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My engine recently started developing less power and the tach jumps up and down (200 RPM either way) at idle and somewhat less a higher speeds. I pulled the plug wires individually while the engine was running - no apparent effect when I pulled the #2 cylinder lead but the engine nearly stalled when any of the other were pulled. Any ideas other than a coil issue?

Thanks
 

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Junior Grue
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I doubt if it's the coil as that should affect both #2 and #1 but a faulty spark plug, plug cap or high tension wire could be a possibility.
 

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Stillmight be a coil issue.. As Ken says, 1/2 in series and 3/4 in series so pulling a wire should "dump" both.... and since #2 didn't act like that, the coil could be shorted near the #2 high tension lead side (maybe a crack in the case?)... of course, check Ken's list first.. as they are most applicable.

Just for information, it is not a good idea to check spark by just pulling plugs. It causes very much higher voltage in the coil that trys to find a path to ground.. and that might be weak insulation in the coil.. a better way is to remove all plugs.. keep them grounded and crank the engine to observe spark... (becareful of fire potential at plug ports..
 

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sandiegobrass wrote:
Stillmight be a coil issue.. As Ken says, 1/2 in series and 3/4 in series so pulling a wire should "dump" both...
Jim are you sure that it would kill both?
On two cylinder opposed twins with one coil wasted spark I have seen one spark plug boot knocked off and the engine continued to run although poorly.
Of course the loose boot could have been arcing to ground but that was many moons ago and Oldtimes has reared its ugly head here.:lash:
 

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Ken Bergen wrote:
sandiegobrass wrote:
Stillmight be a coil issue.. As Ken says, 1/2 in series and 3/4 in series so pulling a wire should "dump" both...
Jim are you sure that it would kill both?
On two cylinder opposed twins with one coil wasted spark I have seen one spark plug boot knocked off and the engine continued to run although poorly.
Of course the loose boot could have been arcing to ground but that was many moons ago and Oldtimes has reared its ugly head here.:lash:


Yes.. sure in the sense that this is the way it was designed.... but because of shorts and weak insulation (jumping sparks) this is not always the observation.

The secondary of the GL coil does not have a center tap and can only complete the circuit if both plugs are connected or if there is a short or jumping spark where there should not be one. It is these latter items (shorts and jumping sparks) that would allow it to run.. and of course, not knowing the bike you are referring to, I can't say if the plugs/coil were in series or parallel (with a coil center tap) on the twin.

And the above is also the reason that you chould not just pull off a lead..

There was a "warm" discussion on this several years ago with some diagrams.. if I can find it, I'll link it HERE. There are some brken links for images in that thread that I'll try to find/fix later..

I'll add a diagram from the GL1100 manual that shows the series arrangements of the plugs and coil in the secondary.. basically all GLs are the same in the secondary high voltage loop.. [some differences in the primary loop.. (multiple spark boxes, ballast resistor, etc.) but not pertinent to this discussion.]

 

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More likely a clogged idle jet..easier check; spray a little gas into the air box while it's running..does #2 pick up? What does #2 plug look like?? wet, dry , black dry, sooty?
 

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Might be a good idea to check the main ground for a good connection.
 

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Thanks guys, you've given me some good points to start investigating. I'll let you know how it turns out...
 

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I've pretty much ruled out fire as the culprit and have pulled the carbs. Something I've always wanted to tackle and decided why not now. The first thing I've discovered is that where I can now see into the chamber in the head, under the carbs, the whole cavity and exposed valve stems have quite a bit of sludgy looking build up (that's a technical term). I'm not sure what it should look like - comments please. And is it something I should try and clean? How? If I get the chance I'll try and post a picture or two.
 

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