Ignition problem sorted but I'm left curious.....


Ignition problem sorted but I'm left curious.....

Postby CBX1000chris » Thu Nov 14, 2019 11:50 am

.... and I hate that!

Hello everybody. I have a thread running under Engines - low compression - that gives some background on the 'special' I'm working on for a friend. Anyway, as it's a special being built from scratch I'd previously checked the ignition wiring (some of it home grown, some Honda original) and found a pair of coil wires swapped so I'd reversed them (didn't make a jot of difference as far as starting bike was concerned). While the engine bits are away for checking I returned to the ignition system for another look, I'd decided to do the test of making a bridge between the pulsar rotor and each pick up in turn and actually make sure the right plugs were sparking. It occurred to me that laying the plugs on the head and turning the engine over tells you have sparks but they might actually be the wrong ones! For those Brits who remember Eric, "I am playing all the right notes, but not necessarily in the right order", that sort of thing. I'd found the previous wiring error through visual inspection. Anyway, enough of that.

Working on the right side of the bike I popped a plug in leads 4, 5 and 6 and rested them on the engine (what's left of it), plug leads 1, 2 and 3 were lying across the top of the frame with no plugs fitted. To my horror when I bridged to the #1,6 pick up, plugs 4, 5 and 6 all sparked. Now I don't claim to be a world class diagnostician (admit it, part of us would all like to be House MD) but I was pretty sure the CBX has effectively 3 independent ignition circuits so this had me foxed. I convinced myself it was cross talk in the non-standard wiring between the pick ups and the spark units, so much so I disconnected the #2,5 and #3,4 pick ups, (an electrical broad spectrum antibiotic?), I was surprised when I bridged to the #1,6 pick up and again plugs 4, 5 and 6 all sparked (maybe it was Lupus after all!). With 20/20 hindsight I should have realised the signal strength from the pick up couldn't induce any signal into anything other than what it was meant to. I realised at this point I must be doing something stupid so referred to the web and in particular Mike Nixon's articles on ignition - thank you Mike.

Almost straight away I realised that leaving the plugs out on the 1, 2 and 3 cylinders meant that all three the secondary (HT) circuits were basically open circuit as far as a spark was concerned given the way the CBX HT system works (two plugs in series). When I fitted plugs to all the HT leads and all plugs were earthed the problem went away, in fact even without the plugs fitted the problem went away if I moved the leads away from the top of the frame and ...... wait for it ...... the coils. I've pondered this for a while and the only explanation I can come up with is that with plugs missing from the 1,2 and 3 leads I created an aerial in each of the three secondary circuits so all three reacted to the collapsing field on the #1,6 coil when the leads were in close proximity to the coil and produced a spark, albeit a weak one.

Does this make for an sound explanation or am I talking ball cocks?

Sorry for the ramble but I'm no longer fighting a problem, just curious and chewing the fat. For those who want a trip down memory lane I've pasted a link below, try not to think it was nearly 50 years ago. How on earth did that happen?

https://www.youtube.com/watch?v=uMPEUcVyJsc

Regards,
Chris.
CBX1000chris
New Member & Happy To Be Here
New Member & Happy To Be Here
 
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Location: Devizes

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