[Healeys] Spark Scatter over 4000rpm

[Andy Thorp]

I'm having an issue with timing mark jumping around on a BN4 engine at high
rpm and I can't decided whether it is due to a dodgy timing light or something
internal. The mark jumps around maybe +/- 10 degrees randomly over approx
4000rpm but is otherwise steady at lower speeds. I removed the advance weights
entirely and locked the upper spindle to the main shaft as well as locking the
vac advance plate but no change to the behaviour.

The distributor is original Lucas with Pertronix +ve earth module. There is a
tiny bit of slack in the shaft if I disassemble it and wash out all the grease
but it is not noticeable when greased so I'm thinking that should not be the
cause of the issue. There is the normal amount of rotational slack from the
drive dogs and bevel gear. Checking on an oscilloscope didn't show more than
500us difference in switching events and had no change when the timing mark
jumps around. The rest of the ignition system is stock except for NGK 5k ohm
resistor spark plug caps.

Timing light is Optilux which is by all accounts a reputable brand although it
does show a similar thing on another engine but nowhere near as bad.

Engine is fairly stock Longbridge with 10:1cr and about 2000 miles since a
full rebuild

Anyone else experienced this?

Andy.