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post #1 of 71 (permalink) Old 07-27-2010, 05:49 PM Thread Starter
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Engine Stutter Solved (Interesting Discovery)

For the past few months I've been experiencing stuttering from my 2009 EFI Thruxton under certain conditions. Through trial and observation I concluded that the problem is most probably due to a weak spark. The observations leading to this conclusion were:

-Stutter coincided with rich fuel mixtures, especially with sudden acceleration and fast increasing rpm's (sudden hard acceleration in low gears from low speed).

-It was also worse when the engine was cold and still on richer warm-up mixtures.

-Using higher octane fuel made the stutter worse.

All this suggested a weak spark struggling to ignite rich (or higher octane) mixtures.

So last night I decided to quickly check the basics around the coil in the hope of confirming my suspicions. Everything checked out OK except for the resistance between the coil core (HT ground) and the bike frame, I got about 1.5Ω when it should be very close to 0. So that confirmed the weak spark theory and just left finding the culprit.

Almost all bikes I've owned have just had the coil bolted via a bracket to the frame, thus grounding it. While simple and elegant this is a far from ideal setup, the brackets are usually steel and thus painted, not an ideal contact surface for electrical conductance. It's for this very reason that some people run a ground wire directly from the coil to the battery, claiming improved engine response.

Triumph's setup is even worse, I was amazed to find that the coil bracket is grounded to the frame via a little tab at the front merely touching the frame. The tab is slightly spring loaded against the frame by the flex in the bracket. That's it! The tiny contact area was, of course, rusted on my bike as well, explaining much, especially why the bike also tended to misfire more in the rain...

So, a short wire shunt was quickly constructed and firmly bolted between the coil core and the frame. Et voilà, resistance down to zero, and I'm happy to report the stutter is gone, and the bike running better than ever.

I wonder how many of the bikes stalling in the rain is also due to this poor coil grounding...???
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post #2 of 71 (permalink) Old 07-27-2010, 06:04 PM
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Thanks for your persistance and input on this issue. When I get to doing a valve clearance check and have the tank off, I'm ging to do a check of my own coils and their grounding.

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post #3 of 71 (permalink) Old 07-27-2010, 06:08 PM
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Interesting indeed. Probably the best thing is to run a wire to the battery negative. Is the coil in the EFIs the same as the one in the carbed versions, in which case I wonder how many of the previous wet running and other problems blamed on the pickup coil might have actually been this silly little issue.

EDIT: My Nology case is plastic. It can't be grounded. Duh.

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Last edited by NorthernThrux; 07-27-2010 at 06:41 PM. Reason: I'm a bozo
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post #4 of 71 (permalink) Old 07-27-2010, 06:12 PM
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I'm sorry, but most respectfully, I think you are seeing a placebo effect.
The case of the coil is not electrically part of the system;
The primary has positive power and the 'ground' is switched by the ECM;
The HT output of course is obvious and its return path is via engine ground and ultimately to the same grounding point as the ECM which completes the circuit.
Indeed many coils have plastic cases (have no direct experience of what the Thruxton one is: but from your report would believe you that it is metal - however irrelevant)
- as in point demonstrated by Northern Thrux example in post above.
The 'grounding wire to the can' mod is urban legend across multiple motorcycle sites - but it has no basis of fact in its actual operation and does absolutely nothing.

What your described stutter/surging problem really points to is actually a lean mixture as opposed to rich - this is very typical throughout all the Triumph range in attempt to meet emissions standards.
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post #5 of 71 (permalink) Old 07-27-2010, 06:50 PM
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I think Decosse might well be right but I must say that the wiring diagram dosnt help (at least mine) as it do's not show the spark plug thus the high tension earth via the spark plug?
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post #6 of 71 (permalink) Old 07-27-2010, 11:15 PM
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He got it working right, and that sure as hell beats walking home!

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post #7 of 71 (permalink) Old 07-28-2010, 12:14 PM
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decosse is right if the wires are hooked up it will fire hanging in the air.

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post #8 of 71 (permalink) Old 07-28-2010, 01:38 PM Thread Starter
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Nope, you guys are dead wrong about this. No placebo effect at all, with the ground wire disconnected, it misfires/stutters seriously, to the point of almost stalling, no way that's in my head. Connect the wire, stuttering is gone. Pity you guys are so far away, I could demonstrate it quite easily for you.

Yes, the housing is made of plastic, but look closely and you will clearly see the metal core of the secondary windings sticking out at both ends. The coil is in fact bolted down via the core ends. Why would Triumph have endeavored to ground the coil core if it's of absolutely no consequence?

All the above is fact boys, theorize over it as much as you like or call me a liar if you must. It's just a 10cm piece of wire that takes 3 minutes to fit. Certainly worth a try for anyone experiencing similar symptoms to mine, wouldn't you agree?
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post #9 of 71 (permalink) Old 07-28-2010, 01:56 PM
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I think guys here are assuming the coils fitted to EFI models are the same familiar all-plastic encapsulated ones from either Gill, PVL, Nology, etc

Our coils are entirely different, Japanese, made by TEC, model MB-10

The fixings are entirely different and although I haven't had the opportunity of having a close look at it, I certainly will now...

I still can't see why grounding the laminated core makes a difference as the primary and secondary windings are isolated from it (I think). With some single-ended coils this has to be done via the fixing bolts, but with our double-ended ones the secondary is isolated from the core.

Last edited by Forchetto; 07-28-2010 at 01:58 PM.
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post #10 of 71 (permalink) Old 07-28-2010, 02:49 PM Thread Starter
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Forchetto, it is not uncommon for the primary and secondary windings to be completely isolated from one another within the coil and thus NOT sharing a common ground. When this is the case, the secondary windings are often grounded to the core. I suspect this is most likely the case with our Tec coils, hence why Triumph bothered to ground the core (albeit poorly), and hence why my grounding exercise solved my misfire.

From the experts:
"Conventional canister or can style coils used with older distributor ignition systems usually have a common primary and secondary ground connection. High energy coils may use a similar design or have isolated primary and secondary windings. DIS coils may have isolated primary and secondary windings (typical of the waste spark systems), or a common primary circuit with an isolated secondary circuit. COP and CNP coils usually have a common primary and secondary ground junction."

"The metal core in a ignition coil could be part of the circuit and is the common or return or ground."

I should have checked this while I had the coil out... I will most certainly do so next time, when I fit my newly constructed (and highly conductive) aluminium coil bracket.

Last edited by Grubscrew; 07-28-2010 at 02:52 PM.
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