Thread: Ignition 101
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Old 12-05-2013, 02:46 AM
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Ignition 101

I wrote this summary of (distributor/points) ignition, based on my recent exchanges here. Hopefully others will find it helpful.

How (old school) ignition works.
Or – what I learned from Dave Neptune, Neil Dutton, Hanley Clifford, John Cookson and others.

In a 4 stroke engine, the piston rises and falls twice for each explosion of fuel. Slightly simplistically, its like this -
TDC (Top Dead Center)
Inlet stroke – piston goes down, inlet valve open, gas in.
BDC
Compression stroke - piston goes up, valves closed
TDC - spark ignition of gas mixture
Power stroke - piston goes down, valves closed
BDC
Exhaust stroke - piston goes up, exhaust valve open.
(Both valves are open at the top of the exhaust stroke - referred to as "overlap".)

Battery voltage is supplied to the coil – intermittently- by the points, which are actuated by a cam on the distributor shaft. When the points close, power is supplied to the primary of the coil. The coil, an inductive device, produces a high voltage spike at the moment of the collapse of the induced magnetic field – called flyback voltage. Voltage in multiplied by the primary/secondary relationship.

Points gap is the maximum opening of the points, which is when the shoe rides on the highest point of the cam.

Dwell is the period of time the points are closed, measured in degrees of distributor shaft rotation. Ie this is the period during which the coil ‘charges up’.

The Condensor or capacitor. The primary winding and the capacitor form a ‘tuned circuit’. Stored energy oscillates between the inductor formed by the coil and the capacitor. The condenser, which is connected across the points, is also said to absorb back emf, protecting the points from arcing/burning.

The high voltage spike is passed to respective spark plug by the rotor of the Distributor. As the rotor rotates it contacts the contact for each spark plug lead in succession. The distributor rotates at 1/2 the speed of the crankshaft – thereby synchronizing with every second TDC of each piston!

The distributor is thus two separate machines which happen to be on the same shaft. The points supply power (12v) to the coil, the rotor supplies 20000 volts to the sparkplugs.

Timing refers to the moment of spark ignition with respect to piston TDC. Advanced timing is when ignition occurs before TDC. Retarded ignition when ignition occurs after TDC.

Centrifugal advance is caused by weights and springs under the plate in the distributor upon which the points and condenser are mounted.

There is a useful video at http://www.youtube.com/watch?v=W94iksaQwUo
This discussion is also helpful http://www.ratwell.com/mirror/users....r/ketterin.htm

Timing Adjustment
Timing can be adjusted by loosening the clamp which lock distributor body to block, turning it a few degrees - when the engine is running. Best position is maximum smooth revs for any throttle position.

The manual says - "Take the boat out for a trial run end after bringing engine to normal operating temperature and the boat running at top speed. Loosen the distributor clamp bolt and caefully advance the ignition timing by slowly rotating the distributor body counter-clockwise until the RPM begins to fall off. Then rotate the distributor body in the opposite direction to obtain the greatest number of RPMs without rough running of the engine. The timing is now set properly in the advanced position. The spark automatically adjusts as the engine speed changes."

Distributor Cam Wear
Dave says- The lobe of the cam that wears over the years as it also wears out the many sets of points contact shoes. As the lobe wears there is far less lift (opening) required for the points to come back together at the same spot on the cam lobe, so when setting the "gap" (lift) to "spec" the points will stay open much longer (more degrees of rotation) on what is left of the "cam lobe". This holds the points open longer and gives them far less degrees of rotation closed which is the dwell timing that saturates the coil. (With an EI the timing is fixed and once set does not ever need adjusting unless moved). Once you have set a set of points and set the timing the timing is constantly on the move as the cam shoe on the points and the contacts themselves wear, that is why in the old days if you really took care of your car or boat you constantly had the timing readjusted because it is constantly moving as it wears.

Hanley says- Just make sure that you set your points by means of a dwell meter without regard to point gap specification; then check the actual point gap that yields your required dwell and post that number for us. This will provide useful information about the condition of your distributor cam.

Testing Spark.
Romantic comedy says – Take (spark plug) lead and hold 1/4 inch away from a ground (block). Remember that the rubber cap that fits on the plug will hold the lead away from the ground. Many use a spark plug hooked up to a plug lead, and grounded. Any plug will do. Blue is the best spark. Nice and fat if you can get it.

You can also do a quick and dirty test to the coil. Just hook up 12 volts to the positive side. Then hook up a ground wire to the other side. (no other wires are connected) Take the ground wire and touch to ground and off ground. This will cause a spark. Maybe not the best spark, but a quick test.
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