1990 K1 Exhaust Rattle At Low RPM

Apparently my search-fu is weak today as I cannot find anything on this topic.

My 1990 K1 recently developed an annoying rattle/knock at idle. It goes away as the revs ease above ~2,000 rpm.

This has been diagnosed by an experienced BMW mechanic as my catalytic converter coming loose within its encasement.

It was the only time I have ever seen a hammer used on a BMW---although it was rubber headed. Thought that was only appropriate for my H-Ds!

Given that this is a one-piece exhaust from header to muffler tip, any experience with this development and/or solutions?

I would like to keep the original look for now, if possible, for 2025 AMCA judging purposes.

OEM is, of course, unobtanium.

Any thoughts or experiences welcome!

The complete exhaust system for K1 and K100RS bikes shows the same part number for both in the 1990 model year. Lots of K100RS bikes were sold, and many had their exhaust systems replaced with aftermarket. So the first thing I’d recommend is a “Wanted” ad in the BMWMOA Marketplace, for a good used K100RS exhaust. You could try that same ad on other venues as well, like k-bikes.com or Bmw sport touring.com, etc. Also check with the parters like Beemerboneyard.com, etc.

If you can’t find a used system, then you’re likely looking at an aftermarket exhaust—if you can find one—or cutting open your existing system and reattaching the internal parts. That’s a proposition that probably only works if you’d plan on doing it yourself, as most welding or exhaust shops won’t touch a cat due to the hazardous metallic particulates found inside.

Good luck,
DeVern

A stainless self-drilling screw or two can do it.
OM

I'll second what DeVern said. Lot's of 4-valve Ks out there, and I think they all use the same exhaust. I can't believe a stock exhaust is that hard to come by. I understand that aftermarket is not an option for a collector bike.

That said, I can't say I've heard of this happening on the older Ks, so you might want to make sure that's the real issue. Putting a stethoscope or even a metal rod up against the cat while rattling, and listen to be sure that's where it's coming from.