Timing light advance knob correction ratio
 

In perusing a couple old threads in this forum on timing, and using a light with a built in advance advance knob; I noticed in one post it said the figure was 2 to 1; in another it siad 1.5 to1. In both cases the posters agreed the the light would read a lower degree of advance than the engine actually had.

Anyone know which is correct?

On a related note, does anyone know the degree marks on a mazdacomp 12a pulley? (Sorry if that's an obvious question; I don't have the pulley in my hands yet)

thanks!!
Marcus

I remember someone saying that the stock 12A timing pulley was 36 cm diameter. This would mean that each mm would be 1 degree. Other than that, I have no idea. Wish I could help more.

1 mm of diameter or circumference? 36 cm diameter would mean ~113 cm circumference.. I think I'm missing something..

Sorry, I'm no help on the original question right now..

36cm diameter = 14.17", that is a figging large pulley!

If 36cm was to be circumfrance, then the diameter would be 4.5" which is reasonable. Then 36cm = 360 mm thus 1 mm = 1 degree.

Let's try to be accurate!

I have small black anodized aluminum pulleys from Mazdacomp and Racing Beat, and it seems like they each had 3 marks:

TDC
12 deg BTDC
24 deg BTDC

If its wasted spark, its 2 sparks per crank rev on the leading, equally phased at 180*. Compare this to a 4 stroke, which is 1 spark per 2 revs. Voila, the factor is 4. If you don't have wasted spark, or you use the trailing plug wire (on FC), then the factor is 2.

Thus, on my FC, using the leading plug: dialing 20* on the 4-stroke timing light = 5* of advance.

I meant circumference. Sorry about the confusion there.

FC pullies are 360mm around. I remarked my pullie 50mm from the L1 mark so i can set my timming while idling at a happy 45*

The correction factor is 1.5
A rotary goes thought a full Otto cycle every 1080 degrees.
A piston eng. does the same in 720 *'s
Do the math.
When the light reads say 30*'s, BTDC, it is actually 20*'s. ect.
BUT, Your best bet is to get a CHEAP timming light, and use it ! ;)

To avoid any mistakes, or any errors from the electronics in the timing light, I leave the knob set on Zero, and I use the degree marks on my pulley.