How do timing lights work?

[Neil Bardsley]

Another numpty question. How does the advance degree feature work on timing lights? If I set the advance to 30 degree how does the light know much earlier to flash? Does it build a wave from previous impulses or does it just average the last n impulses? Either way this seems to me like it could be quite unstable as the speed of rpm aren't constant. Is this why some, I think I've heard Vic said this, prefer not to use advance on the timing light but mark the max advance on the crank pulley?

[Wes Bender]

Congratulations on your 356th post! No, I don't know how those timing lights work. Harry Pellow used to swear by them, but I'm with Vic. I've painted two additional timing marks on my pulley. One at 30 degrees and one at 35 degrees. This lets one set the timing accurately. I set mine at 33 degrees which is easy to interpolate. That seems to suit my 1720 cc C engine with a CR of 9.5:1.
I'm sure others will chime in with their own preferred timing and maybe even explain to both of us how that advanced timing light works.

Wes

[Neil Bardsley]

Thank you Wes. This evening I did a test and set the advance to zero them marked the pulley at 4.25cm. At max advance my mark was static under the timing light. Where previously it been jumping around when I set the advance to 34 degrees. I don't think my timing light is clever enough to smooth the advance so I think I will be sticking to the zero advance method. I advanced the timing a little 34/35 and its running better again.

[Dennis ODonnell]

As you've probably guessed by now the knob on the light just moves "zero" where the strobe fires on a pistol shaped inductive timing light. It's accuracy depends on the knob & scale location & attachments. Filing or painting accurate timing marks on the pulley always works.