Hello All! I collect and sell the old, "SEARS PENSKE" timing light's! Non dial back! If you use any other timing light, you need to rev engine past 3,000 rpm to set timing!! Because the MSD ignition fire's the plug, 3 time's in 20 degrees at idle! If your timing light doe's not pick up the first fire your total timing will be off!!! With the dial back and up timing light's they may pick up who now's what signal!!!