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 voilą, 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...???