Using an inductive timing light for rpm

[Frank The Tank]

I'm playing with my carbs and since I don't have a tachometer on the bike right now, I'm using my timing light to tell me the RPM. I'm assuming since xs650s have a wasted spark that I need to take the RPM it displays and divide them by two to get the proper engine speed?

[Travis]

Yep sounds right to me. But I guess it depends on your timing light. Some have a switch for 4-cycle and 2-cycle/wasted spark. If yours doesn’t then I think it’s safe to assume you can cut the rpms in half.

If you don’t have that switch, you also have to be careful if you can advance the timing on your light because you will have to double it to get a proper reading. An adjustable timing light uses the time between sparks to compute the engine speed and then calculates how long to delay the flash. The wasted spark means it’s sparking twice as often as the timing light expects for a given rpm. It thinks the engine is running twice as fast so the delay is half what it should be for any given advance setting.

That’s how I understand it anyway. On my timing light you can adjust advance but you can’t adjust for 4 stroke and 2 stroke. I keep it at zero so I don’t have any problems. And if I’m reading the rpm off the display, I divide it in half.

[Frank The Tank]

Ok cool. I was just using it for the rpms and it doesn't have 2 and 4 cycle option. Thanks for the info.