Rough Engine Performance

[kris]

Hi all,
I've put about 300+ miles (500km) on my brick since re-construction. She has always backfired irregularly, mostly at idle and seems to not do this so much of late. She is great at highway speeds and seems smooth there. City driving anywhere around 2800 to 3000 rpm does not go smoothly. She doesn't seem happy in a 'mid-load' situation. More throttle and full load feels fine. It's the inbetween that is rough. Any thoughts? 61,200 miles (approx. 100,000 km) on the clock. 1986 K100RT. Just checked that valves - number 4 intake out slightly and corrected. Number 1 throttle body has a leak. I am awaiting 4 new 'bushings' from Germany. Injector O rings appear tight. Z crankcase breather hose replaced.

Thanks in advance. I am a little frustrated in that I figured this fine German bike should (and hopefully will) run as smooth as silk.

[Scott_]

I'd say you are on the right track.
See how the performance is after you get the bushings replaced.

[gazman]

Suckin air would do that. Bushings should fix.

[Motorhobo]

First do the unlit propane torch test to see where the vacuum leak is. Might be time to replace the rubber boots on the throttle bodies.

[Scott_]

First do the unlit propane torch test to see where the vacuum leak is. Might be time to replace the rubber boots on the throttle bodies.

I do believe that is what he is referring to. They are called bushings in the parts reference.

[kris]

Thanks very much, guys. Yes, the boots will go. At that point I'll examine any other rubber components relating to vacuum...or anything else. All looked good when I had her apart but I didn't actually test the components. I used "Quick Start" which is basically an ether-type spray used to coax stubborn engines into starting...usually cold weather. The stuff worked well for this leak test application.  After 29 years (I'm assuming the TB's are original) they can be forgiven for giving it up! I'll keep you posted.

Thanks again....happy motoring!

[kris]

Replacing the throttle body "bushings" made a big/ huge difference in my low rpm performance. The engine now runs smoothly and isn't searching for a comfortable spot. I used some vacuum gauges to balance the TBs but I'll be looking for a fluid (mercury, if I can get it) instrument to do this with before I'll be totally satisfied.  For those looking to replace TBs and not get robbed by BMW for Oetiker clamps you can use Tridon worm gear (hose) clamps. You will be looking for a band diameter of 8mm or just slightly less....I think the guy spec'd them at 7.83mm or just around .3 inches. They are like $1.00 to $1.50 per.  Note to the wise....if your TBs need replacing then also replace the little rubber caps (again, you can buy replacements for around 20 cents each at an auto parts store - they are called caps) and also the vacuum line wrapped in that springy looking thing that goes onto the #4 TB on the K100. Getting the TBs onto the airbox isn't too bad but then installing the entire unit can present a little more of a challenge. I used silicon spray on parts before assembly. Could probably use dielectric grease also. Don't use any petroleum based products on your rubber. Synthetic greases could also work.

Have fun!

[kris]

Just one more thing on the topic...I had dismantled this bike pretty thoroughly and, therefore, just checked on the exhaust bolts after running the bike 300+ miles (500+km). Six out of eight nuts needed tightening...(21 Nm torque). I had heard that they are prone to loosening up. And a loose exhaust system could apparently also contribute to a little backfiring/ rough running.

For what it's worth to y'all....

[kris]

A quick follow up on my initial topic. I think the rattling mechanical noise is the air flow valve above the air filter. Could anyone tell me why that thing would clang away at times and not at other times? Again, it is mostly under half load conditions. I don't believe there is any vacuum directly involved but could it be a result of vacuum leak elsewhere? Or some other malady? Does it do any harm for it to clang like that? I really don't like the noise and I'm sure it wasn't designed to run that way.

Thanks in advance!

[kylemb]

A quick follow up on my initial topic. I think the rattling mechanical noise is the air flow valve above the air filter. Could anyone tell me why that thing would clang away at times and not at other times? Again, it is mostly under half load conditions. I don't believe there is any vacuum directly involved but could it be a result of vacuum leak elsewhere? Or some other malady? Does it do any harm for it to clang like that? I really don't like the noise and I'm sure it wasn't designed to run that way.

Thanks in advance!

did you ever get this bike squared away?