Saturday, November 20, 2010

Airhead Bean Cans and Fog Banks

Recently, I'd bought a used Bosch Ignition Trigger Unit from a guy on who was parting out a 1983 BMW R80RT motorcycle.  His motorcycle had only 30K miles on it when he decided to part it out and so the ignition trigger unit or "bean can" as it is nicknamed was less "used" than Brigitta's own bean can.

You see, once you put enough miles of use on a Beemer Airhead's bean can (over 90K on Brigitta), you have to take them apart and lubricate some linkages inside.  Otherwise, you sometimes get some surging type behavior while riding at speed, very annoying.  Last year, the indie mechanic I take Brigitta to had done this and recent behavior on the part of Brigitta had led me to purchase a working spare.

The job of the bean can, is to trigger the coil's generation of electricity to the spark plugs I believe, there's a mechanism inside, which spins in conjuction with the engine's crank shaft and as it moves, another mechanism senses its travel and "triggers" a signal to the coil to do its thing.  Not sure if this entirely correct but you get the idea.

Why not a new one you ask?  Well, BMW or Bosch are really proud of this unit, and it costs well north of $500!  The cost of a used unit was $100.

This morning, I followed the manual's procedures in removing the front engine cover on Brigitta and exposing her bean can and alternator:

The bean can is the smaller can shaped object, the thing with all the wires is the alternator

 Rear view of the old bean can

Front view of the old bean can

Removal and installation are pretty straightforward, the hardest part being to disconnect the old unit from the ignition system.  It's only hard because its located in an ackward location.  Just take it slow and careful so you don't break off the locking tabs.  Make damn sure you disconnect, as it says in the manual, the battery's ground cable when removing/installing the front don't want an accidental electrical short frying your ignition system module.

The new(old) unit in place, I fired Brigitta up and checked the timing mark.  It was not right, so a slight rotation of the new bean can and I could see the timing S mark centered.  I ran the revs up to 3200 rpm and saw the Z timing mark as expected.  Cool!.  I tightened things down, buttoned her up and of course took her out for a ride.

I went to the nearby prairies which are north of the Aurora Water Reservoir and Park.  Brigitta was running a bit smoother than yesterday and running nicely by the way.  I wandered about the dirt trails and posed Brigitta as usual atop on of the small hills:

Sunny skies and Colorado Prairie

 Distant fog bank

Notice that fog bank in the picture above, I went to see if  I could get closer to it.  It was a thick fog bank rising from the water reservoir, caused by the temperatures in the low to mid 30s we were enjoying here in the east side of the Denver Metro area.

A little bit more dirt riding, along trails which occasionally showed deeply rutted channels where trucks and such had probably gotten stuck when it was muddy.  The end of the trail was a fenceline barring further progress to the reservoir itself.

I turned around, topped another small hill and posed Brigitta while we were in the fog bank itself:

Inside the fog bank

The fog bank moves on 

As you can see, the slight wind I was feeling was moving the fog right along and soon I was under sunny skies once again.  I rode back towards pavement and noted while doing so that my Brigitta's idle was a bit lower than usual.  The tachometer only registered in the 800 rpm range and while not causing issues, I knew it was suppossed to be closer to 1100 rpm.

Rode home, turned the idle stop screws 1/4 turn inward, now she idled closer to 1000 rpm.  A quick followup ride and she was just fine and dandy.  It'll take more riding to see if swapping out beancans will eliminate the surging I'd experienced.  The behavior is very random you see, some days you get it, some you don't.

Now I can see if I can do the linkage lubrication myself or have it completely rebuilt for about $150, still much cheaper than a new unit.

Hope you got some riding in today.....


Raftnn said...

Nice write up, glad you got out for a ride. No such luck for me this weekend , work is frantic this time of year. Pouring with rain anyway!

irondad said...

Imagine my disappointment when I saw bean can wasn't a food but another boring mechanical procedure.

At least the great fog pictures made up for it!

Jack Riepe said...

Dear Charlie6 (Dom):

I have never hesitated to call the Beemer Boneyard to get used parts, when presented with the opportunity to save big bucks. This was true when I upgraded to much stronger alternator on my 1986 K75 (Blueballs). The 1995 one is all new stuff, but I haven't really had a serious break-down yet.

The big plus for you is the accompanying education. I don't need to be that close to my machine. Happy Thanksgiving. Glad to see the snow isn't on the ground there yet.

Fondest regards,
Jack • reep • Toad
Twisted Roads