Proper maintenance, inspection, and timing of a reciprocating engine ignition system are essential for reliable engine operation. Maintenance procedures include checking magneto internal timing and E-gap, timing the magnetos to the engine, performing operational ignition system checks, and inspecting ignition leads and harnesses for defects. Accurate adjustment and careful testing help ensure that the ignition system delivers a strong spark at the correct point in the engine cycle.
Checking the Internal Timing of a Magneto
When replacing a magneto or preparing one for installation, the first consideration is its internal timing. For each magneto model, the manufacturer specifies the angular position beyond magnetic neutral at which a pole of the rotating magnet should be located to produce maximum voltage at the instant the breaker points open. This angular displacement from the neutral position, known as the E-gap angle, varies with different magneto models.
On some magneto models, a step cut into the end of the breaker cam is used to check internal timing. When a straightedge placed along this step aligns with the timing marks on the rim of the breaker housing, the magneto rotor is in the E-gap position and the breaker points should just begin to open.
Another method for checking E-gap is to align a timing mark with a pointed chamfered tooth. [Figure 1] The breaker points should be just starting to open when these marks line up.
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| Figure 1. Timing marks indicate the number one firing position of a magneto |
In a third method, the E-gap position is established by installing a timing pin and aligning the red marks visible through a vent hole in the side of the magneto case. [Figure 2] The contact points should be just opening when the rotor is in the position just described.
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| Figure 2. Checking magneto E-gap |
Bench timing a magneto, or setting its internal timing, involves positioning the rotor at the specified E-gap position and adjusting the breaker points to begin opening when the applicable timing marks are correctly aligned.
High-Tension Magneto E-Gap Setting (Bench Timing)
The following steps are taken to check and adjust the timing of the breaker points for the S-200 magneto, which does not have timing marks in the breaker compartment:
- Remove the timing inspection plug from the top of the magneto. Turn the rotating magnet in its normal direction of rotation until the painted, chamfered tooth on the distributor gear is approximately in the center of the inspection window. Then, turn the magnet back a few degrees until it is in its neutral position. Because of its magnetism, the rotating magnet holds itself in the neutral position.
- Install the timing kit and place the pointer in zero position. [Figure 3]
- Connect a suitable timing light across the main breaker points and turn the magnet in its normal direction of rotation 10° as indicated by the pointer. This is the E-gap position. The main breaker points should be adjusted to open at this point.
- Turn the rotating magnet until the cam follower is at the high point on the cam lobe, and measure the clearance between the breaker points. This clearance must be 0.018 ± 0.006 inch (0.46 ± 0.15 mm). If the breaker point clearance is outside these limits, the points must be adjusted to the specified clearance. It is then necessary to recheck and readjust the timing for breaker opening. If the breaker points cannot be adjusted to open at the correct time, they should be replaced.
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| Figure 3. Installing timing kit |
Timing the High-Tension Magneto to the Engine
When replacing magnetos on aircraft engines, two factors must be considered:
- The internal timing of the magneto, including breaker point adjustment, must be correct to obtain maximum available voltage.
- The crankshaft position at which the spark occurs. The engine is usually timed by using the No. 1 cylinder on the compression stroke.
The magneto must be timed by first adjusting or checking the internal timing with the magneto off the engine. This is done by checking and adjusting the breaker points so that they begin to open at the E-gap position. The chamfered tooth, which serves as the magneto reference timing mark, should align with the center of the timing window. The magneto is set to fire the No. 1 cylinder.
Remove the most accessible spark plug from the No. 1 cylinder. Rotate the propeller in the normal direction of rotation until the No. 1 piston is moving upward on the compression stroke. This can be determined by holding a thumb over the spark plug hole until compression pressure is felt.
Set the engine crankshaft at the prescribed number of degrees ahead of true top dead center as specified in the applicable manufacturer’s instructions, usually using the timing marks on the engine.
With the engine set at a prescribed number of degrees ahead of true top dead center on the compression stroke and with the final movement toward the timing position made in the normal direction of engine rotation, the magneto can be installed on the engine. [Figure 4]
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| Figure 4. Timing marks aligned |
While holding the magneto drive in the firing position for the No. 1 cylinder as indicated by the alignment of the reference marks for the magneto, install the magneto drive into the engine drive. It should be installed in the middle of its slotted flange to allow for fine timing of the magneto to the engine.
Attach a timing light to both magnetos. With the engine still in the firing position, the magnetos should be timed by moving them in the flange slots until the breaker points in the magneto just open.
If the slots in the mounting flange of the magneto do not permit sufficient movement to obtain the correct breaker point opening position for the No. 1 cylinder, move the magneto out of position far enough to permit turning the magneto drive shaft. Then, install the magneto in position again and repeat the previous check for point opening.
Install the magneto attaching nuts on the studs and tighten slightly. The nuts should remain loose enough to permit small timing adjustments of the magneto in accordance with the applicable manufacturer's procedure.
Reconnect the timing light to the magneto and breaker points. With the light and ignition switch turned on, rotate the magneto assembly first in the direction of rotation and then in the opposite direction. This movement helps determine the exact position at which the breaker points begin to open.
After completing this adjustment, tighten the mounting nuts. Move the propeller sufficiently opposite the normal direction of rotation, then rotate it forward in the normal direction while observing the timing light until the prescribed timing position is reached. Ensure that the timing lights for both sets of breaker points indicate point opening within the prescribed timing position.
When the right and left sets of breaker points open at the specified timing position, proper magneto-to-engine timing is established and the ignition systems are correctly synchronized.
Some early engines had what was referred to as staggered timing where one magneto would fire at a different number of degrees before top dead center on the compression stroke. In this case, each magneto had to be timed separately. If staggered ignition timing is used, the spark plug nearest the exhaust valve will fire first.
Timing Light Example
In the following example, a timing light is used for timing the magneto to the engine. The timing light is designed in such a way that one of two lights comes on when the points open. The timing light incorporates two lights.
When connecting the timing light to the magneto, the leads should be connected so that the light on the right side of the box represents the breaker points on the right magneto, and the light on the left side represents the left magneto breaker points. The black lead or ground lead must be attached to the engine or an effective ground.
When using the timing light to check a magneto in a complete ignition system installed on the aircraft, the engine ignition switch must be placed in the BOTH position. Otherwise, the lights do not indicate breaker point opening.
Performing Ignition System Checks
The ignition system is checked during the engine run-up performed before flight. The magneto check, as it is usually referred to, is performed during the engine run-up checklist.
The ignition system check is used to check the individual magnetos, harnesses, and spark plugs. After reaching the engine rpm specified for the ignition system check, allow the rpm to stabilize.
Place the ignition switch in the RIGHT position and note the rpm drop on the tachometer. Return the switch to the BOTH position. Allow the switch to remain in the BOTH position for a few seconds so that the rpm stabilizes again.
Place the ignition switch in the LEFT position and again note the rpm drop. Return the ignition switch to the BOTH position. Note the rpm drop for each magneto position.
The rpm drop should be within the limits specified for the aircraft and engine, and the difference between the two magneto drops should also remain within the prescribed limit. Always refer to the applicable aircraft operating manual for specific values.
The rpm decreases because combustion is generally less efficient when the engine operates on one magneto than when both ignition systems provide sparks in each cylinder.
Remember, this tests not only the magnetos but also the ignition leads and spark plugs. If either magneto has excessive rpm drop while operating by itself, the ignition system needs to be checked for problems.
If an excessive rpm drop occurs when operating on only one magneto, the fault may be isolated to the ignition system associated with that magneto. This ignition system check is usually performed at the beginning of the engine run-up because rpm drops not within the prescribed limits affect later checks.
Ignition Switch Check
The ignition switch check is performed to see that all magneto ground leads are electrically grounded. The ignition switch check is usually made at 700 rpm. On those aircraft engine installations that do not idle at this low rpm, set the engine speed to the lowest possible to perform this check. When the speed to perform this check is obtained, momentarily turn the ignition switch to the OFF position. The engine should completely quit firing.
After a drop of 200–300 rpm is observed, return the switch to the BOTH position as rapidly as possible. Do this quickly to eliminate the possibility of afterfire and backfire when the ignition switch is returned to both.
If the ignition switch is not returned quickly enough, the engine rpm drops off completely and the engine stops. In this case, leave the ignition switch in the OFF position and place the mixture control in the idle-cutoff position to avoid overloading the cylinders and exhaust system with raw fuel. When the engine has completely stopped, allow it to remain inoperative for a short time before restarting.
If the engine continues firing with the ignition switch in the OFF position, one or more magneto grounding circuits may be open or otherwise defective and must be corrected before further operation. This means that one or more of the magnetos are not being shut off even when the ignition switch is in the OFF position. Turning the propeller of this engine can result in personnel injury or death. If the propeller is turned in this condition, the engine can start with personnel within the propeller arc.
Maintenance and Inspection of Ignition Leads
Inspection of ignition leads should include both a visual and an electrical test. During the visual test, the lead cover should be inspected for cracks or other damage, abrasion, damaged shielding braid, or other physical damage. Inspect leads for overheating if routed close to exhaust stacks.
Disconnect the harness coupling nuts from the top of the spark plugs and remove the ignition leads from the spark plug wells. Inspect the contact springs and compression springs for any damage or distortion and the sleeves for cracks or carbon tracking. The coupling nut that connects to the spark plug should be inspected for damaged threads or other defects.
Each lead should be checked for continuity using a high-tension lead tester by connecting the black lead to the contact spring and the red lead to the eyelet of the same lead in the cover. The tester's continuity lamp should illuminate if continuity is present.
The insulation resistance test of each lead is accomplished using the high-tension lead tester by attaching the red, or high-voltage, lead to the spring of the harness lead. Then, attach the black lead to the ferrule of the same lead. Depress the press-to-test push button switch on the lead tester. Observe that the indicator lamp flashes and the test gap fires simultaneously as long as the press-to-test switch is held in the depressed position.
If the indicator lamp flashes and the gap fails to fire, the lead under test is defective and must be replaced. The indicator lamp flashes to show that a high-voltage impulse was sent out. If the indicator lamp flashes but the test gap does not fire, the high-voltage impulse is leaking through the insulation of the ignition lead, indicating a defective lead.
When defective leads are revealed by an ignition harness test, continue the test to determine whether the ignition lead or the distributor block is defective. If the difficulty is in an individual ignition lead, determine whether the electrical leak is at the spark plug elbow or elsewhere.
Remove the elbow, pull the ignition lead out of the manifold a slight amount, and repeat the harness test on the defective lead. If this stops the leakage, cut away the defective portion of the lead and reinstall the elbow assembly, integral seal, and terminal connector (sometimes referred to as a “cigarette”). [Figure 5]
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| Figure 5. Replacement procedure for ignition lead terminals |
When installing ignition leads, avoid sharp or excessive bends that could create weak points in the insulation and allow high-tension current to leak.
Single Ignition Lead Replacement
If the lead is too short to repair in the manner described, or the electrical leak is inside the harness, replace the defective lead. Single ignition lead replacement procedures are as follows:
- Disassemble the magneto or distributor so that the distributor block is accessible.
- Loosen the piercing screw in the distributor block for the lead to be replaced, and remove the lead from the distributor block.
- Remove approximately 1 inch of insulation from the distributor block end of the defective lead and approximately 1 inch of insulation from the end of the replacement cable. Splice this end to the end of the lead to be replaced and solder the splice.
- Remove the elbow adapter from the spark plug end of the defective lead, then use the old lead to pull the replacement lead through the ignition harness. While the old lead is being pulled through the harness, have an assistant feed the replacement lead into the ignition manifold from the distributor end to reduce the force required.
- When the replacement lead has been pulled completely through the manifold, feed additional lead into the manifold from the distributor block end to provide extra length for possible future repairs, such as those required because of chafing at the spark plug elbow.
- Remove approximately 3⁄8 inch of insulation from the distributor block end. Bend the ends of the wire back and prepare the ends of the cable for installation into the distributor block well. Insert the lead in the distributor and tighten the piercing screw.
- Remove approximately 1⁄4 inch of insulation from the spark plug end of the lead and install the elbow, integral seal, and cigarette. [Figure 5]
- Install a marker on the distributor end of the cable to identify its cylinder number. If a new marker is not available, use the marker removed from the defective cable.
Replacement of Ignition Harness
Replace a complete ignition harness only when the shielding of the manifold is damaged or when the number of defective leads makes it more practical to replace the harness than to replace the individual leads. Replace a cast-filled harness only when leakage in the cast-filled portion is indicated. Before replacing any harness to correct engine malfunctioning, make extensive ignition harness tests. Typical procedures for installing an ignition harness are detailed in the following paragraphs.
Install the ignition harness on the engine. Tighten and safety the hold-down nuts and bolts and install and tighten the individual lead brackets according to instructions. The ignition harness is then ready for connection of the individual leads to the distributor block. An identification band is attached to each lead at the distributor end of the harness to indicate the corresponding cylinder. However, each lead should be checked individually with a continuity or timing light prior to connecting it.
Check for continuity by grounding the lead at the cylinder and then checking at the distributor block end to establish that the lead grounded is as designated on the band for the lead. After checking all leads for proper identification, cut them to the proper length for installation into the distributor block.
Before cutting the leads, however, force them back into the manifold as far as possible to provide extra lead length within the ignition manifold. This extra wire may be needed at a later date in the event that chafing of a lead at the spark plug elbow necessitates cutting a short section of wire from the spark plug end of the harness.
After cutting each lead to length, remove approximately 3⁄8 inch of insulation from the end and prepare the lead for insertion into the distributor block. Before installing the lead, back out the set screw in the distributor block far enough to permit slipping the end of the wire into the hole without force.
Insert the lead into the block and tighten the set screw. Connect the leads to the distributor block in the correct firing order, with the first cylinder in the firing sequence connected to the No. 1 position, the second to the No. 2 position, and so forth.
After connecting each lead, check continuity between the lead and its distributor block electrode with a continuity light or timing light. To perform the test, connect one test lead to the ignition lead and touch the other test lead to the corresponding distributor block electrode.
If the light does not indicate a complete circuit, the set screw is not making contact with the ignition wire or the lead is connected to the wrong block location. Correct any faulty connections before installing the distributor block.
Induction Vibrator Checks
To check the induction vibrator, ensure that the manual mixture control is in IDLE CUTOFF, the fuel shutoff valve and booster pump for that engine are in the OFF position, and the battery switch is ON. Since the induction vibrator buzzes whether the ignition switch is on or off, leave the switch off during the check.
If the engine is equipped with an inertia or combination starter, make the check by closing the engage-mesh switch; if the engine is equipped with a direct-cranking starter, see that the propeller is clear and close the start switch. An assistant stationed close to the induction vibrator should listen for an audible buzzing sound. If the unit buzzes when the starter is engaged or cranked, the induction vibrator is operating properly.