Inline spark tester for small engine ignition diagnosis. Fits any engine with a conventional spark plug and plug boot, two stroke or four stroke. HOW IT WORKS. The tester goes in series: it pushes onto the spark plug, and the plug boot pushes onto the tester. The plug stays in the cylinder and stays connected. Pull the rope and watch the window for a flash. WHY IN SERIES MATTERS. Because the plug is still fitted and still firing, you can watch the tester while the engine cranks or runs. That is what finds a heat related failure - the coil that works cold, quits once the engine warms, then works again after it cools - and the intermittent dropout under load. A test with the plug removed cannot show you either one. WHAT IT DOES NOT DO. It reports spark present or spark absent. It does not measure spark energy, voltage, timing or shot to shot consistency, and it does not test the spark against cylinder compression. Compression sharply raises the voltage needed to jump the gap, so a coil that fires in open air can still fail in the running engine. A lit tester is not a certificate of health. If you have spark here and the engine still will not run, an adjustable gap tester or a peak voltage measurement is the next step. DIAGNOSTIC ORDER THAT SAVES MONEY. Before buying an ignition module, check the stop or kill wire for a rub through against sheet metal - this is among the most common no spark causes and it is free to find. Then the plug gap and insulator. Then the high tension lead and boot. Then the flywheel to coil air gap, typically 0.25 to 0.40mm on two stroke equipment. GROUNDING. The module completes its secondary circuit through the engine itself, usually through its mounting screws into the cylinder. Corrosion, paint, a loose screw or a broken ground lead produces weak or absent spark that mimics a failed coil exactly. Confirm a clean tight ground before replacing parts.
- WIRES IN BETWEEN THE BOOT AND THE PLUG - Push the tester onto the spark plug, push the plug boot onto the tester, pull the rope and watch the window. A flash means the ignition is firing. Nothing means it is not. Works on any engine with a conventional plug and boot, two stroke or four stroke - saws, trimmers, blowers, edgers, tillers, mowers, generators and pressure washers.
- THE ONE THING A PLUG-OUT TEST CANNOT DO - Because it stays in circuit with the plug fitted, you can watch it while the engine is cranking or actually running. That is how you catch the coil that fires fine cold and quits once the cylinder gets hot, and the intermittent cut-out that never shows up on a bench test. Pull the plug out instead and you lose the one symptom you were chasing.
- WHAT IT WILL NOT TELL YOU - This is a go or no-go spark indicator, not a coil tester. It does not measure spark energy, voltage, timing or consistency, and it does not test the spark against cylinder compression, which is where a marginal coil actually fails. A lit tester does not prove a healthy ignition. If it flashes and the engine still will not run, the next tool is an adjustable-gap tester or a peak voltage reading.
- CHECK THESE FOUR THINGS BEFORE YOU BUY A COIL - A stop or kill wire rubbed through against sheet metal is one of the most common causes of no spark, and it costs nothing to find. Then the plug gap and a cracked insulator. Then the high tension lead and boot for cracks. Then the flywheel to coil air gap, usually 0.25 to 0.40mm on a two stroke.
- GROUND IS PART OF THE CIRCUIT - The ignition module completes its circuit through the engine metal, normally through its own mounting screws into the cylinder. Paint, corrosion, a loose screw or a damaged ground lead will give you weak spark or none at all, and it looks exactly like a dead coil. Check the mounting screws are clean and tight before you condemn anything.