Intermittent Miss

treoleileagh said:
Since the pattern of the miss is to drop back to two and then one cylinder, I'm starting to think the double coil might be a candidate for replacement.
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There's a small flaw in your logic. If it drops back to two cylinders it can't be the double coil, otherwise you'd only be left with one cylinder straight off. Not two then one, because the two plugs are wired in series. If the coil fails then you lose both L and R spark plugs at the same time.

When my jota had an ignition failure on the centre cylinder, it reminded me of my old 1967 Triumph Bonneville. It went into parallel twin mode with a very similar exhaust note to the old Bonnie. Had about the same power too. :)


Edit: Having a bit of a head scratch on this. Having it fail in stages (first one cylinder then two) is a bit strange. Given that your bike has two independent ignition systems, it would require an intermittent failure in both systems, but not at the same time (otherwise you'd lose all 3 cylinders). That leads me to the conclusion that it's not an ignition problem, unless you have two dodgy spark plugs.

Likewise, it has 3 independent carbies. So if the jetting is OK, you'd need a problem in two carbs to cause your issues. That's not so unlikely. If you've had a shitty load of fuel, there could be gunk in the carbs.

Look at things that are common to all cylinders. Carb jetting, fuel quality, fuel starvation from a blocked tank breather, etc ...
 
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Dellortoman said:
There's a small flaw in your logic. If it drops back to two cylinders it can't be the double coil, otherwise you'd only be left with one cylinder straight off. Not two then one, because the two plugs are wired in series. If the coil fails then you lose both L and R spark plugs at the same time.

Look at things that are common to all cylinders. Carb jetting, fuel quality, fuel starvation from a blocked tank breather, etc ...
Click to expand...

Or dodgy earths.
 
I doubt it is fuel starvation from a blocked breather as the bike runs fine on high revs if I read it corectly and only produces the issue when going back to lower revs (below 4.000)

re-reading the first post, you have changed the idle jet already. Have you checked for water in the tank? i had a very similar symptome and it turned out to be Water in the tank. Water is heavier than gas and so it collects at the bottom of the tank. The pet-cocks tube might "just" reach into the level where the fuel sits and when you turn, the water level on one side rises and the water gets sucked into the carbs where they can block the idle jet. High revs open the main jet which is bigger and sucks the water through and burns it. Next time you go into a turn (below 4.000 revs I suppose...), water again can get into the carbs.

if it is not water, it might be insulation of the pickup-wires. Or a cable connection that is not 100% perfect sitting in the plug.

next to check: a broken contact at the ignition key-lock. Also check the fuse box, my brother had a similar issue with a fuse holder that wasn't tight.
 
Have had a good read of the original post and one thing that strikes me is that the fault occurs around the 3-4000 rpm mark which could mean that you are hitting a specific vibration band.
Would be checking all your cabling connections and earths.
 
By-pass the ignition switch and kill switch, check that you have a really good solid contact on your grounded cables, check the cable lugs on the battery cables. It is definitely an electrical problem, are you still running the original charging coils with the very suspect aged insulated wiring?HTH
 
One tricky issue I had coming back from the first Tassie trip was a broken earth wire about 50mm along the wire, I had a slight woopsy doing a Uturn on a steep hill and the battery hold-down strap broke allowing the battery to move and broke THAT earth wire. So give your earths a good solid shacking with the engine running and see what happens.
 
My Duc had a miss at 3K RPM. Turned out to be a bad earth/ground on one coil. There was just a slight bit of corrosion where it bolted to the frame.
 
most ignition related intermittent misses on the overrun tend to be due to induction side air leaks... spray a bit of ether around the manifolds and if the revs pick up you've found your ignition problem...
 
Dellortoman said:
There's a small flaw in your logic. If it drops back to two cylinders it can't be the double coil, otherwise you'd only be left with one cylinder straight off. Not two then one, because the two plugs are wired in series. If the coil fails then you lose both L and R spark plugs at the same time.

When my jota had an ignition failure on the centre cylinder, it reminded me of my old 1967 Triumph Bonneville. It went into parallel twin mode with a very similar exhaust note to the old Bonnie. Had about the same power too. :)


Edit: Having a bit of a head scratch on this. Having it fail in stages (first one cylinder then two) is a bit strange. Given that your bike has two independent ignition systems, it would require an intermittent failure in both systems, but not at the same time (otherwise you'd lose all 3 cylinders). That leads me to the conclusion that it's not an ignition problem, unless you have two dodgy spark plugs.

Likewise, it has 3 independent carbies. So if the jetting is OK, you'd need a problem in two carbs to cause your issues. That's not so unlikely. If you've had a shitty load of fuel, there could be gunk in the carbs.

Look at things that are common to all cylinders. Carb jetting, fuel quality, fuel starvation from a blocked tank breather, etc ...
Click to expand...
Hi Dellortoman,
Thanks for your further thoughts.
Well, I've fixed it, and it turned out to be a curly one.

In fact there were TWO separate issues with the double coil. The coil itself was breaking down after about ten to fifteen minutes which, as you pointed out, would leave only nr 2 cylinder firing. However, on top of that problem, the HT wire on one side of the double coil had come free of its electrode and was rattling around loose in its hole. As I figure it, when the dodgy coil's power weakened after a few kms, there was insufficient oomph from the coil to jump electrode to loose HT wire. Then as the coil weakened further, it would eventually fail altogether on both sides, leaving only nr 2 firing. When the coil would recover after being shut down for a few kms, the cycle would begin again.
Anyhow, I'm chuffed to have it running again.
Thanks again for helping me think it through and thanks to everyone who offered your ideas. I was growing desperate and I really appreciate the support.
Cheers
 
treoleileagh said:
However, on top of that problem, the HT wire on one side of the double coil had come free of its electrode and was rattling around loose in its hole. As I figure it, when the dodgy coil's power weakened after a few kms, there was insufficient oomph from the coil to jump electrode to loose HT wire. Then as the coil weakened further, it would eventually fail altogether on both sides, leaving only nr 2 firing. When the coil would recover after being shut down for a few kms, the cycle would begin again.
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interesting thing about the dual coil, lets call it the energy, it runs down lets say No 1 spark plug lead, jumps from the electrode across to the spark plug body, travels across the engine to No 3 spark plug area, jumps from the head itself to the electrode, travels back up the spark plug lead to the coil and completes the circuit
if you interrupt any part of that circuit you will lose both cylinders either totally or intermittently
 
redax5 said:
interesting thing about the dual coil, lets call it the energy, it runs down lets say No 1 spark plug lead, jumps from the electrode across to the spark plug body, travels across the engine to No 3 spark plug area, jumps from the head itself to the electrode, travels back up the spark plug lead to the coil and completes the circuit
if you interrupt any part of that circuit you will lose both cylinders either totally or intermittently
Click to expand...
I agree Red. You can't lose spark on just one plug in that system. It's both plugs working or both not working.

However, I can imagine a (theoretical) failure mode that would appear as though only one cylinder has failed. If a fault in a cable is making and breaking the circuit due to engine vibration, then it's possible that at certain frequencies, the intermittent break in the circuit could be synchronous with crank revs. So it might be connected when one of the outer cylinders comes to compression, but not when the other one does. Yeah, I know it sounds pretty unlikely, but stranger things have happened.
 
Dellortoman said:
I agree Red. You can't lose spark on just one plug in that system. It's both plugs working or both not working.

However, I can imagine a (theoretical) failure mode that would appear as though only one cylinder has failed. If a fault in a cable is making and breaking the circuit due to engine vibration, then it's possible that at certain frequencies, the intermittent break in the circuit could be synchronous with crank revs. So it might be connected when one of the outer cylinders comes to compression, but not when the other one does. Yeah, I know it sounds pretty unlikely, but stranger things have happened.
Click to expand...
How about this explanation... the breakdown caused voltage drop and consequent variable spark.
Glad it’s fixed, tricky things to test are coils. I’ve never actually had a faulty one myself, although I’ve replaced a few! 😁
 
This conversation has been valuable. I did not know the theory as Red explained it. I had thought the circuit was battery > coil > motor > battery, not coil > motor> coil.
My main reason for suspecting a coil breaking down was the ten to fifteen minutes or so from a cold start, before the problem would emerge.
However, I don't understand how a failing coil will perform better at high engine speed. Or for that matter, how the coil will overcome a bad connection to an HT wire at high engine speed. Is there more to it? Or perhaps it was just how I happened to have the throttle wide open when the coil recovered?
 
High voltage can jump gaps and blow past bad connections or poor insulation etc. If the fault is borderline then it may only appear intermittently, in the worst conditions eg hot, or in rain, or vibrating, or with just the right voltage on the primary circuits. In your case I can only guess at one or more of those, but the main thing is that the problem is now in the bin 😉
For the ignition type on the double coil, look up “wasted spark” if you wanted to do more research 👍
 
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