Help me get my '85 k100 running again

[K1300S]

Sounds like loose connection, bad kill switch, or bad fuel pump.

[Laitch]

Any other thoughts out there?

Start a separate thread for your mechanical problems so that answers aren't confused with CCalms's issues on this thread that he started.

[gpcrane]

I started a new thread called "Run while on the center stand".

[CCalms]

Unfortunately, replacing the faulty air flow sensor with a functioning one did not solve my problem :(

[Laitch]

Have you performed the compression and vacuum tests yet or have you decided to replace every part of the engine until it works correctly, CC? :hehehe

As far as I'm concerned, and that may not be very far, I think a recap is in order here. How about listing—in a simple linear form—everything you've done so far to remedy this?

[CCalms]

I feel the vane in the air flow sensor oscillating a good amount as the bike tries to idle. Should the vane be more steady? Is this evidence of an air leak? If I hold the vane open with my finger, the bike idles very nicely.

[rbm]

It should not oscillate at idle; it should be steady.  Probably indicating a leak somewhere in the intake or stiction in the mechanism.  Search Youtube for videos on troubleshooting the AFM if you are not immediately successful in isolating a leak.

[The Mighty Gryphon]

Since it is oscillating I am going to guess the problem is in 1 or possibly 2 cylinders, and not in the airbox or the breather.   One of the cylinders is not pulling in air through the flow sensor.

Can you do a compression test?  That would quickly identify a bad cylinder if the problem is rings or a bad valve.  A bad valve is probably going to be a burnt exhaust valve that allows air in from the exhaust system.  If that is the problem, I would expect the spark plug in that cylinder to either be more or less wet that the others because the flow of fuel will not be the same as the cylinders that are getting good air flow from the intake plenum. 

If not a bad valve or rings, the problem is going to be either a leak above or below a throttle body or possibly a problem with the linkage that does not open one or more of the butterflies. 

[tsnap]

would pulling  and reconnecting plug wires while running  and watching the flapper assist in this diagnosis?

tom

[Laitch]

would pulling  and reconnecting plug wires while running  and watching the flapper assist in this diagnosis?
tom

That certainly could be added as another stop in the tour around Robin Hood's barn, tom. 

[tsnap]

going around the barn a few times could result in meeting ALL the farmers daughters, rather than limiting oneself to the one working on the most demanding  udthers

[Laitch]

going around the barn a few times could result in meeting ALL the farmers daughters, rather than limiting . . .

They'd all be charmed, I'm sure but packing a lunch is essential. It's a pretty big place.

[CCalms]

If not a bad valve or rings, the problem is going to be either a leak above or below a throttle body

I definitely should do a compression test. The bike has a confirmed 20k miles so I am not too worried about the valves, but that's obviously no reason not to do an easy and important test.

[tsnap]

Never let the facts (or data) get in the way of a good theory.


tom

[The Mighty Gryphon]

If not a bad valve or rings, the problem is going to be either a leak above or below a throttle body

The hard plastic manifolds below the throttle bodies are visibly cracked and audibly leaking. I've changed all my other leaks, and I'd be happy to change these out too, but BMW didn't have the foresight to make these manifolds out of rubber by '85 and the hard plastic ones are a bit expensive. Unfortunately I'll be leaving the country for two months but those will get changed when I get back.

That is what I was expecting you would find.  Those cracks and the unmetered air they let in cause a real lean mixture that won't run worth a crap.  If they are at the point where they are starting to break up, I wouldn't be surprised that the more it runs, the worse the leaks are.  A temporary test to see if that is the problem would be to clean them well, and then put a coat of silicone caulk on them. 

I have found pretty good used lower bushings on eBay for very cheap compared to new ones from the factory.  They are cheaper when you buy them attached to the throttle bodies along with a fuel pressure regulator and a throttle position switch.  Sometimes you can even get a "choke" indicator light switch in the bargain.  I have bought K75 throttle bodies with all the above stuff for less than the price of one new lower bushing.

[TimTyler]

If you buy a used throttle body setup, find one that still has the blue paint on the butterfly adjust screws. That way you'll know it probably hasn't been adjusted improperly.

[caveman]

CC,
So what have you done since Feb. I am trying to bring a 85 100 back from the dead myself and am having the same symptoms as you (starts but dies off idle). Reading this thread gives me a few things to try and maybe we can help each other out.