K75 vibration

[rydfar45]

I have a 1994 K75 that has about 42,000 miles on it. Starting about a year ago I precieved a increase in the normal buzz\vibration to the extent that mentionable and unmentionable parts of my body would become numb. I took the bike to my BMW dealer where they comfirmed this condition and localized it to the engine but could not come up with a fix. I was wondering if anyone out there has had a similar problem and if a fix was found.

Thanks

[jdiaz]

Check the torque on the mounting bolts where the engine and trans mount to the frame. I think there are five locationss.

Pull the alternator and look at the drive fins on the die-cast cup. They may have broken out.

Was there a change in tires or tire balance? I sometimes get the same numbness from using knobbies on the street.

[Brckridnbob]

On my K11, I would periodically get an increase in buziness over time that was REMARKABLY reduced by changing the spark plugs and/or adjusting the valves. The dealer may have already checked these things, but if not something else to try.

[Paul_F]

I keep a pair of work gloves directly under the seat (in case I ever learn how to work on the bike). When I last picked up my bike from a tune up, there was a lot of vibration under my seat that I had never noticed before. Once I got home and put my regular gear back into the tail and gloves under seat, voila, the vibration disappeared. It may not be eliminating the source of the vibration but it stopped the vibration where I felt it.

[breyfogle]

IMHO, any K75 that vibrates to the degree you indicate.... is broken, somewhere. I'd try another dealer.

[PGlaves]

First - determine if the vibration varies with engine speed or with road speed by riding - when you feel the vibration pull the clutch and let the engine drop to idle. If it goes away when you drop the engine speed it is engine vibration. If not it is from the clutch disk back. Note that the clutch cover and pressure plate are bolted to the engine - the clutch disk, transmission etc are connected to the wheel.

Retorque the engine and transmission mounting bolts to the frame.

That done - vibration is caused by either a mechanical imbalance or combustion imbalance. While it is possible for mechanical imbalance to creep up on you it is very unusual. Two possible causes are a buildup of carbon on the pistons or badly worn clutch components.

The most likely mechanical cause however is in the alternator drive. The driving vanes or driven cup may be broken, and the rubber cush drive which conects the alternator to the engine may be all chewed up. Or you may have bad alternator drive bearings, either in the engine auxilliary case or the alternator itself. This is usually, but not always, associated with funny noises. Check this first.

Almost everything else mechanical goes out of balance with a rather large bang.

Combustion imbalance simply means that one or more of the combustion chambers is producing power pulses weaker than the others. There are several possible causes which are normally addressed with a precise tuneup. These include such things as: spark plugs, plug wires, combustion chamber deposits, valve adjustment, and injector cleanliness.

If the alternator drive checks out OK, your bike needs to be correctly tuned before you do anything else chasing this, assuming it is engine imbalance. If it is driveline imbalance it could be a U joint or bad transmission bearing.