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Timing a 2-Stroke Outboard, Finding TDC

Updated: Nov 24, 2023


Ignition timing for a 2-stroke engine is determined by the height of the piston before top dead center (TDC). The ignition timing is indicated by the distance from TDC, not the angle of the crankshaft.


TDC is the position of a piston when it is at the very top of its stroke. The elusive part is caused by piston dwell, where there are a number of rotational degrees, before and after TDC; making it difficult to measure piston height differences, even with the most precise dial indicator.


You should still use a dial indicator, but to address this issue, we measure two points and split the difference to find TDC:

  1. Insert the dial indicator into a spark plug adapter.

  2. Rotate the engine clockwise until the dial indicator drops to a certain reading.

  3. Mark that reading on the flywheel.

  4. Rotate the engine counterclockwise until the dial indicator reaches the same reading.

  5. Mark that reading on the flywheel.

  6. TDC is halfway between the two marks.

On a Mercury 2-stroke V6 outboard, we typically use 25 degrees (roughly 150 thousand of an inch) before and after TDC and indicate 25 degrees on the Buckshot Racing Super Timing Wheels (Magnet for the stock OEM and Light Race Flywheel or Decal for the Power Performance Engineering (PPE) Flywheel with Titanium flexplate) to find and compare the most accurate timing across all 6 cylinders.


Timing varies from cylinder to cylinder (based on multiple factors), flywheel to flywheel (based on the placement of magnets), and from powerhead to powerhead (based on compression, fuel octane, rich or lean). Please call with any questions.


Advancing ignition timing increases high-end power and decreases low-end power. It also helps the spark get past the ignition delay and run at peak power.


Retarding ignition timing causes the spark plug to fire later in the compression stroke, reducing power. This is because the cylinder pressure won't reach its max until the piston is already headed back down on the Power Stroke.


Overly advanced timing can reduce power and make the engine overheat. It can also cause detonation, which can lead to a hole melted in the piston.






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