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Arcing points

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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:

--- Quote from: Aye Gee 1977 on March 22, 2024, 03:23:08 PM ---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

--- End quote ---

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!

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