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Question on ignition coil

Mark K

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Evening All,


I've been experiencing a slight and occasional misfire at idle this year that I don't remember from last...


Not wanting to remove the tank at this point, I changed the plugs and dumped some Techron in the tank. While there, I thought I'd check the resistance from wire end to wire end (left side to right side) and came up with 20.4k. This is at odds with what my Haynes manual indicates by quite a lot if I'm reading it right (it says 13k per side). What should I be expecting??

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Evening Mark


That depends on the color of your coil & temp of coil/wires--


Coil (winding) temp has some effect on coil resistance so close is usually good enough.


The resistance in the old (gray) coil is usually pretty close to the manuals I have at 13K ohm for the secondary and 0.5 for the primary.

The (later) black (black/reddish)coils are around 7K for the secondary and about 1 ohm (or just under) for the primary.



Plug wires are kind of iffy but most BMW plug wires are in the 5 - 5.5K range.


Another test is to open an old spark plug up to have a 3/16"+ electrode air gap then see if the thing will put a (snappy) bluish spark across that gap. If not then suspect an internal problem in the coil windings.


Coils very seldom if ever cause problems at idle that they don't act much worse under higher RPM's & under higher engine load.


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Thank you for your response.


I can't recall the color of the coil, but from what you describe, and from reading some of your other related posts, it seems unlikely that a bad coil is the culprit.


Again using the minimally invasive approach, I removed my (yellow) CCP and went for a spirited ride. Idle speed dropped to the point where I had to engage the 'choke' to the first stop to prevent it from dying at stops, so it was impossible to determine if idle consistency had changed.


At this point, I should mention that I recently rebuilt both throttle bodies using Dan Cata's kit and had to set the TPS at the lowest end of the acceptable range in an attempt to bring down RPMs at idle (yet still remained a little high IMO). This can be easily remedied.....


Replaced the throttle cables in the process and TBs are very precisely balanced using a water manometer. Motor pulls consistently and very strong up to rev limiter at EXTRA-legal speeds. Now I am leaning toward a flaky O2 sensor.


Am I on the right track?

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Morning Mark


Something's not right there-- As a rule removing the CPP should raise the idle RPM's not lower it.


It's possible you have a lazy or bad o2 sensor so that is a good place to look. (on the 1100 removing CCP deactivates o2 sensor use)


My suggestion is to get the TPS set back to where it belongs (that should raise the "CCP-Removed" idle RPM's a little higher. TPS re-setting to near max usually makes them run better also (don't go above .395v at curb idle choke off)


If you can't get the idle RPM's back down to where they belong with a .390v-.395v TPS setting then re-verify that you have the throttle plates centered in the throttle bores & that the base idle screws are set correctly (if you used 0=0) that doesn't always work correctly without a little extra tweaking.






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Afternoon dirtrider,


Finally getting back to this...


I left the CCP out and reset the TPS to .393 as you recommended (originally set to .370. I turned the BBSs out to 1 1/2 turns from closed and, once warmed up a bit, rebalanced at idle using the BBSs. Idle RPMs increased, but only very slightly. Reinstalled the CCP and idle RPMs come up once again. The miss at idle remains. There is something else going on here as you said.


I loosened all the cables and idle stops and removed the throttle bodies again to inspect the plates by shining a strong flashlight through them from the opposite end. I was not happy with one and tried to reinstall it the other way (it appears there are only two ways these can be installed due to the beveled edges). This looked worse, so I put it back the way it was. This is as good as it gets.


At this point, I think I'm down to a careful 0=0 and resync, hoping for a better outcome.




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Afternoon Mark


That throttle plate fit doesn't look all that good (any signs of throttle body throat wear from the old plates moving latterly & wearing the sides?)


That could explain your idle RPM issue but shouldn't really cause a miss at idle as long as the side to side TB balance is close.


Did you try loosening the screws, backing the idle stop screw out then repeatedly lightly closing the throttle as far as possible to see if that will center the plate in the bore?


On your idle miss-- I would maybe look more to a dripping fuel injector, or poor injector spray pattern, or a loose intake valve to valve guide, or a worn cam lobe (something down those lines anyhow)


OR-- is it actually misfiring at idle or do you just hear a punt/punt or light pop in the exhaust note? If it is more audible than actually feeling the bike shake then maybe look for an air leak into the exhaust at idle causing some post firing. Look for a crack in top rear of cat converter (somewhat common), or a leaking joint, or look into the rear of muffler for a rotting inner pipe or holes in the muffler innards.


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  • 1 month later...

Finally getting back to this.....short answer is all is well again.


I first checked out each of dirtrider's suggestions and found no glaring deficiencies. I then went back for a complete do-over of everything I had touched since the bike ran well (new throttle cables, plugs, Zumo install, TB rebuild and balance, TPS setup and 0=0 -- there may be more, but those are the highlights). I believe it was a bad 0=0 initially, but I can't be sure as I did not check running status as I went. The bike runs perfectly now, as it always has. In fact, it may run even smoother than usual (especially at idle) as I can do a much more precise sync with the new TB shafts and bushings.


Thank you again for your directives and insights. They once again proved invaluable.

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