Often it is an added ingredient that makes problems come to life. . .
I was pretty sure you'd find the problem there. Going forward I'm going to caution others - you can't look at plug and say its OK. Yes there can be some obvious defects that make a plug bad like:
* broken upper insulator
* missing or smashed ground electrodes
* gaps filled with carbon
* broken lower insulator
What you can't see and measure is shorted insulators as most plugs have a built in resistor to suppress electrical noise interference. And you can't tell its wet inside or shorted so its best to replace a plug thats misfired for sustained period of time as it builds up deposits inside when it doesn't fire. (Obviously MUCH worse in 2 strokes)
Why its worse at low RPM:
At low RPM the time interval between ignition discharge is slower. Because its slower, the coils have more dwell time (charge time if you will) and saturate delivering more energy when the secondary core collapses producing the discharge arc. In essence the firing rate is reduced but the time allowed to fill the bucket full of water went up! The result is more energy is available at low RPM. SO - At idle you can easily have arc-over misfires because the energy is Greater and its looking for the shortest least resistance path to ground! And that may be through the plugs body, the cap, the coil wire to ground, or even the ignition coil itself as the housing material breaks down and leaks the high voltage to ground.
Air is good insulator. Dust, dirt, contaminated grease, oxidation, salt, water, all compromise the isolation barrier making arc-over possible. So, Keep the electrical system clean and seated!
blah blah blah - I wrote a book! Hopefully it helps someone understand WHY things happen this way.
I was pretty sure you'd find the problem there. Going forward I'm going to caution others - you can't look at plug and say its OK. Yes there can be some obvious defects that make a plug bad like:
* broken upper insulator
* missing or smashed ground electrodes
* gaps filled with carbon
* broken lower insulator
What you can't see and measure is shorted insulators as most plugs have a built in resistor to suppress electrical noise interference. And you can't tell its wet inside or shorted so its best to replace a plug thats misfired for sustained period of time as it builds up deposits inside when it doesn't fire. (Obviously MUCH worse in 2 strokes)
Why its worse at low RPM:
At low RPM the time interval between ignition discharge is slower. Because its slower, the coils have more dwell time (charge time if you will) and saturate delivering more energy when the secondary core collapses producing the discharge arc. In essence the firing rate is reduced but the time allowed to fill the bucket full of water went up! The result is more energy is available at low RPM. SO - At idle you can easily have arc-over misfires because the energy is Greater and its looking for the shortest least resistance path to ground! And that may be through the plugs body, the cap, the coil wire to ground, or even the ignition coil itself as the housing material breaks down and leaks the high voltage to ground.
Air is good insulator. Dust, dirt, contaminated grease, oxidation, salt, water, all compromise the isolation barrier making arc-over possible. So, Keep the electrical system clean and seated!
blah blah blah - I wrote a book! Hopefully it helps someone understand WHY things happen this way.
