When asked to troubleshoot a tuning problem, probably the first thing I will ask is “Where is your timing set?” Without a timing light, you just don’t know for sure. The cheap in-line ones use spark voltage to fire a bulb, and are not particularly useful, though I suppose they are better than nothing. I would recommend one that has a pair of alligator clips for 12v power and an inductive clamp to clip over a plug wire to trigger the light. The best ones,called “dial-back-to-zero” timing lights or just "dial-backs", also have a knob on the back of the unit. Since most engines do not have timing marks at every degree, these lamps have a circuit that allows you to “dial back” the flashing light until the mark on the crank lines up with the the TDC pointer on the engine. Accurate timing readings are then read off of the dial to the exact degree.