Engine Kickback - Causes and Correction

Taming the Threat to Your Starter's Drive Train

Kickback has several meanings: graft in politics, feet up with shirt collar open, and sudden reversed engine rotation at starting.

Each is a "condition" caused by some defect. The first is greed, the second indolence, and the third "defect" is a set of inter-related factors, possibly damaging to the engine starter train.

There are three simultaneous and necessary conditions for engine kickback at starting:

• gas vapor in the cylinder that "kicks,"
• spark with sufficient voltage to jump the plug gap and enough current to ignite that vapor,
• the piston in the ignited cylinder to be some crankshaft degrees before top-dead-center (BTDC).

Engine kickback, if severe, can cause damage to the starter mechanincal train, usually the Bendix Drive spring/gear or shaft, so the condition should be avoided.

As noted, there must be three simultaneous and necessary conditions at start for kickback. The only one that can cause the effect is ignition and sufficient gas expansion while a piston is moving up in some cylinder. This gas expansion force must be great enough to overcome the starter motor torque and momentum of the engine rotating masses.

It has been well established by both theory and measurement that the gas vapor is completely burned in approximately 0.002 seconds of time. At an engine cranking speed of 150 RPM, the crankshaft has moved only 1.8°, not a significant change in piston position or gas compression .

That small rotation determines that kickback is caused by spark, gas ignition, and burning well before top-dead-center (BTDC) piston position. If it were after TDC (ATDC), the engine would start, idle or run normally.

To experimentally investigate that simple analysis, tests were recently conducted on two representative "A's." Both were stock 1931's, 6 volt positive ground.

Tests were made both with point contact and condenser ignition and with a transistor system. The gas vapor and burning should not know which spark started ignition, but the tests would confirm if any difference existed.

The test programs follow:
Program A:

• Initial timing set to 5° ATDC, full retard at steering column.
• Run at set timing for 15 seconds.
• 6 consecutive starts - normal start sequence (Ignition "ON" before starter is actuated).
• 30 to 60 seconds between starts.
• After the 6th run, reset timing by advancing spark control rod and check setting with timing light.
• Repeat at 0°, 5°, 10°, 15°, 20°, 25°, and then at 30 and 32-1/2°, all BTDC or heavy bucking occurs.

All timings were measured with timing light and crankshaft indicator as noted in reference¹.

Program B:

• Repeat all above with starter actuated, next followed by ignition switch turn "ON" for ignition.

Record: Smooth start and run?
            Any evidence of hard start?
            Any "buck/kickback?"

Test Data Results and Comments
There was no "kickback" using the normal start procedure (ignition "ON" and then engaging the starter) through timing set to 25° BTDC.

Repeat tests with the starter engaged and then turning the ignition switch "ON" were substanitally the same as the first tests. When advance was set to 30° BTDC, sporadic and slight "kickback" started to occur. At 32-1/2° BTDC, HEAVY "KICKBACK" was experienced. Further advance could result in starter damage so the tests were stopped at this point.

To investigate if cold or hot engine temperatures were a factor, the tests were repeated after a 12 hour interval. They repeated as above within the limits of observation.

Conclustions:
The tests (well over 100 trials) showed NO KICKBACKS and start and run with timing set at less than approximately 25° BTDC. At greater setting, approximately 30° plus or minus 2-1/2° BTDC, "kickbacks" may well occur on start, with damage to the starter or other components. So, engine start should be made with the column spark control set at FULL RETARD (UP) and timing set to 5° to 10° ATDC for ease of starting.

More on Piston Position
Where do the pistons come to rest in relation to TDC when the engine stops after ignition turn "OFF?" It was decided to investigate.

The procedure was-start, idle, ignition "OFF," then measure the crankshaft angle from TDC on the cylinder that is coming "up" to "fire." Repeat the sequence for 10 trials. The results are recorded on one engine were:

Trial No.	Piston Coming Up on Cyl.# to Fire	Crankshaft Degrees BTDC
1	#3	60°
2	#2	130°
3	#2	130°
4	#2	50°
5	#3	60°
6	#1	130°
7	#1	60°
8	#3	130°
9	#2	130°
10	#2	60°

A graph of the results is shown in Fig.1. Note that the final positions were grouped at 50°/60° and 130° BTDC. Also note that this particular engine rotated through the #4 cylinder compression/ firing position. Does that indicate that cylinder #4 has low compression and thus passed throught TDC?

The final sequential piston positions were relatively random in order. The two groupings of 50°/60° and 130° were roughly equally distributed. They were likey related to when the ignition was turned "OFF" and the momentum of the rotating engine masses determined the final positions.

If there is an affect to be considered, it might be associated with rapidity/ease of start. The engine, driven by the starter, intakes a gas charge, compresses it, then the gas is ignited and "start" rotation disengages the starter drive. The number of degrees of engine rotation before "ignition" and "start" is related to the position of the piston(s) at the beginning of starter engagement.

There is little influence the driver has in determining the "rest" piston positions so this part of the investigation has been of academic interest only.

A Visual Timing Procedure
To assure that timing is set for an initial retard position, a simple visual timing method follows:
There are some 10 detent positions of the column spark lever. The total timing range is 40° at the crankshaft (20° at the distributor). Each detent position is thus approximately 40°/10 or 4° of crankshaft rotation. So

1. Establish #1 piston TDC position.
2. Set the spark control lever to full retard, then move it 2 detent steps down (advancing it approximately 8° crankshaft rotation.
3. If a point-cam system:
Rotate the cam in the clockwise direction to remove the gear train's slack and the contact points to go from "open" to just "close." The rotor slot must be pointing in the general direction of the #1 plug contact of the distributor body. Tighten the cam hold down screw while holding the cam from turning.
Recheck that the points just "open" when the cam is rotated from the clockwise toward the just slightly counter-clockwise direction to assure the cam has not moved in "position" when the hold down screw was tightened.
If a transistor ignition sytem:
Follow the instructions which were recommended by the manufacturer and contained in any kit.

When the column spark control is moved to the full retard (UP) position, the ignition spark now occurs at approximately 8° After #1 Top-Dead-Center (ATDC) position for slow idle and easy start. Timing measurement by timing light is recommended and most desirable to confirm a retard setting for start and slow idle.

The control range, so set, is approximately 8° ATDC at full retard, and 32° BTDC at full advance. At 2400 RPM, the advance is then 28.8°

A Case History
An "A" owner in Southern California recently reported he was experiencing a kickback problem on start. His car is a '30 with a Hi-Compression Miller Overhead Valve Assembly and capable of speeds of 70 MPH and greater.

Recently he experienced "kickback" at start. He retarded the spark setting to a measured (timing light) value of 15°/20° ATDC (Full Retard) and it still occurred sporadically.

Conversation with him and measurements led to the diagnosis and conclusion-the fault was due to "cross-firing" within the distributor body. Cross-firing occurs when the spark travels to an adjacent distributor contact and thus to a wrong spark plug. It is caused by a carbon conductive track, open crack, or other defect within the distributor body. It only gets worse with time.

It was suggested that a different body be installed. Follow-up confirmed that the body was intermittently "cross-firing." The new body corrected the mal-performance - normal timing was reset and proper operation was attained.

Summary
Kickback at start can thus occur by -

• spark rod control inadvertently left near a full advance setting
• timing mis-set to advance greater than approximately 28° BTDC
• cross-firing within a defective distributor body/cap.

¹B.W. Abrams, "Ignition Timing the A," MODEL A TRADER, Jan/Feb 1992, pgs 2,3.

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