Tuesday, January 31, 2012

Motorcycle Woes

On a motorcycle, the usual battery is a 12 volt alternating current. 12 volts isn't sufficient, though, to create a spark that will ignite gas to cause a piston-moving explosion in the engine. The ignition coils of a motorcycle solve this problem by creating and collapsing a magnetic field many times a second and sending out bursts of much higher voltage along the circuitry to the spark plugs. To time these magnetic field creations and collapses along with the upward and downward motion of the pistons so as to get a high-voltage spark at the right instant, the circuitry is built into a rotating "crankshaft". As the crankshaft is turned by the pistons, the breaker points on the crankshaft connect and disconnect, alternately completing and opening the circuit connected to the ignition coils.

See picture of my breaker points below:

I noticed the other day, when trying to get my bike to start up properly, that the ignition coils were severely overheating. Taking off the cover to the crankshaft and rotating it several full rotations with a wrench, I realized that at no point during the crankshaft cycle were the breaker points actually opening. The circuit was never being opened! That meant the coils' magnetic fields were never collapsing, never creating the proper high voltage necessary to start and power the engine.

I adjusted the breaker points so that at their farthest apart they'd be .3 to .4 millimeters wide, the specs according to my manual. Then, according to the manual, I was supposed to do what's called a "continuity test" to make sure that the circuit was being broken at the proper time in the crankshaft cycle. The way you're supposed to do this is to ground one end of the tester to the engine, and attach the other wire of the tester to one side of the breaker point. The you turn the crankshaft until you see the meter needle drop, indicating that there is no power flowing through the circuit.

The needle never dropped! It looked like, even with the correct gap between the points, the circuit was ALWAYS complete. The magnetic field still shouldn't be collapsing! But after that first gap adjustment, the bike turned on just fine!

How was the bike turning on while the circuit was constantly complete? More importantly, why is the circuit constantly complete! Electricity isn't crossing that gap between the breaker points when they're open. It must be crossing straight across the base plate that holds the points in place. Some sort of insulation isn't working properly in the circuit, allowing electricity to flow where it shouldn't.

Here's a picture of the way the wires are connected to the breaker plates. It's a zoomed in version of the picture above, zooming in just to below and to the right of the central hex bolt:

So the electricity flows up the black and white wires onto the each of the two screws at the connections indicated by the red circles. It goes onto the thin spring breaker plate (visible on the left connection coming out to the left and heading up-left off the picture) and, theoretically, along the plate up to the point. It also, in parallel, goes into the condensers (the metal cylinders seen at the bottom of the picture). At the same time, though, it looks like the electricity is flowing onto the bracket that holds the screw in place, and down the bracket onto the large circular baseplate holding this whole contraption in place.

So that's the first problem! In summary, I can't do a continuity test to make sure the circuit's connecting and breaking at the right points of the crankshaft cycle because, according to my instrument, the connection is always being made, even when there is a visible gap between the breaker points with no spark crossing.

Remember, all of this is without the bike running. When I start the bike up (which it does easily), I see sparks arching across the breaker points. This isn't supposed to happen. The voltage should not be high enough for a spark to cross a gap in the air - it's damaging to the breaker points. With a spark crossing many times a minute, breaker points develop pitting over time that can destroy them.

That's the second problem.

The third problem is that whenever I rev the engine over around 2.5k revolutions per minute (rpm), the engine sputters out of and dies. We believe that this is due to a timing issue - as the crankshaft turns faster and faster, you want the spark to be exploding gas in the cylinders at a different point of the piston's motion. This probably has something to do with the breaker points as well, but we can't check the timing if we can't do a continuity test, which we can't do since we constantly have a closed circuit!

And those are all the issues with my bike right now!

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