JD LX178 weak spark? bad coil?

If the circuit is broken, it'll jump the gap out to 1/4 inch or thereabouts. Seems to me that that sort of problem would go unnoticed unless you accidentally caught it with an ohm check of the leads, or visually.

Mark

On the resistor.

Interesting that they chose to do that. Guess their coils are more reliable to connect up (windings to output lead) with solid wire rather than the wire we have on our trucks.

Now the resistor would be (see I'm rambling) there for only 2 reasons that I know.

1. Limit radio interference. But, I don't know of a lawnmower with a radio or other electronics..... .other than digital status monitors, which could be the reason.

2. Or it could be to limit the current thru the circuit. When the plug fires, the voltage across the gap goes to a very low value. There is nothing but the winding resistance of the coil to limit the current. This could be bad for the points and the plug electrodes in terms of erosion. The resistor would limit this current to a definite maximum. Current limit is normally in the primary circuit, but resistance in the secondary can be reflected to the primary and make it appear to be much more resistive than it actually (physically) is.

One last blab. If the resistor burns open, there becomes 2 gaps to jump (the gap of the open resistor and the gap across the plug's terminals).

Probably the ignition system has enough whoopie to jump both at first. Then as the resistor gets eaten away, the gap there gets larger and larger and then with the gaps too large, the voltage can't jump across both and the plug starts fouling.

That's all fine, but it doesn't explain the fact that the engine in question here, ran better with (yet another gap) the cap backed off some.

Mark

Any spark booster in the plug is an additional gap. What the additional gap does is this. Back to theory; sorry.

In a coil, when you attempt to interrupt current (points open in this case) the nature of the beast says this: I'm going to generate whatever voltage I have to in order to keep that current flowing (within it's limitations).

Nothing much happens in the secondary (high voltage side) until the points open. When this happens, since it is abrupt, it generates an abrupt voltage in the high voltage secondary. If the voltage on the primary raises to keep the current flowing, so also does it in the secondary.

Theoretically, if I double the gap I essentially double the voltage that the coil will generate in an attempt to keep the current flowing. This allows the coil output to raise to say 30,000 volts rather than the 18,000 it usually goes to to jump a .030 plug gap.

This is useful in firing a fouled plug as a fouled plug will usually bleed the voltage off as fast as the coil delivers it and it is a snowballing process. With this setup, once it's high enough it jumps both gaps and you get a spark in the combustion chamber whereas you wouldn't otherwise.

"and that's the rest of the story" (Paul Harvey) Grin.

Mark

You allowed the voltage output of the coil to build to a higher value before it was absorbed in firing the plug.

So what builds the voltage higher in the coil otherwise? The Voltage across an inductor is produced by L di/dt. Which is the inductance of the coil times the change in current divided by the time in which all this occurs.

So what can affect what?

If the coil had shorted turns, it wouldn't have the correct value of inductance (L). The output voltage would be low.

To switch the current at the given rate, you need a switch capable of doing that. If you have an electronic ignition module that is semiconductor driven, the semiconductor could be lazy or not getting the correct drive. This would produce a lower than normal current and resultant low voltage.

If it was a point condenser thing, the points could be pitted or the condenser could be leaking (partially shorted) bypassing the snap action of the points. If the condenser slows this interruption enough, there is no interruption (so to speak) and no output voltage produced.
Since you were able to increase the gap, and the engine ran fine, the coil apparently operated satisfactorily at a higher voltage which suggests that there is not an internal short.

That leaves the switching mechanism...ignition module (points or condenser/scr whatever).

Or, there is something about the plugs that allow the charge to bleed off before it gets to a high enough potential for sufficient plug firing. I would first think of dirty insulators, but having changed them twice, doesn't lend it'self to being the problem.

Course dirty plugs could very well be the problem caused by excessive fuel, which also causes black sooty smoke, caused by an improperly set float valve.

Or a SLOPPY FLOAT LIKE IS PREVALENT NOWADAYS WITH NYLON SUPPORTS RATHER THAN BRASS. The nylon allows the float to rise to the top of the bowl without supplying adequate pressure to the inlet needle valve and shutting the fuel off.

Gapping the wire just made a gap (which you apparently didn't have in the plug due to dirty insulator bleeding voltage off before it could jump the plug gap) allowing the coil to get to voltage which it couldn't when black sooted insulators were bleeding it off. Been there done that.

Nuf Said.....now that you are probably totally confused. Start with the float (and clean plugs) then the switch (ign module) and last the coil.

Mark