Hi,

I have an ignition problem which has got me a bit stumped.

I have a K100 which has been stood outside quite a while. It's mostly working but misfiring a bit on one cylinder.

I checked it with a strobe light and 3 cylinders look fine but the fourth if firing rather intermittently. I swapped the HT leads and the problem stayed with the same HT output. So that seems to eliminate the HT lead and plug.

I suspected a winding fault on one side of the coil. I swapped the coil for a spare but the problem persisted.

I then inverted the two low tension connections to the coil and the fault followed the green/yellow connection.

I'm having trouble making sense of that finding since, AFAIKT, there is just a single primary triggered by a single connection to the electronic ignition unit.

Now this does not make sense to me but maybe someone can tell me what I'm misunderstanding here.

Does this make sense to anyone?

TIA.

Beamer wrote:I then inverted the two low tension connections to the coil and the fault followed the green/yellow connection.

Would you explain this statement using the attached diagram extracted from the schematic on this site's Tech Page, or does this extract not conform to your moto's wiring?

I inverted the black/red and green/yellow wires.

When I did that, the misfire moved to the opposite cylinder on the same coil. In this case it's the coil serving cylinders 2&3.


The green/yellow was originally at the rear of the coil and the plug served by the rear HT was defective.
When I inverted the wiring the green/yellow was nearest to me and the misfire was on the HT nearest me.

On the failing coil, what is the resistance of the primary? What is the resistance of the secondary? Is there continuity between any secondary output and the ground wire?

primary = aprox. 2.2 Ohms
secondary = apron. 10,000 Ohms
coil to ground = infinite Ohms

Swap the coils and see if the problem follows the coil. Dud coil is my guess as its wasted spark, both leads on the coil fire at the same time.

robmack wrote:On the failing coil, what is the resistance of the primary?  What is the resistance of the secondary?  Is there continuity between any secondary output and the ground wire?

primary = aprox. 2.2 Ohms
secondary = apron. 10,000 Ohms
coil to ground = infinite Ohms

Thanks Rob.  I have 2 ohm primary and infinite from primary to ground connector , I'll check the secondary when I'm back on the  job.

Olaf wrote:Swap the coils and see if the problem follows the coil. Dud coil is my guess as its wasted spark, both leads on the coil fire at the same time.

I mentioned that in the top post:

Me wrote:
I suspected a winding fault on one side of the coil. I swapped the coil for a spare but the problem persisted.

The second coil was of unknown state but the way that  I see the same , rather odd, behaviour suggests the problem is not coil.

What is puzzling me is that the problem goes to the other output when I change to polarity of the input.

What is the internal structure of these coils? There is a single, isolated primary, that's pretty standard but I don't get how the secondary is structured.  It seems to be a single coil outputting a positive and a negative HT spike ( I have to invert the strobe lights pickup clip when going from one output to the other). This implies a centre tap to ground. That is not shown in the schematic.

Since the schematic shows a single secondary winding, I have difficulty seeing why this is affecting one output of the two but not the same output if I invert the wiring.

This does not make sense to me which suggests I have a misconception about something.

Beamer wrote:What is the internal structure of these coils? There is a single, isolated primary, that's pretty standard but I don't get how the secondary is structured.  It seems to be a single coil outputting a positive and a negative HT spike ( I have to invert the strobe lights pickup clip when going from one output to the other). This implies a centre tap to ground. That is not shown in the schematic.

Since the schematic shows a single secondary winding, I have difficulty seeing why this is affecting one output of the two but not the same output if I invert the wiring.

This does not make sense to me which suggests I have a misconception about something.

This is a schematic of the coil:



Each side of the secondary goes to a spark plug and thence to ground to complete the secondary circuit.  The primary is connected to a source of +12V on one side and to ground through the ICU on the other side.  The ICU coil driver is normally open but momentarily gets grounded and then opened again. When the current in the primary collapses, it induces a voltage in the secondary.  The current in the secondary travels in one direction, through the plug wires, arcing across both spark plug gaps. The energy dissipates quickly;

The ground wire goes directly to the iron core of the coil.  It's there for safety.

• P1 and P2 are primary connections.
  
• S1 and S2 are secondary connections.
  
• There should be no continuity from any primary connection to any secondary connection.
  
• There should be no continuity from any connection to the case, core, or mounting points of the coil.
  
• P1 to P2 will show continuity and will have very low resistance ~2.2 Ohms
  
• S1 to S2 will show continuity and will have very high resistance. ~10K Ohms
  

Beamer wrote:

robmack wrote:On the failing coil, what is the resistance of the primary?  What is the resistance of the secondary?  Is there continuity between any secondary output and the ground wire?

primary = aprox. 2.2 Ohms
secondary = apron. 10,000 Ohms
coil to ground = infinite Ohms

Thanks Rob.  I have 2 ohm primary and infinite from primary to ground connector , I'll check the secondary when I'm back on the  job.

Olaf wrote:Swap the coils and see if the problem follows the coil. Dud coil is my guess as its wasted spark, both leads on the coil fire at the same time.

I mentioned that in the top post:

Me wrote:
I suspected a winding fault on one side of the coil. I swapped the coil for a spare but the problem persisted.

The second coil was of unknown state but the way that  I see the same , rather odd, behaviour suggests the problem is not coil.

What is puzzling me is that the problem goes to the other output when I change to polarity of the input.

What is the internal structure of these coils? There is a single, isolated primary, that's pretty standard but I don't get how the secondary is structured.  It seems to be a single coil outputting a positive and a negative HT spike ( I have to invert the strobe lights pickup clip when going from one output to the other). This implies a centre tap to ground. That is not shown in the schematic.

Since the schematic shows a single secondary winding, I have difficulty seeing why this is affecting one output of the two but not the same output if I invert the wiring.

This does not make sense to me which suggests I have a misconception about something.

These coils do go faulty and in particular the earlier coils. its no surprise to get a second faulty one, been there on that one. I have spares and had to go to a third coil to get one not faulty. If one cylinder of a coil is good then the problem is in the coil or from the coil to the plug. Swap the plug leads on the coil and that identifies if its in the coil or the plug leads/plug/cylinder. Swap the coils and see does the problem move. If it does, then its coil issue.

Many thanks Rob, you seem to have a good level on knowledge on this.

What you show is the config shown on K100 schematic so that was my original assumption but I don't understand how that can be consistent with what I'm seeing.

On one coil output I have a good, regular flash on the strobe light. Picking up on the other HT lead, I get an intermittent flash.

My strobe light has magnetic pickup which clips around the HT lead so works from picking up the current in the lead. It's polarity specific and marked "this side toward plug".  I have to turn the clip the other way when I go from one output to  the other.

This is consistent with it being one current loop. The HT spike in the secondary pushes a current down one HT through the head and back up the other.

But in that case, you cannot have a spark on one plug and not the other. All or nothing, it's one circuit.

The strobe gets a regular signal from both outputs on the 1-4 coil as long as I flip the clip. ( If it's wrong, the strobe does not trigger at all ).

I cannot make sense of what I'm seeing here.

Olaf wrote:Swap the plug leads on the coil and that identifies if its in the coil or the plug leads/plug/cylinder.

Thanks. Already tried that before posting. Please reread the top post.

Me wrote:
I swapped the HT leads and the problem stayed with the same HT output. So that seems to eliminate the HT lead and plug.

Deleted, misunderstood the question.

To me that means on one side is to the coil and the other is from the coil that's why you have to turn it around.

Exactly. That's what I said.

So why do I see the strobe triggering correctly on one HT and irregularly on the other?

I wouldn't swap wires 15 and one around, that's going against the wiring diagram.

If you do not understand the significance of doing it, that would be wise.

Beamer wrote:
But in that case, you cannot have a spark on one plug and not the other. All or nothing, it's one circuit.

Current in the secondary is travelling around the circuit at the speed of light so you're going to see spark at both plugs at the same time.  If you had a very special high speed quantum camera, you'd see spark appear at one plug first and then the other.

Also remember that you can't see current; you can only see the effect of current.  A spark is the air around the gap igniting as a result of the current.  Current may still flow through the plug but not in a path that ignites the air.  What if the plug was contaminated around the insulator and current flowed through the contamination instead of across the gap?

Beamer wrote:
The strobe gets a regular signal from both outputs on the 1-4 coil as long as I flip the clip. ( If it's wrong, the strobe does not trigger at all ).

I cannot make sense of what I'm seeing here.

The pickup coil in your tool is polarized and needs to be oriented correctly to work.  That's why it works in one orientation but not the other, even on a good coil.

What was the result of swapping plugs?

 If you had a very special high speed quantum camera, you'd see spark appear at one plug first and then the other.

LOL, I'll leave relativistic quantum mechanics aside for now

Current may still flow through the plug but not in a path that ignites the air.  What if the plug was contaminated around the insulator and current flowed through the contamination instead of across the gap?

So far, I'm using the strobe lamp as a proxy for the presence of a spark. The plugs are still in place. I did clean and gap the plugs before even starting the bike, since I had them out for a compression check. I checked there was a good blue spark on each one but that was before I fired up and found the irregularity. That's probably more visible on the strobe than at the plug anyway.

I'm aware the pickup cct is polarised, that's why they mark it on the clip.

What was the result of swapping plugs?

I did not swap the plugs in the head but swapped HT leads at the coil. As reported above , the fault remained with the same HT output, thus excluding the HT leads and plugs as the problem. ( Now I realise it's one loop , that was probably a waste of time anyway ).

What I am unable to explain is why inverting the LT connections to primary moves the fault to the other outlet.

I tested primary. Both original coil and the unknown replacement were about 2 ohm. However, the original coil was open cct at the secondary. It looks like it has a fine break which is able to bridge under HT.  The replacement has 12 kohm, in line with what you indicated above.

I substituted the replacement coil but I'm still seeing the SAME oddities.

I'm unable to chase this down because I cannot make sense of what I'm seeing.

It does not make sense to me that I can pick up the current in one HT wire with the strobe but on the other wire its erratic. ( Obviously I invert the pick-up clip to preserve the sense ).

If you can make sense of that it's would be great.

Thanks.

If you are on NGK plugs don't dismiss the possibility of on being faulty out of the box. Been there.

But going on the previous posts the only thing I can point to is a bad coil and the replacement being bad. Both coil outputs should be the same and spark at the same time, if that's not happening there isn't another cause. 

In my opinion a replacement coil has a better than 50% chance of being faulty, especially if its the early type. I also found that I can't get the coil cover on if I mix early and later coils, they have to be the one type to get the coil cover to fit.

Yes, I obviously have to suspect the condition of the unknown replacement. That is why I said it was of an unknown state as soon as I mentioned it.

I think I have another pair somewhere, can't find them.

The windings of the ignition coil are wound in a certain direction around the core, and so primary and secondary coils are polarised relative to each other. If both windings are wound in the same direction, clockwise current flow in the primary will induce clockwise current flow in the secondary. The opposite is true if the two coils are wound opposite to each other; clockwise current flow in the primary will induce counter-clockwise flow in the secondary..

This is just a theory and I don't have proof.  Let's say the secondary winding is somehow defective; maybe it does not produce enough potential because of insulation breakdown.  So connecting the LT leads in one orientation causes clockwise current flow in both primary and secondary.  There is enough potential to bridge the first plug gap but not enough to bridge the second.  When you reverse the LT leads, the current direction reverses in both primary and secondary.  Now, with the weak potential, the other plug sparks but not the formerly working one.

Can you test by moving the 1&4 coil to the 2&3 spot, and visa versa?  The problem should move with the coil.

Rob wrote:There is enough potential to bridge the first plug gap but not enough to bridge the second.

I avoided biting on that first time but I really don't think it works like that. That would imply a build up of charge somewhere in the loop.

I'm wondering about a short to the core inside the coil.  I wondered about pulling off the brown earth lead to test it. That should mean any leak to core does not go to chassis and they both fire. 

Can you test by moving the 1&4 coil to the 2&3 spot, and visa versa? The problem should move with the coil.

Good idea. That means pulling the damned things off again because the HTs are not long enough for no. 1 and 2.

Grr.

Its only a case of swapping the coils over but only disconnect the ht leads from the coils, don't swap them over at the plugs.

The problem should follow the coils.

only disconnect the ht leads from the coils, don't swap them over at the plugs.

Yeah, that's the problem. My no. 1 and 2 leads are short and will not stretch to the 2/3 coil. So I have to dismount the coils and swap them.

Beamer wrote:I avoided biting on that first time but I really don't think it works like that. That would imply a build up of charge somewhere in the loop. .

It does work that way.  Compromised Insulation does breakdown under high voltage but its effects are not evident under low voltage.  That's why HIPOT testers exist.  The breakdown in insulation represents a resistance in the circuit that would not be there if the insulation was intact.  So at collapse, the voltage is spread across the various resistors in the circuit in proportion to their resistance and the resulting current -- the internal resistance of the secondary coil, the resistance of the spark plug wires, the resistance of the engine block between the two spark plugs,  the resistance of the air between the spark plug gaps and the resistance of the phantom insulation breakdown. If the insulation didn't break down, there would be no phantom resistance that appears under high voltage stress and therefor the other plug would fire. Again, I stress this is just a theory trying to explain the evidence you are observing.

I seemed to have missed it, but what year is Beamer's K100? If indeed it is an early model, I concur that the coil has an internal insulation breakdown.

I've lost track of the number of bikes that have passed through this forum with bad coils in the years I've been here. Seems like a couple a year, at least. Always an early coil. That must be why BMW changed them so quickly in the model progression.

Beamer's K100 is an '85 RT.

Always an early coil.

what do the later ones look like ?  Can't find my spare ones so I'm going to be looking.

Beamer wrote:

Always an early coil.

what do the later ones look like ?  Can't find my spare ones so I'm going to be looking.

The secondary terminal housings on the later coils usually are orange as illustrated here. The early coils are all-black.

many thanks.

That is exactly what I have been referring to as the replacement coil above. I was wondering where it came from but it was paired with an old style K100 coil and had the same mountings so I guessed it was a BMW coil.

I'm seeing the same behaviour, except this one is not o/c on the secondary as the first one was.

Can you use it to replace the 2&3 coil which is presently working? That way you can gain confidence that it is not defective.

robmack wrote:Can you use it to replace the 2&3 coil which is presently working?  That way you can gain confidence that it is not defective.

That is what I've already done !

As I just posted that is the coil I was calling the replacement above.  I have exactly the same odd behaviour.

It seems better in that it is not o/c on secondary winding. Primary is good. But I still get the same intermittent spark, exactly as described above. It's worrying that it is so similar.

I can't see this not being the coil and since I've worked out that a short to the core could explain this , that seems like the only logical conclusion.

A little gendanken experiment seems to say that an intermittent short to the core iron somewhere along the secondary could explain what I'm seeing.

Sadly, I don't have a third coil to substitute to test that hypothesis.

But if you exchange the 1/4 and 2/3 coils and the problem stays with the coil then it confirms the problem as being the coil.

OK, let me pause and reflect what I think the situation is based on this thread.

- Cylinders 1,2 & 3 are firing great.  Cylinder 4 is intermittent (first post you wrote: "I checked it with a strobe light and 3 cylinders look fine but the fourth if firing rather intermittently")
- But then you clarified that the failing cylinder was either 2 or 3 which contradicts your first statement (in post 3 you wrote: "I inverted the black/red and green/yellow wires. When I did that, the misfire moved to the opposite cylinder on the same coil. In this case it's the coil serving cylinders 2&3.")
- you swapped out the bad coil for a replacement but it also failed in the same way (In post 3 you wrote: "I swapped the coil for a spare but the problem persisted.")
- you confirmed that you have a spare coil of a later vintage with Orange secondary posts

So, here's what I'm asking.
- identify the coil pack that is failing.  Point at it.
- now, point to the other coil (which we are assuming is working perfectly). Remove the coil you're pointing at completely from the motorcycle
- take your later model coil with the orange secondary posts and install it in place of the one you just removed
- make doubly sure all the connections to all coils is to BMW specification (primary wires and spark plug wires connected properly)
- test your engine.

One of two things will happen:
1. the engine will work fine except that the cylinder that was acting up before the change continues to act up after the change.
2. now two cylinders will be acting up

If #1 happens, you have confirmed that your later model coil is in fine shape.  Take the original failing coil off the bike and replace it with the previously known good one
if #2 happens, you have confirmed that your later model coil is bad and you have to find one that is in good shape.  Swap back in the previously known good coil.

Me wrote:"I checked it with a strobe light and 3 cylinders look fine but the fourth [is] firing rather intermittently"

3 cylinders look fine, I did not specify which three ! The fourth means the other one, not #4.

Me wrote:the opposite cylinder on the same coil. In this case it's the coil serving cylinders 2&3."

Now I did specify which coil was playing up. No contradiction.

I'll follow your suggestion to put the two suspect coils on but not tonight. It's gone midnight here.

Thanks for the suggestion. That should give more info.

Was it cylinder two that was originally misfiring? I think you say that spark plug three was defective but that each plug had a good blue spark? Was it replaced with an identical one? Anyway I'd swap the coils rather than the primary wires, like with Olaf's first post. 

If the problem remains unchanged after the coil tests by Robert and Olaf, could it be a defective spark plug? (or HT lead) 

A faulty plug might send a weak pulse to the good plug with a weak strobe light on the lead to the good plug. If swapping the primary wires reverses the current direction, the good plug shows good and the weak pug has a weak strobe light. Or it's an alternating type thing. Swapping the plugs with 1 and 4 might verify it.

Could it be?

I think you say that spark plug three was defective

I realise all you fellas are trying to help but it frustrating the number of times folks are reading stuff which isn't there.

All I said about plugs is that they were all cleaned and gapped before I even attempted to start the motor and that they all had a good strong spark.

A faulty plug might send a weak pulse to the good plug with a weak strobe light on the lead to the good plug.

That's not how electricity works. If you want a simple analogy think of a pipe full of water. The electrons are already in the circuit, they don't need to travel all the way and go through one plug then the other. As soon as current is flowing in one end, current is flowing in both spark plug gaps and current is coming out of the other end. It's a bit more subtle than that due to the capacitative nature of electrical breakdown of air entering the plasma state ( the spark ) and the inductive circuit of a coil but loose talk of one plug "sending" something to the next one if not informative.

The only explanation I can find for the two HT leads showing differently on the strobe light is a secondary circuit being formed by intermittent, internal leakage to the core of the coil.

All the wisdom here suggests the newer coils don't have that defect. Sadly this one looks the same.

How's about reading your reply in post three where you said "the green/yellow was originally at the rear of the coil and the plug served by the rear HT was defective"

We're trying to help but if you considered the help instead of opposing it, you would be helping yourself. For example you opposed Olaf's early suggestion to swap the coils by saying you already did by replacing one of the coils. Replacing one of the coils is not the same as swapping the coils.

We still don't even know which cylinder was originally faulty. 

Most people are happy for offers of help.

Me wrote:The green/yellow was originally at the rear of the coil and the plug served by the rear HT was defective.
When I inverted the wiring the green/yellow was nearest to me and the misfire was on the HT nearest me.

OK, I can see how that was poorly worded but if you take both lines together, instead of taking one out of context. It is clear the plug itself was not defective since problem shifted when the LT wiring was changed.

Again, please note all testing is done with bike running and using pick-up clip strobe light to detect presence of a spark or not.

Plugs were not directly observed after initial clean and test where all showed good strong spark.

OK, testing two unknown coils as suggested by Robmack.

new model coil in place of the suspect old coil cyl 2/3

old suspect coil in place of good coil cyl 1/4

Results:

old suspect coil: Intermittent spark on HT outlet closest the green/yellow primary ; good on other HT.
new style coil : unable to trigger strobe light on HT outlet closest the green/yellow primary; good on other HT.

Please note "intermittent" spark is referring to detection by strobe light, not visual observation of a spark. 

The motor still starts and runs rather roughly , not obviously different than before, though intermittent "spark" was present a good proportion of the time but irregularly so.

Old suspect coil showing same behaviour now in position 1/4. Problem follows coil not HT leads / plugs.

Maybe for some odd reason the strobe is triggering less reliably on one side of these dual output coils and the uneven running is from some other cause !

There is definitely a fault on the old 2/3 coil since its secondary shows o/c on a resistance test, despite actually working under HT. It does need replacing but I'm less sure that's the real problem.

Uneven running.....

Bobbins missing/loose?
Cracks inside HT lads [especially if original]?
Either of the above can bugger a coil too.

Z pipe cracked?
Vacuum pipe to FPR cracked?
Vacuum cap/caps cracked?

Thanks Olaf.

I've just patched up one of the little rubber caps which was cracked. I had ordered some new ones from tills.de but the order got a bit messed up and they got left off. Probably my fault .
Vacuum pipe to FPR OK.

What's the Z-pipe ? What are bobbins?

There is a vent pipe to the left of the coils, goes from the engine into the air intake box. If it cracks it buggers the tickover and running too. It's cheap and can be replaced from the coil side. If it's old it cracks easily and the moving about coils may have affected it.

The vacuum pipe to the FPR can do the same and crack inside the protective spring or at the connection on to the FPR.

Yours should not have this, but in case it does, on some Ks there is another vacuum pipe at No 1, goes around to a vacuum sensor. Disconnect and put a vacuum cap on it.

Beamer wrote:What's the Z-pipe ? What are bobbins?

It's a small molded hose that connects the crankcase to the plenum for the purpose of recycling blow-by combustion gas into the fuel air mixture. If not involved in textile production, bobbins are like doodads only with engineering specificity. 

Photos are from Euro Motoelectrics, a USA source of BMW moto aftermarket parts.

Thanks Laitch.

Yours should not have this, but in case it does, on some Ks there is another vacuum pipe at No 1, goes around to a vacuum sensor. Disconnect and put a vacuum cap on it.

Yes, mine does have that. Though the pressure switch is not connected :?

I'd trimmed the end which was a bit cracked, put it on the list for replacement. Good idea to blank it off. 


What was bobbins about ?

Thanks for the pic Laitch.

One item that hasn't been mentioned during our failing attempts at assuaging your engine performance frustration, Beamer, are the terminals of the spark plugs to which the HT leads attach. If your spark plugs don't have terminal nuts—which, incidentally, fall into the bobbin classification—but simply have a screw shank as a terminal, the plugs likely make only intermittent contact with the HT lead terminal, depending upon the HT lead's terminal design. Sometimes that contact is prolonged but sometimes it creates leakage by being loose. That's enough to cause coil death by chronic hissy-fits.

The HT terminals should fit tightly onto the spark plug terminal with neither wiggle nor wobble.

By bobbins, I think Olaaf might be referring to the rubber caps used to plug the brass vacuum pipes on the throttle bodies.  Those tend to crack or go missing, leading to unmetered air going into the engine and causing rough running.

robmack wrote:By bobbins, I think Olaaf might be referring to the rubber caps . . .

Those parts certainly fall into the bobbinhood category. 

Laitch wrote:One item that hasn't been mentioned during our failing attempts at assuaging your engine performance frustration, Beamer, are the terminals of the spark plugs to which the HT leads attach. If your spark plugs don't have terminal nuts—which, incidentally, fall into the bobbin classification—but simply have a screw shank as a terminal, the plugs likely make only intermittent contact with the HT lead terminal, depending upon the HT lead's terminal design. Sometimes that contact is prolonged but sometimes it creates leakage by being loose. That's enough to cause coil death by chronic hissy-fits.

The HT terminals should fit tightly onto the spark plug terminal with neither wiggle nor wobble.

That's good point. When I bought this bike I got it cheap because it was making a horrible clacking noise on tickover. The owner had given it to a couple of local "bike repairers" who obviously had zero experience on these bikes.

The first thing I found was an HT cap completely eroded inside. It was obviously arcing and got eaten away. I don't see that would be a problem for the coil. If the gap got really large it may breakdown at a higher HT than the plug. But the coil should handle that.

There was also a rotten depression tube to FPR a badly split sleeve on the injector ramp.

All this was making tickover very lump and causing the sprague clutch on the starter to catch. I sounded like it was about to drop a rod.

Currently #1 cap is a little slack , though it does snap on. However, I've already established through swapping HT leads and LT supply that the plug-HT combos are not what was causing the ignition anomalies.

I did have a rather flakey Z-pipe which I've swapped with a better conditioned spare one. Good idea by Olaf.

Also just discovered that the throttle by-pass screws do not make any difference on the vaccuum gauges and the throttle stop screw is not hitting the stop. I need to chase all this out.

Time for a cool beer and another try ...

OK the throttle cable was probably not seated properly where it went into the ramp. By the time I took air filter out to check it , it had fallen back into place.

All running a lot better now.  Looks like the strobe light was misleading me. I'll worry about explaining that another day. At least I picked up a coil which was o/c and needed replacement.

So it's better but not right.

To balance the vacuum gauges I needed to pull #3 3.5 turns out and #4 pretty much all the way home. #1 and #2 about 1/5 turns, OK.

Also can't set the bypass on the air flow meter.  It does not seem to affect tickover. So some things clearly not right yet.

It is running fairly well at tickover and throttle response is good. Haven't done road test yet because the front break needs an overhaul too.

Have you checked the valve clearances? They have an effect on throttle body balance, minor, but noticeable. You might want to remove the air screws and shoot a bunch of carb cleaner down the holes.

If all that doesn't help, you probably have air leaks in the boots that connect the throttle bodies to the plenum chamber and the cylinder head.

One thing to note on the plug caps.

Sounds like they are original. BUT, the condition they are in can and does destroy the coils. If original a good idea to replace them as they are damaged if loose fitting or used without the bobbins. The description of the inside of the cap eaten away confirms they got used without the bobbins. This is going to cause issues on sparking to needs to be fixed before going too far into other areas. Us of carb cleaner can verify if you do actually have any leaks.

The bobbins are needed for most of the NGK plugs if they show screw threads on the top of the plug, if not fitted you get arcing [ticking sound] and that's got bad consequences for the plug caps and the coils.

BSK Speedworks at Leighton Buzzard can send you HT leads.

Also, if you go checking valve clearances, be very very very careful. The bolts on the cam/valve cover are meant to be very little more than hand tight. Your K has been sitting out so the 'rubber' seats are very likely hardened. Not cheap but replacement of the rubbers is recommended because new ones are soft. One of these bolts goes into a camshaft bearing cap and is very easily stripped with disastrous consequences. New soft washers and little more than hand tightening should mean no problems.