I receive a lot of technical questions from members; some by telephone, some by mail or e-mail, and some by way of the BMW MOA Forum. Many times the questions and answers deserve a larger audience than just the person who asked the question. The Forum, on the BMW MOA Web site does address a somewhat larger audience than just the questioner, but the Owners News has even broader readership. So I have decided that from time to time I will use a Q & A format in Benchwrenching.
Here is a sample:
Q: I keep encountering more and more stations that have 10% ethanol added to the fuel. Can I use this in my BMW motorcycle?
A: In many places, ethanol in fuel is almost unavoidable. Some states—Kansas comes to mind—have eliminated the requirement that it be announced with a sign at the pump. So it is hard to know if it is there or not.
Now for the bad news: If you have an Airhead, the ethanol will destroy your OEM fuel floats in short order. It dissolves the lacquer finish on the floats. The floats then absorb fuel and get heavy. Then they sink, causing a high fuel level or causing the bowls to overflow fuel all over your boots. Bing has an "alcohol resistant" float setup that doesn't dissolve, but doesn't meter the fuel level as well either. OEM fuel hoses soften and break down. Bing sells alcohol resistant fuel line, too.
Classic K bikes (K1100, K100, and K75) have a rubber mounting cushion called a “vibration damper” that holds the fuel pump in place. Alcohol attacks this particular rubber piece and soon turns it into a thick, black goo! After a while the goo gets literally sucked into the fuel pump, ruining the fuel pump; possibly into the injectors too. The fuel line seems to be OK. (Note to BMW: there is no excuse for not changing the rubber composition of this piece so it resists alcohol.) On our K75s, I check this piece at least twice a year and replace it if it has softened before it turns completely to goo and ruins things.
I haven't discovered any real mechanical issues with the Oilheads caused by alcohol in the fuel except if the bike is prone to a lean surge the alcohol (having less energy per squirt) may make the surging and rough running worse. If yours happens to run on the rich side this is generally not noticeable.
I think the following from my K75/K100 Haynes Manual is instructive:
"Owners of machines used in the United States should note that pure gasoline only is recommended—fuels containing a percentage of alcohol must not be used since alcohol will cause corrosion in aluminium, brass, rubber, and plastic components and can cause severe engine damage, It may also cause bad starting and performance problems such as misfires or erratic idling."
This manual was written in 1988 and I think it somewhat overstates the problems, but is instructive nonetheless.
Q: I recently replaced the front brakes on my 1999 R1100RT. Now when I brake hard I hear a clunking sound like clunk, clunk, clunk and I wonder if it is the little bobbins for the floating disks. The clunking is a noise between clicking and a deeper clunk and I only hear it when I'm in a quiet area—not a bunch of other vehicles around and it has to be during harder braking not a soft slowdown.
A: The disk roll pins usually click when rolling the bike around, not while braking. A deeper clunk makes me think something is moving. I would recheck the placement of the pads in the calipers, and recheck the tightness of all the mounting hardware.
Q: I have a problem with my 1991 K75. I have had this bike since new and never had problems until now. The engine starts and runs nice and easy but, after half mile down the road, the engine starts missing and sometimes it quits running and back fires, I checked the fuel pump and it works. I cleaned fuel filter and put in fresh fuel and still have the same problem.
A: This problem screams "water in the fuel." It settles into the low sump beneath the fuel pump and sits there with the bike stationary. After it jostles around a bit in that first mile or so droplets get picked up by the pump. Which causes anything from a misfire to rough running, or the bike just quits. After the water settles back out it can get good gas for a little while, so restarts. Repeat! This is what I suspect is happening. The first thing I would do is use some gas dryer in the fuel tank. IsoHeet in the red bottle would be my choice, but regular Heet in the yellow bottle would suffice.
That is not the only possible cause, but water in the fuel seems more likely than an electronic failure to me because electronics that fail intermittently usually take more than a mile or so to warm up enough to fail. Usually!
Next time it acts up check for spark at the spark plugs. I would do a quick check with an inductive timing light. Clip the inductor around a plug wire and shine the light onto a surface where you can see if it is flashing, and flashing regularly. It is very easy to see a misfire using a timing light this way. An in-line flashing spark tester can also be used. I try to avoid needing to remove a spark plug to check for spark, and either of these methods will work well.
If you find it is ignition related, then a problem with the Hall sensor (crankshaft position sensor) or the Ignition Control Unit up on the backbone under the fuel tank is possible. Since K75s have three coils I would think a coil problem would cause a misfire but wouldn’t make the engine quit completely.