K100 running on 3 cylinders when hot

[zipster68]

Hi all,
Wondering if anyone can shed any light on this.  My '86 K100RS seems to be running on 3 cylinders (rough idle and runs rough taking off from stop) when it's hot.  Runs perfect until it reaches full operating temp.  Runs perfect at rpm over 2k. 

I've done the following:
DeOxit cleaned the Motronic connectors and both 4 pin connectors
checked the vent tube
sprayed around the throttle body boots to check for leaks. 
I've run injector cleaner through a few tanks of gas, but the tank is super clean inside. 

This is a low mileage bike.  If anyone has any ideas or leads I could check out before taking this to the dealer and throwing money at it, I'd appreciate it.  Hopefully it isn't the Jetronic or Motronic units.  Maybe coils?

Cheers,
Ray

[wmax351]

Check the hall sensors. There should be a writeup here.

[kennybobby]

What do the spark plugs show--dark and sooty, or dry and ashy, etc...?

[alcorelli]

Cracked plug ceramic insulator?
Bad plug (s)?
Ignition wire?
I wouldn't rule out the rubber around the throttle bodies.

Sent from my SM-N920P using Tapatalk

[zipster68]

throttle body rubber shows no leak when sprayed with gum-out.  It looks new.  Haven't checked plugs as they are pretty fresh, but that's easy and will look at it tonight. 

Regarding Hall effect sensor, wouldn't it kill the whole engine, and why would it get better at higher rpm's?  I do see that they're temperature sensitive when they fail though.

[wmax351]

Regarding Hall effect sensor, wouldn't it kill the whole engine, and why would it get better at higher rpm's?  I do see that they're temperature sensitive when they fail though.

There are 2 of the sensors, 2 cylinders mapped to each, and when they fail, it isn't always an all or none deal. So you can get a 2/4 cylinders out, or a partial loss on 2 of the 4, causing rough running. Easy to test too.

[zipster68]

Aha, that sounds more like it.

There are 2 of the sensors, 2 cylinders mapped to each, and when they fail, it isn't always an all or none deal. So you can get a 2/4 cylinders out, or a partial loss on 2 of the 4, causing rough running. Easy to test too.

[Scott_]

How many miles, and when we're the valves checked last?

[zipster68]

22,000 miles, and unknown regarding valves.

How many miles, and when we're the valves checked last?

[The Mighty Gryphon]

A 30 year old bike with 22,000 miles has spent a lot of time with stale gas in the fuel system.

The injectors are triggered in unison, and the plugs are fired in pairs, so I would think the problem is downstream of the engine control unit and the coils. 

First thing is to find which cylinder isn't firing, and replace the plug or swap it with another cylinder and see if the problem follows the plug.  Problem might also be the plug wire.

Second, the stale fuel deposits can take thousands of miles to completely remove.  I would begin a regimen of Techron and, if you can easily find it, ethanol free gas.  It could take as many as 10 tanks to clean everything up, but I would expect a noticeable improvement in as little as a couple weeks and 500-1000 miles. 

Third, if you can identify the cylinder, you could pull the injector and clean it.  There are videos on YouTube showing ways to clean them.

[zipster68]

Thanks Gryphon,
The tank cleanliness is pristine, but a clogged injector is a possibility (the way it clears out at higher rpm).  Not sure why it would only be an issue when hot though.  I'm going to try to isolate which cylinder this weekend.

Cheers,
Ray

A 30 year old bike with 22,000 miles has spent a lot of time with stale gas in the fuel system.

The injectors are triggered in unison, and the plugs are fired in pairs, so I would think the problem is downstream of the engine control unit and the coils. 

First thing is to find which cylinder isn't firing, and replace the plug or swap it with another cylinder and see if the problem follows the plug.  Problem might also be the plug wire.

Second, the stale fuel deposits can take thousands of miles to completely remove.  I would begin a regimen of Techron and, if you can easily find it, ethanol free gas.  It could take as many as 10 tanks to clean everything up, but I would expect a noticeable improvement in as little as a couple weeks and 500-1000 miles. 

Third, if you can identify the cylinder, you could pull the injector and clean it.  There are videos on YouTube showing ways to clean them.

[The Mighty Gryphon]

It's possible that when cold or at high rpm's the injectors are being fired harder to put out more fuel.  At low rpm's when the engine is hot the mixture is leaner and less fuel is needed so the injector signal doesn't hit them hard enough to fully open a sticky pintle.

[The Dude]

Best check valve clearences,whilst your at it.Very easy with feeler gauges.

[zipster68]

I've done vintage Gold Wings, but never a Brick.  At least they're all on one side (i'm lazy) on a brick.  Guess I need to buy some of those BMW feelers and read up on it.

Best check valve clearences,whilst your at it.Very easy with feeler gauges.

[The Dude]

Just buy the feeler guages from any engineering suppliers.The adjustment is done by shims(BMW)but it is to know that that is  not the cause  of your problem

[The Mighty Gryphon]

You only need eight feeler gauges, .006", .007", .008", .009" for intake and .010", .011", .012" and .013" for the exhaust. 

[Laitch]

You only need eight feeler gauges, .006", .007", .008", .009" for intake and .010", .011", .012" and .013" for the exhaust.

I use a 0.15mm and 0.20mm for intake; a 0.25mm and 0.30mm for exhaust.

[zipster68]

Thanks guys.  These are solid leads.  My plan.
1.   Find out which cylinder isn’t firing when hot (sequentially disconnect wire while running)
2.   Run in the dark and look for arcing
3.   Check valve clearances (more involved)
4.   Plugs, wires (easy), coil, control unit
              a.   The ignition is working with a wasted spark...firing on every revolution...one coil serves cylinder 1 & 4
                        the other cylinder 2 & 3.  Try to swap cylinder 2 and 3 at the coil terminals, if the problem moves to #3...the coil is your bad guy
                        (check the output terminal for corrosion, also).
                        If the problem is still at #2, the check wire, cap (and plug, could be a bad one in the batch of new ones).
5.   Hall effect sensor (there are two)
6.   Partly clogged injector

[Scott_]

Not sure that pulling a plug wire will help you much. Like you mentioned and others here know it is a wasted spark system. The coil needs both leads connected to plugs to properly complete the circuit and fire. Pull one wire and you kill the spark on 2 cyl.
A bad wire you could have, worth checking, but I would suspect that would show at any temperature.

[zipster68]

Really, you pull one wire and they both stop firing?  Vintage Gold Wings use wasted spark, and they don't do that, pull one the the other keeps firing.
-Ray

Not sure that pulling a plug wire will help you much. Like you mentioned and others here know it is a wasted spark system. The coil needs both leads connected to plugs to properly complete the circuit and fire. Pull one wire and you kill the spark on 2 cyl.
A bad wire you could have, worth checking, but I would suspect that would show at any temperature.

[Martin]

You need to keep the wire you are testing earthed otherwise damage to the ignition unit can occur. I have a cheap laser temperature gun that is handy for showing up faulty cylinders. I just point it at the exhaust header one at a time, if any one is significantly cooler than the others there is a problem. I paid about $80.00 Au about 5 years ago a mate got two from China for $30.00 Au each, and they are a useful diagnostic tool for quite a few applications.
Regards Martin

[Brad-Man]

If only one cylinder is not firing the problem is not in the coils or hall sensors.

If one of those 2 cylinders would not be firing.

Spark plug lead would be cheapest/easiest to ascertain.

From there you have valve or compression issue.

[zipster68]

Replaced coils, running great now!
Thanks for your input.
-ray

[K1300S]

glad you got it going!

and thanks for checking back in with the solution.  that will help someone in the future.

[The Mighty Gryphon]

Another case of early ignition coils acting up.  More proof that they are the first place to look when an early K100 starts acting up.