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Wiring harness 1974 sportster???

Bought a new coil and completly removed the old harness and installed a new coil and still no fire to the plug??? need help

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I am assuming your bike is stock, or close to it. In 1974, all h-d's used a battery coil ignition with wasted spark. Battery positive flows from the ignition switch to the coil, through the coil primary winding to the points, then open or shorted to negative (through the chassis) depending on the points being open or closed. When the engine is turning over, this is what happens: The small cam that operates the points has the points in the closed position, so battery current is flowing through the coil primary, through the closed points to negative. This causes the primary winding inside the coil to set up a magnetic field. When the cam starts to open the points, the current tries to bridge the gap. If we let this happen, the big arc between the point surfaces would burn up in a hurry, and the plugs would not fire for the following reason: The ignition coil is a transformer. It has a primary and secondary winding.The secondary winding has many more turns of wire than the primary. When the secondary winding "cuts" magnetic lines of force a larger voltage is induced in the secondary. In the case on your sporty, we are boosting 12 volts to over 10,000 volts. This depends on 2 things: The number of lines of force and the speed they are cut. So, when the points just start to open, the condenser absorbes the current until the points are open enough to prevent arcing. Then the current flow through the primary winding stops. The magnetic field quickly collapses, cutting through the secondary windings that are connected to the spark plugs. The resulting high voltage (pressure) Jumps the gap on both plugs and lights the fire. One of the cylinders is on the exhaust stroke so that spark is wasted. So: ignition on : 12v at coil positive. With points closed, 0 volts at coil negative. If you read voltage here, the points are dirty or open or the wire from the coil to the points is open. Points open: 12 volts at coil negative. To check for spark, you don't have to crank the engine. Just manually opening the points should fire the plugs. After you get the plugs firing, post again and I will tell you how to razor tune this thing. It should start instantly with a perfect idle. Hope this is clear.

Posted on Jan 01, 2009

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