Hey guys

1985 K100RT

I've had this issue over the last few hundred miles and have tried various fixes. It feels like I'm running out of gas even with a full tank. I lose any throttle response and the bike becomes jerky. It's doesn't feel mechanical. It feels like a fuel/air/mixture problem. If the bike is cold, it will take 35-40 highway miles to start showing symptoms.

Attempted fixes:

-Fresh air filter
-Fresh fuel filter
-A few rounds of injector cleaner
-New fuel pump (purolator 2042 mod well documented here: http://www.motobrick.com/index.php/topic,2058.25.html)
-Checked electrical going into the tank

I'd love any suggestions. Home is Los Angeles but I'm currently in Yakima, Washigton. I'm going to start checking the fuel rail and injectors and I'll update when I can.

Have you tested the gas tank vent?

A vacuum build up can act like this...

I've had venting issues in the past and it doesn't seem to be the case now, but yes, very similar symptoms as a venting issue.

Ignition key too heavy, or kill switch not used much, or the four pronged connector to the tank under the R'H side cover may be corroded, cleaning or fiddling with these would eliminate most reasons for it being electrical

K75cster wrote:Ignition key too heavy, or kill switch not used much, or the four pronged connector to the tank under the R'H side cover may be corroded, cleaning or fiddling with these would eliminate most reasons for it being electrical

Could also be located front left of the fuel tank on early 85? But yes I had this problem. If the connections go poor it can result in arcing inside the connector and running as you described. Easy fix to close up the connectors a little so there is a better connection.

desperado wrote:
...... I lose any throttle response and the bike becomes jerky. It's doesn't feel mechanical. It feels like a fuel/air/mixture problem. If the bike is cold, it will take 35-40 highway miles to start showing symptoms.
.......

If the problem only happens after the engine has reached operating temperature, and you feel its fuel related, it's probably the temperature sensor.
The testing procedure is described in full on this link about ¾ way down the page....
EFI/Electronic ignition troubleshooting

Easy to do and will rule it out if nothing else.

Thanks KH, I missed the bit about operating temperature. As you correctly say, operating temperature is the key.

Anyone else here suspect a faulty Hall Effect Sensor?

With the description that is where i'd be looking.

Thanks for the tips guys. I'm going to check out the Hall effect sensor this morning. We'll see how it goes.

Ok I'm back. Like I said before, I'm 1100 miles from home and don't have quick access to a heat gun, but I read on another forum to test it with freeze spray. I just went on a 60ish mile ride and stopped three times to spray the hall sensor cover, but the issue kept happening. I tried a hair dryer but I don't think it would have gotten hot enough to make a difference.

I also tried riding with the gas cap open to make sure it isn't a tank venting problem.

I'm not completely sure the Hall Sensor isn't the issue. The freeze spray seems like a vaguely crude way to test so that may still be at play.

I'm going to let the bike cool off a bit and then take the fairing and the tank off so I can start checking things there. I'll check plugs then as well.

Any other suggestions are fully welcome.

You're wasting your time spraying the cover with freezer. You should take it off and spray the hall sensors directly.

desperado wrote:but I read on another forum to test it with freeze spray. I just went on a 60ish mile ride and stopped three times to spray the hall sensor cover, but the issue kept happening.

You need to shoot the spray directly onto the hall sensors, not the cover.  When you go out, do it without the belly pan(You have a K75S, correct?) and only two screws holding the cover on so you can get it off quickly.  Carry the allen wrench and spray where you can get to them quickly when you stop.

Run the bike until it barely runs, stop, pull the cover and spray under the "cup" to get as much as you can on the stuff under it.  Try to start the engine and see if it runs better without waiting the normal amount of time needed to cool down.  If the engine runs better as soon as you cool the sensors that will tell you that they are the problem.

The freeze spray is a diagnostic tool, not a fix.  As soon as the sensors heat up again the problem will come back.

Yeah I'm definitely not expecting a fix from it, just trying to track down my issue, fix it, and then get home. Thanks for your help.

And it's actually an 85 K100RT

Oops... Senior moment.

I think if it was the HES you'd lose ignition and the engine would just die. They work or the don't.
My money is on the temperature sensor (re my post #6). Pull the FICU plug and measure the resistance between pin #10 (sensor) and pin #13 (earth). You should get a reading of 2.5 kilohm at 20^oc and 300 ohms at 80^oc. Don't worry too much about the temp of the engine... just cold and hot.
Count the pins from the lead end of the plug including the blanks.
It's a very quick and easy test that takes just 1 minute, even if just to rule it out before you go to the trouble of playing with the HES.