85 K100 RT - Why would idle drop

[montec]

Anyone have thoughts or suggestions on what would cause the idle rpm to drop on an 85 K100RT?

My typical ride is a commute of 20-ish miles to work. It includes regular stop-n-go traffic such that the engine gets a chance to drop to an idle on most trips. The idle has historically been between 900 and 1000 rpm and has not required any adjustments. The last adjustment was done in the fall of 2009 when I adjusted the idle, choke and TPS. I've added about 20k miles to the odometer since then without any changes (Current mileage 95,727 miles).

I noticed last week that the idle rpm has dropped to the point that the engine will stop if I don't keep the throttle partially open. The first notch of the "choke" (fast idle) is almost enough to keep the engine running. The second notch advances the RPM to about 1500. I have not noticed any changes in performance otherwise.

The simple solution would be to readjust the idle, but I'd like to know what caused the change to ensure that I'm not masking another problem.

To catch up on some routine maintenance while looking at the idle issue:
1) Checked valves (all in spec)
2) Changed fuel filter
3) Cleaned air filter
4) Checked the spark plugs (light coat of soot, nothing extreme, attributed to "smokey" startups while researching idle issue)
5) Checked for vacuum leaks (no visible cracks or changes in RPM when moving hoses, crankcase breather was replaced last year)
6) No obvious signs that the idle adjustment had worked loose or changed recently
7) Changed fork oil (unrelated)
8) Replaced fairing bracket (unrelated)

I estimate that it would take apx 1mm of adjustment on the idle screw to compensate for the decrease in idle speed with adjustments to the choke and TPS as needed.

Unfortunately the odometer decided to quit last week also, so I'll be investigating that issue this week also. It likely needs new gears but I haven't opened it up to confirm that yet.


In the mean time, are there any thoughts or suggestions on the what would make the idle RPM drop?

Thanks,
Monte

 

[cele0001]

Check the vacuum line going for the fuel regulator to the cylinder 3. Sometimes fuel regulator membrane will leak into it, causing rich run condition at idle, causing the idle to drop.

 

[montec]

Thanks for the insight.

I'll check the vacuum line and fuel pressure regulator next.

It may be a few days before I have time to get back to it.

Thanks,
Monte

 

[montec]

Update -

I finished the reassembly of the odometer last night and was able to do some additional testing of the fuel pressure regulator and vacuum line this morning.

I'm not sure that I properly tested for the regulator membrane leak, but this is what I have done.

1) I was able to pull a vacuum on the line going to the regulator and have it hold for 30 seconds or so. I did not notice any sounds that would indicate a vacuum leak.
2) Inserted a 2 foot piece of clear tubing into the vacuum line to the regulator to allow visual confirmation of a membrane leak.
3) Disconnect the vacuum line on cylinder #1 going to the Pressure Relief switch (early 85 model) and capped the inlet to eliminate a potential leak. Research indicated that the switch was eliminated in later models and that it could be disconnected.
4) Started the bike, 2nd level fast idle with RPM around 1k. Let the bike warm up for a minute (maybe 2).
5) Disconnected the clear tubing at cylinder #4 and capped the inlet - did not notice any change in engine performance
6) Reconnected the clear tubing to cylinder #4 and let bike run for a couple more minutes. - no change in performance
7) Reconnected the vacuum line on cylinder #1 - no change in performance
8) Did not notice any fuel in the clear tubing between cylinder #4 and the pressure regulator.

Any additional ideas on how to test the fuel pressure regulator?

Any other thoughts/suggestions on the source of the idle RPM drop?

Thanks in advance,
Monte

 

[98lee]

Do a couple of easy tests:

Get the bike thouroghly warmed up (go for a 15minute ride).

While the bike is idleing, push the starter button. If the idle increases, the bike is running lean (at idle).

Release the starter button and let the idle stabalize. Then insert the altitude plug (or jumper the altitude connector). If the idle increases, the bike is running rich (at idle).

Under normal conditions, the results of both tests will be a slight (like 50 rpm) drop in idle for BOTH tests or no idle change, indicating proper mixture.

Knowing whether the problem is too lean (vacuum leak) or too rich (injector, fuel pressure, or temp sensor issues) will help point you in the right direction.

 

[ChicagoGeo]

Reseat the brain connector?

My K1100 seems to idle smoother if I reseat the motronics connector. The L-Jetronic connector is sensitive, too, I bet.

 

[montec]

Additional testing this morning.

Reseated the L-Jetronic connector. Did not notice any signs of corrosion or other issues.

Reconnected the vacuum lines for the Fuel Pressure Regulator and Pressure Relief Switch and went for a short ride to warm everything up.

Initially the idle was low and upon departure the engine sputtered and experienced some after-fire (combustion in the exhaust). It was rough enough that I turned around after a tenth of a mile (or so) and headed back to the garage. Once back in the garage the idle had stabilized and returned to "normal" or at least what I expect for proper operation. No Choke at 900 RPM, 1st notch 1200 RPM, 2nd notch 2500+ RPM. I went back out to ensure that everything was warmed up and did not notice any additional missing, sputtering or after-fire.

After returning from the ride:
Pressing the starter caused a very minor increase in RPM.
Inserting a jumper into the altitude plug caused a very minor decrease in RPM.

The minor change in RPM was barely noticeable by ear.

For comparison, I pulled the vacuum line to the pressure regulator to ensure that there was a vacuum leak. Pressing the starter button with a known vacuum leak generated a much more noticeable change in RPM.

Any additional thoughts on what to check to try to pinpoint what happened?

Thanks,
Monte

 

[98lee]

So you are running slightly lean at idle.

That would suggest a minor vacuum leak.

Check the tank vent hose to the nipple at the front center top of the block just below the fan for cracks. If this hose no longer runs to the top of the block (a popular modification) check that the nipple is properly capped and there are no cracks in the cap.

Check the three vacuum caps on the vacuum ports on the three throttle bodies that don't go to the fuel regulator for cracks.

Reseat the computer plug a couple more times (take BOTH battery covers off and hold the backside of the computer while pushing the plug from the front until you hear a distinct click).

Try running some Techron fuel injector cleaner through the system.

You might want to check the fuel pressure (should be 38psi)

 

[montec]

Thanks Lee and Cele.

I had new fuel lines on my list of maintenance items for this year, so I replaced them along with the vacuum lines to the pressure regulator and the pressure relief switch.

Replacing the vacuum lines seems to have resolved the slight change in RPM when pressing the starter or jumping the altitude plug.


I replaced the caps on the throttle body ports last year and they are still in good condition.

I did not find a nipple or hose on the front center of the block. There is a place that looks like it could have been drilled for one. The vent from my tank drains into a cup that connects to a hose that runs down the right hand frame rail. Maybe mine never had the vent going to the crankcase?

I've reseated the computer plug a couple more times (as directed) and have added Techron to the current tank of fuel.

The fuel pressure checks out between 35 and 40 psi with the gauge that I have.


The curious thing is that the idle RPM drop with the vacuum line disconnected (now) is nowhere near as significant as the drop that started this thread.

I don't know how to troubleshoot a problem that isn't there anymore, so I suppose that I'll have to wait and see if the original issue returns and go from there.

Please share any additional thoughts that you have.

Thanks again for your help.

Monte