn together so much that they are no longer parallel.
In normal operation, the engine shakes, and this part takes some beating. In time, the shaking can cause the circular part to take a different shape around the frame tube. This “new” shape conforms better to round than when new. Now the short sleeve is too long to allow the clamp to be drawn up close enough to be tight on the frame. Now the whole thing can rattle. As the engine rocks at idle, this part hammers away at the frame. The short-sleeve now is keeping the clamp from being tightened up enough. The solution is straightforward, remove the short sleeve, shorten it a bit more and reinstall. The pain is that the fuel in the tank must be drained before the tank can be removed to get to the engine mount.
Isn’t that better than buying a new crankshaft? That is a worst-case situation. The mount can be only slightly loose and barely rattle lightly. Then it is harder to find. Reach up under the tank, with the bike at idle, touch both the clamp above and the frame. You should feel no movement between the two parts. Test yours at idle, as it only takes seconds.
Valve noise from the head fins
In 70 and 71, the fins on the heads radiated a lot of the valve noise. In 72, BMW came out with heads with roll pins inserted through the fins. These pins dampened the sound a lot, but some remained. The rubber inserts or round plugs can be purchased and inserted. One can even make up some rubber plugs to slide between the fins.
Both great sketches done by Chris, thanks
This sketch shows the recommended position of the inserts on 70-71 heads without the roll pins. It matters little about the exact placement. They could also be inserted between every fin and staggered.
This placement will also work on the 72 and later heads. This will only make the noise slightly less. The largest change is to tighten up the blocks on the rocker arm shafts. Do it first and only add in the plugs if you want even less noise.
Updated 6 Oct 2019