Thursday, February 2, 2012

BMW Airhead Throttle repair

The hard-starting problem on the R100RS finally made itself obvious. One of the choke (well, excuse me--ENRICHENER. Geez.) cables broke the rest of the way, and it really, really only ran on one cylinder. The fix for that was obvious--order new cables. No problem.

The "While We're At It" syndrome set in, of course, and improvement of the throttle was well overdue. I ordered the throttle cam/gear, to go with the newish throttle tube I had already gotten from Ebay long ago. Here's how it went:

This can't be right!

The original cam/gear had a stepped bore to match that stepped post in the throttle.

Checked with the folks at Bob's BMW, and it turns out that this cam replaces the old one, but you also have to buy the cover to make it work.


'Nother $25, 'nother few days.


I could justify my ownership of an antique lathe, and make this little spacer.

It looks like this going together:

And this after it's in place.

Uh-Oh. Now the throttle cover doesn't hold the new cam down like it's supposed to. The big hole in the cam just slides right up over this projection on the cover.

Another lathe job. Rummaged up a washer that's the right thickness, and cut a big chamfer on it to clear the radius at the bottom of the peg.


Here it is, in place, held by the miracle of superglue. Works fine. Way more trouble than just buying the dang cover, but better than spending and waiting.

Two of my least favorite activities, those.

OOoooooooooooo.....another opportunity!

That throttle tube is just hideous. Way too big around, and it's a gross, as-cast surface with huge parting lines sticking up.

That's why the right grip always seemed too big and lumpy.

Chuck it up in the lathe, and......

OK, now THAT looks like a part that should be on a fine German machine.

And when I get around to putting on my nice, thin, diamond-knurled Renthal grips, they might actually come out the right size.

Here's another un-German oversight.

They stamped the part number into the handlebar. That's nice.

Did it right where the throttle tube is supposed to rotate smoothly. Not so nice, because when they stamped it, it made a raised edged all around every letter.

A few minutes with a honing stone has it all smooth.

Added a few other tweaks, like a new rubber boot on the cable where it goes into the splitter, and safety-wire on the two cables leaving the splitter, to keep them from ever trying to jump out and make the throttle stick.

And now....NOW, the throttle works VERY easy and VERY smooth, and if you let go of the grip, it CLACKS! back to idle the way I like.

Another pet peave addressed.

Oh, yeah, the original project, putting on new choke cables, also worked fine.

Just had to make one part:

That's what the parts breakdown calls a "Piston" in the cable splitter. The plastic one came out is several pieces. Can't buy that part, only the entire assembly. So, rather than spend and wait, just make one out of aluminum!

How did I ever live without a lathe?

Bike works so well now that I feel compelled to paint it.

Jess, Winter of 2012