Originally Posted by ben2go
This is exactly what I need to know.I just don't don't know what wires and caps to use.I know that my plugs are NGK DPR8EA-9 which I believe to be non resistor type plugs.The problem I am having is an occasional back fire on the left bank and an occasional dead left cylinder.Coils check out,CDI has been swapped to a known good unit,and crank triggers have been swapped out for known good units.Oh yeah,plugs have been swapped several times for new ones.Battery and R/R has tested good.I did the relay mod to power the coils directly from the battery,and ran new grounds directly from each coil mounting post to the battery negative terminal.All connections are clean and secure.The only thing I haven't changed is the coil wires and plug caps.7mm is the largest wire that will fit down inside my coils on the screw posts.
With a couple of bikes that were ignition problem children, I've connected a timing light.
Then taped the trigger down, and taped the light to the gas tank so I could see it flash
while riding the bike.... Of course you have to stop and change leads from one plug wire
to another. That pins down the grumpy cylinder. I also use a home-made spark tester
almost daily, which adjusts so I can see how strong (long) and regular each spark is.
Resistance-type plug wires can break internally if they are ancient and/or twisted all around.
Plug wire size matters little unless the coil output is super high, as used in some
very high performance hot rods/race bikes...
If you already know which cylinder misbehaves, you may be able to swap plug leads between
cylinders, or even coils/etc. If the problem moves along with the relocation..........
There are other causes of backfires. Too lean, which can be a due to vacuum leaks, partially
plugged jet or injector, valve out of adjustment or slightly burned, change of exhaust system and/or
intake systems, leaky head gasket. Vacuum leaks are common between carburetors and heads.
One recent Yamaha here popped and banged while running and when the air box was pulled back,
the carb fell off. The rubber manifolds had perished. Easy to check for vac leaks with a propane
torch (hey, not lit!)
Point is, since swapping parts is pricey, you really need to pin down what's what before maybe
buying more stuff you don't need...