Feedback needed, K100rs

[2old2rockNroll]

I've spent the week getting the K-bike ready for it's garage nap...( I give it up at 20 degrees) and there are a couple of things that I want to get accomplished over the winter months to get ready for the New Orleans run in April!

Over the last month or so The bike ('85 K100rs, 37K mileage) has been developing a bad idle. A noticeable stumble on the first 1/4 of the throttle range and a tendency to die while idling at a long traffic signal. Once off idle and past the stumble the engine responds well and the bike runs strong...but recently I have noticed the motor seems 'loose' ... no other way to put it, not a rattle but a general feeling of ...well...looseness. Yeah I know, pretty vague.

The bike has strong compression across all four and the electrics are up to spec on output. I rebuilt the airbox plastic this summer, so the airtrain is tight. Plugs always look good...nice and tan and dry.
There is an exhaust leak as well as a dented pipe which will get fixed this winter, I don't know if this is could be a factor or not. I am also going to do a valve adjustment. One thing I have noticed: idle speed seems to drop sometimes while sitting...it usually stays right at 1Krpm but will drop down to 800 or so ... sometimes it 'searches' low and high...this is all new behavior, last summer it was rock steady at 1000 rpm.
Is there anything else I should be looking at? Maybe hidden hoses I missed? Sensors?

I am pretty anal about clean gas and run a few ounces of seafoam through every month, so I'm thinking it is not crap in the system, I also pulled the tank last summer and cleaned it thoroughly and like I said, this problem is only at idle.

If you guys would be so good as to lend me your expertise, what I would like is a systematic breakdown of how to best ferret out this low end problem. I realize a certain amount of ragged bottom end is inherent in this motor, but in my case it getting past being an annoyance and into limiting safe use of the bike. I don't want to haul her into the local BMW dealer: A, I can't afford it and B, If I wrench it then I will know what it needs the next time.

Thanks in advance,
John B

 

[139953]

Sounds like bad spark. Check plugs (or replace), view HT leads in the dark to check for arcing, coils may be going bad.

Scottie

 

[98lee]

A couple of possibilities are valve adjustment, specifically an exhaust that is getting too tight and a cracked crankcase vent hose (above and to the left of the coils). If the hose is more than a few years old, chances are it's cracked. BMW use the thinest flimsiest hose material for this hose and the heat causes them to crack regularly.

10 11151460480 VENT HOSE 1 $19.04

 

[johns3]

John,

Spark, perhaps. However, after you do the valve check/adjustment (as was suggested)- sync the throttle bodies. It might be a valve or two that is out of spec and the TB's will need to be adjusted after that.

But you might find that the valves are OK and the TB's just need to be sync'd.

-OR-

you can't get the TB's sync'd- one or more may be way out of range. You might have an unseen crack in the "boot(s)" between the TB and the cylinder head. #5 in the Max picture:

My RS had a bit of an idle variation and it didn't feel smooth. When I had the gauges on it to sync the TB's, one was way out of range. If I pushed down on the bank of throttle bodies or I sprayed WD-40 below TB #2 it would smooth right out and the gauges would all read OK. One of my many jobs this winter.

Good info here at MOA and at http://www.k11og.org

Good luck- let us know how you do....

John