I recently replaced the spark plugs in a Toro (Loncin) 24.5HP V twin with new non-resistor plugs, the original were Champion RN9YC resistor plugs. The replacement was solely scheduled maintenance as per the book.
The engine was then difficult to start, it took a long time cranking and often backfired whereas previously it had started quickly and certainly. Low speed idle was rough and slow.
So, could the non-resistor plugs be the cause of this sudden change?
A pair of new NGK BPR6ES (resistor plugs) was installed and the engine starting was restored and idle speed was smooth and book speed.
So, the non-resistor plugs did look like the problem, and being of an enquiring mind, I captured the HT waveform with one resistor plug and one non-resistor plug (#2).
It was again difficult to start and idle was slow and rough.
Above is the capture, red is #2 and blue is #1. The lack of symmetry can be seen, there is a distinct blue spike at the beginning of the red burn phase. It seems like the red spike (non-resistor plug) that caused the spark burn has somehow triggered an impulse on the blue circuit.
This 90° V-twin's conrods share a single crankpin, which means that as #2 reaches TDC on the exhaust stroke (and waste spark generated triggering the spurious impulse on #1 spark plug), #1 is mid way on the compression stroke (ie there is fuel air mixture in #1 cylinder) which might explain the problems observed.
Now the magneto coils and HT wiring are really well physically separated, perhaps the strongest coupling of the two ignition sub systems is the kill wire. Perhaps the transient due to breakover of the spark plug has coupled to the other ignition circuit and caused this false impulse, this is a kind of electromagnetic compatibility problem where the two ignition subsystems should be totally independent.
Above is a capture with the two resistor plugs installed, the behavior of each ignition circuit appears quite similar, but independent of each other, ie there appears no false triggering of an impulse from the ‘other' ignition sub system.
In this trace, the different burn phases can be seen, those with compressed fuel air, and those with an 'empty' cylinder (waste spark).
I have used non-resistor plugs in single cylinder small engines with not apparent ill effect, so it seems that the issues here are interference with one cylinder's ignition circuit by the other, something that would not be apparent in a single cylinder engine.
A suggestion by online experts is that using a resistor plug results in a longer burn time, but in this case the burn times were within 1% with and without the resistor.
On further examination
Above is a pic of both ignition coils. It can be seen that the HT wiring is short and direct so the likelihood of cross fire from one HT lead to another is very low. The kill wire is in close proximity to one HT wire, #2 on the right. Differently to a lot of V twins, this does not use a different coil for left and right… good for inventory but it exacerbates the coupling issue being discussed.
These coils probably contain electronics, but even so the probability of cross fire by magnetic coupling or one to the other is pretty remote.
Above, coil #1 has its kill wire (the thin black wire) 5mm or so from the HT wire and parallel to it for a bit, not wonderful, but some separation.
Above, coil #2 has its HT wire pulled tight over the loom which contains the kill wire, which then turns back again crossing close to the HT wire.
IMHO, the mechanism of the observed cross fire is probably coupling of the higher transient using non-resistor plugs to the other coil via the common kill wire… though you would think that a good design would incorporate effective transient suppression on the kill wire.