Arcing points

[Aye Gee 1977]

It's a standard battery and coil system
Points 1-4 are arcing excessively - the spark is much the same as at the plug and it causes a misfire, which clears for a while when the points are cleaned.
The condensers are new pattern DS parts; I've tried swapping them over with the 2-3 set and also returned the original (40 years old) one.  Whilst you can convince yourself that there is a marginal difference, before long it's points 1-4 that are troubling.
The LT resistance of the coil is 4.2 ohms (same as the other coil).  I can't yet see how it can be caused by the HT side but nevertheless it has a resistance of 30.9k ohms (only 23.4k ohms for coil 2-3).  The plug caps are 8.3k & 8.6k (4.3kx2 for plug caps  2-3 - which are a longer reach type).
New set of NGK plugs.
Any thoughts on what is causing the arcing would be most welcome

[taysidedragon]

Arcing at the points usually means a faulty condenser. The new pattern ones don't last long at all.

[McCabe-Thiele (Ted)]

I agree with the most likely cause being the Condenser,

My 500 had an extra pair of condensors fitted externally at the coil ends fitted by the PO as an experiemnt.

As I only rode the bike for about 30 miles before the current rebuild I am unable to say if this improved things.

[K2-K6]

It's a standard battery and coil system
Points 1-4 are arcing excessively - the spark is much the same as at the plug and it causes a misfire, which clears for a while when the points are cleaned.
The condensers are new pattern DS parts; I've tried swapping them over with the 2-3 set and also returned the original (40 years old) one.  Whilst you can convince yourself that there is a marginal difference, before long it's points 1-4 that are troubling.
The LT resistance of the coil is 4.2 ohms (same as the other coil).  I can't yet see how it can be caused by the HT side but nevertheless it has a resistance of 30.9k ohms (only 23.4k ohms for coil 2-3).  The plug caps are 8.3k & 8.6k (4.3kx2 for plug caps  2-3 - which are a longer reach type).
New set of NGK plugs.
Any thoughts on what is causing the arcing would be most welcome

That looks considerably out of range, you'd really need to correct this to assess fully.

You can run the cylinders off opposite points if you want to try and check system variance.  Swap the points cam on advance and retard mech 180 degree, then swap the points to coil wires over between each other ..... cylinder 1&4 will now be running on 2&3 points and visa versa.

[Aye Gee 1977]

Thanks to all.  I accept the condenser is the most likely culprit, and I'll check again.  It's just that having tried three separate condensers, I was looking for another possibility.

The plug caps could be renewed; at this stage I can't quite see how an issue on the HT side can cause a problem on the LT side - but maybe it can!

[K2-K6]

Thanks to all.  I accept the condenser is the most likely culprit, and I'll check again.  It's just that having tried three separate condensers, I was looking for another possibility.

The plug caps could be renewed; at this stage I can't quite see how an issue on the HT side can cause a problem on the LT side - but maybe it can!

Emphatically yes, and called back EMF or sometime referred to as "ringing" in which the point when the plug gap is breached by ionisation in crossing the stratified charge in the combustion chamber, the the whole circuit, HT & LT are affected.
This effect is utilised to drive a longer period of spark duration as the entire system ricochet back and forth between the condenser (capacitor) and the plug gap.  Defective condenser and it'll jump the points gap, significantly shifted resistance from HT side in variance from specification, can give compromised performance.

https://nationalmaglab.org/magnet-academy/watch-play/interactive-tutorials/ignition-coil/#:~:text=The%20collapsing%20field%20also%20induces,spark%20across%20the%20knife%20switch. Mentioned here.

[McCabe-Thiele (Ted)]

The back Electro Motive Force (EMF) doing it best to maintain an electrical circuit as it is switched off.
Takes me back to A level Physics.

[K2-K6]

The points can register up near 400v too in this event,  needs competent condenser to handle that feedback loop.

[Mikep328]

FWIW, on my Norton Commando a set of ORGINAL condensers - now fifty, yes FIFTY...yep you read that right, 50 years old(!) produce a tiny spark at the points.  I replaced them with new condensers a couple of years ago "just because" and the spark was much bigger.  I put the originals back in!  Admittedly, this is just one sample which is of no statistical significance but I have to admit it put me in the "They don't make 'em like they used to" camp!    Same is true of points...

[McCabe-Thiele (Ted)]

I found this interesting - not just bikes that have issues with condenser quality.

https://www.physicsforums.com/threads/death-of-a-condenser-with-autopsy.877969/

[Mikep328]

I have no personal experience but I have read and talked to folks that rather than buying new OEM-type/size condensers (now called capacitors), they have purchased known good quality capacitors from the various electronic suppliers.    They are reported to work well/have a long life.  As far as the spec, I seem to recall reading or hearing something like .22uF/1000V AND due to size/fitment that they had to be mounted externally, NOT on the points plate but I wouldn't trust my memory re any of that! 

[McCabe-Thiele (Ted)]

I've seen car distributors in the past with capacitors mounted on the outside of the casings rather than inside next to the points.

This link shows the 0.22 Micro Farad value.
 https://www.jalopyjournal.com/forum/threads/points-ignition-condenser-question.1237753/

Some interesting comments on this post about the same issues.

https://www.britbike.com/forums/ubbthreads.php/topics/746022/re-what-the-purpose-of-a-condenser-in-the-ing-sys

[McCabe-Thiele (Ted)]

Interesting arguments on this site about the wiring illustration being wrong before it decends into a slanging match!

https://classicmechanic.blogspot.com/2011/03/ignition.html

[Bryanj]

In THEORY it should not matter where on the points lt lead they are fitted and if you fitted an extra in parallel the total capacitance increases so should be better, i tried it on the 500 i sold Ted with 2 rover v8 ones at the coils and the bike did not like it so i disconnected them but as Ted noticed it was too much of a fiddle to remove them!

[K2-K6]

In THEORY it should not matter where on the points lt lead they are fitted and if you fitted an extra in parallel the total capacitance increases so should be better, i tried it on the 500 i sold Ted with 2 rover v8 ones at the coils and the bike did not like it so i disconnected them but as Ted noticed it was too much of a fiddle to remove them!

Interesting practical test Bryan. My hypothesis on this is that the performance of the condenser in these system is related to the characteristic fuel burn time, which has largely stayed the same in petrol (refining, octane rating, combustion conditions etc) such that a condenser within this type of triggered HT delivered system needs to have that in defined range to positively benefit combustion.

There's quite a lot of research available (some really good from GM in R&D of Northstar V8) that discuss in this area. Also interesting is Honda CVCC combustion process from their 70s car engine program, itself a precursor to current F1 pre-chamber combustion of extremely lean mixture burning.