Precision Magneto Timing

1. Ensure magneto switch is OFF. Remove key.

2. Remove cowling as necessary to remove spark plugs.

3. Remove battery ground strap.

4. Remove #1 cylinder spark plug and another spark plug from each cylinder -- top or bottom -- whichever is easiest to access. Be sure to keep track of the copper gaskets.

5. Find compression stroke pre-TDC (top dead center) of the #1 cylinder.

6. Choose the prop blade face that will be convenient for viewing the digital protractor that you will attach in the next step.

7. Attach the protractor with double-back tape and retention tape at the end of the prop blade selected in Step 6. (Double-back tape helps to remove any associated jiggle when "bumping" the prop while checking the timing until point "open" lights.)

8. Install TDC stop plug fully into the #1 cylinder top spark plug port. Turn prop slowly forward until the piston comes to rest on the TDC finder plug. Using the digital protractor, locate the "normal" rotation stop point -- be gentle! Record the location as angle #1 (BTDC - before top dead center).

9. Reverse prop direction and locate stop point, recording location angle #2 (ATDC -- after top dead center). Remove TDC stop pin. Do not move prop. Perform math: (#1 angle + #2 angle)/2 - TDC angle.

When measuring these angles, watch how the angle display changes on the protractor, especially if it transitions quadrants (past 0deg or 90deg). This often happens with a 2- or 4-blade propeller. In this case, the math may need to be adjusted.

10. Rotate the prop forward (normal rotation) to the mag-to-engine ignition timing degree setting you located using the timing sheet form. Ensure protractor reading is "natural" or "normalized" -- do NOT zero (or alternate zero) out the protractor! This angle number is the BTDC angle of mag-to-engine timing.

11. Further check that the impulse couplings have released, and install the timing indicator light tool to both mag "P" leads. DO NOT REMOVE THE "P" LEADS FROM THE MAG. Install the key in the mag switch and turn the key to the "BOTH" position.

12. Install and "activate" the timing indicator unit and check the "point open" light as the propeller is rotated. Record the starting initial timing indications on the digital protractor, i.e. 22.5 degrees left mag and 21.2 degrees right mag. (Log these numbers by mag to evaluate future mag-to-engine timing drift.)

13. Adjust one mag to proper timing (for instance 22 degrees BTDC on a SR22NA) by using the digital protractor indication. NOTE: Common digital protractors have 0.1 degree resolution. Timing to 0.1 degree or less is possible in "mag-to-mag" as well as "mag-to-engine" timing.

14. Most digital protractors automatically "shutdown" after a period of inactivity. You can reactivate (turn "on") the digital protractor to view without worrying about losing the "angle."

15. When timing is set to your satisfaction, remove the protractor, remove the timing indication tool, ensure the mag switch is OFF, and remove the key.

16. Check and service the spark plugs for proper gap, lubricate the threads, replace the copper gaskets, install and torque to proper specs, clean the lead ends with isopropyl alcohol, ensure springs and contact buttons in spark plugs are clean of any corrosion, debris, etc., and install.

17. Install the ground strap to the primary battery, and install the top cowling with all the hardware (screws in front!). Don't forget the landing light connector.

18. Perform a ground run and test the function of the mag switch, engine operation, and mag drop/split. Prepare and complete the required documentation for approval for return to service.

The BIG question: Which way do you rotate the mag to advance or retard the timing?
While standing at the #2-#4-#6 cylinder side (pilot side) of engine:

- PUSH the magneto AWAY from you to RETARD the timing
- PULL the magneto TOWARDS you to ADVANCE the timing

NOTE: The BTDC timing may vary by the engine model. Check latest information for correct timing settings.