Howdy, folks!

I have a new (to me) 1985 K100RT, with about 77k miles - I don't have a ton of mechanical experience, but intend to pick some up working my way through issues with this bike once I have it in known good condition. I've put some $ into getting it back up to road quality (the back brake was a disaster, some idiot in the past had repaired the rear disc with welds and it had a crack in the 4mm steel you could see light through), and it at least is mostly road safe. However...

It's occasionally (at this point, 4 times for a few minutes each, over about 500 miles, and a few more occasional much shorter times) doing something worrisome - the engine suddenly becomes incapable of running over about 1500-2000 rpms. When I throttle up, it starts to struggle and sounds odd (see youtube link) and you can also hear a clacking - maybe like the fuel pump struggling. Most often this has been after going over a substantial bump, like railroad tracks.

Does this ring any bells? I have no idea whether this is just a symptom of gunk working its way through the system, or some more sophisticated mechanical problem that I am going to have to address.

Thanks for any insight you can offer!

-- Todd Masco

start by removing the fuel tank, and then unplug and clean each and every electrical connector you see also there are a number of wires connected to earth(frame) these need to be removed cleaned, don't forget to clean the earth point, also need to page through some of our previous posts on the site. loads of info, we do swear by a product called DEoxit, for all electrical connections.you may need to flush the fuel tank, replace the fuel filter

Now: If you search on this forum, you will notice that there are a hundred different things that can go wrong with these bikes if they haven't been looked after, broken down into: Connections loose or corroded (which ungaas referred to - the bikes run on a low voltage system which is sensitive to poor connections); fuel system clogged; electrical system misfiring; something broken or dead. They're easy to work on and easy to diagnose if you follow a methodical process.

Cleaning and checking every electrical contact, on and around the frame, is a good first step. If you're going over a bump when this happens, a loose connection is a good candidate.

Also, go right around the bike and check that everything that is supposed to be securely bolted, is. This is a good idea anyway.

1,500 to 2,000 rpm can often be the Throttle Position Sensor (TPS) at the back of the fuel rail. It's function is to tell the engine management computers when the throttle is closed or full, and when it malfunctions (usually due to water getting in and shorting the contacts) the ECU panics, thinks you're over-revving, and cuts fuel so you can't rev over 2,000rpm. Try unplugging it, cleaning the plug and putting some vaseline in so there's no shorting, and seeing if problem persists. This is easy and quick to do, and some people disconnect it permanently anyway.

This trouble-shooting doc is a great start to testing, diagnosis, and learning more about these bikes:http://tomwade.me/motorcycles/technical/k100troubleshooting.pdf

my friend, here in poland, had exactly the same problem. it went out to be a partially disconnected computer (under the seat) so start with this mate. then i would also suggest checking connections under the tank and the HT coils (resistance) as they are things which are pretty sensitive when coming to shakes etc. i would say it's definatelly eletctric related.

good luck

OK, great! Thank you all for your info - it sounds like there's wild agreement that the thing to do is remove the tank and check/clean/reseat all of the electrical connection.

And a special thanks for the troubleshooting document pointer! I had not stumbled onto that yet, and I suspect it will be a massive help well into the future.

In the mean time, I am likely to pick up an R1150RTP so I have something to ride while I work on my K-Bike

Thanks!!

Hi Todd,
The treatment of every electrical connector with DeoxIT is a definite must do to restore them to as new. Do not use any abrasive just follow the treatment process on the DeoxIT website Caig Labs. On the 1985 the throttle position sensor is a switch only it was the later bikes which have the potentiometer TPS, do not use Vaseline or dielectric grease on any connector if you are in a corrosive area use DeoxIT S5 after treating with the DeoxIT D5. I suggest you clean the connections to the engine management unit under the seat particularly carefully and ensure the plug is mated to the socket with a click when it fully inserts and latches home. Clean and check the connector to the fuel tank and when you have it off take a look inside the tank to be sure there is no water rolling around in there.
Regards,
K-BIKE

Just watched the video on Youtube, and it almost sound like the throttle switch is stocked.
Does this looks like your problem?
With this system, when decelerating (switch 1 close), the fuel injection is disabled until the rpm goes down to 2000rpm. If the engine is still decelerating and the rpm is lower than 2000, the EFI is re-enabled and the injection is controlled again by the FI computer. As an example, when riding at 5000rpm and cutting the throttle, the injection will be cut off between 5000 and 2000rpm.
This switch assembly only signals idle or full throttle to the EFI. Mid throttle signal to the EFI is controlled by the vacuum switch.
Unplug the throttle switch, and try again. I am pretty confident that the problem comes from there.

CF (The one who wrote the document on the EFI).
The fully updated page is posted on this site:Click here