Re: Re: Re: SPARK PROBLEMS, I dare you to explain this one!
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Posted by Paul Fox on June 21, 1998 at 13:38:03:
In Reply to: Re: Re: SPARK PROBLEMS, I dare you to explain this one! posted by Lee Bradley on June 21, 1998 at 09:28:57:
: : : Farmall Cub 48. New coil in mag,condenser, points,cap,rotor,wires,plugs and carb, timed on the money. This thing just keeps running rough. I pulled the wire from the coil while it was running to look at the spark. If I pull the wire out till I have a 1/2" spark jumping from the coil to the wire the engine runs smooth as silk. If I move the wire into the coil and make connection it runs rough again. I tried widening the spark gap but this didn't help. I've been told I need to add a resistor on my plugs. Can anyone back this up and or explain to me whats happening. Thanks in advance. : : Joe: : : I'm applying basic electrical theory here, and I can't guarantee it's valid, but here goes: Ohm's Law states that Voltage=Current x Resistance (E=IxR). My theory is that the mag is putting out a relatively constant current. When you pull the wire and introduce an air gap, you've essentially increased the resistance. With current constant and resistance increased, the voltage goes up, hence a hotter spark at the plug. This might compensate for a weak plug or an over-rich mixture by creating a hotter spark. I'd be interested to know if others think I'm on the right track here. : Paul, : Good theory but wrong appication. The coil is not a constant current device. Mags and battery ignitions both use a 'tank circut' to operate. The tank circut consistes of a primary coil, capacitor (condensor), and a spark gap (points) when the points are closed the battery/mag charges the primary coil by creating a magnetic field around the coil, when the points open the magnetic field collapses across the secondary and primary windings (faster than the battery/mag can create it) of the coil inducing a high voltage in the primary and secondary coils (several thousand in the secondary and several hundred in the primary) the secondary voltage will rise to a voltage high enough to establish a spark some where, across the rotor/cap gap and the plug gap (normally 15-20K) if things are working right, else where if not. Once the spark is established the voltage required to maintain the spark drops (4-6K) and the charge in the capacitor discharges through the primary coil maintaining the lower voltage spark until
: the capacitor is discharged. The capacitor discharge rate depends on the voltage required to maintain the lower voltage spark. The lower the voltage the longer the spark continues. So having said all that, the system is constant power that is high voltage for a short time or low voltage for a longer time. The available power is determined by charge time and voltage available while the points are closed. As rpm increase the charge time is decreased and spark power drops. Mags have an advantage over battery systems in that the available charging voltage increases as rpm increases this is why the race engines used mag until electronic systems became available.
: Enough, time to go play on my HD-7 AC that has neither system. : As Always,
: Lee Bradley Lee: Thanks for the info. Never messed with a magneto, didn't realize it uses a points/coil/condenser system same as a distributor ignition.
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