Originally Posted by CycleDoc59
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!) Valves checked and in the middle range.Carbs are clean and jetted properly.New carb to head intake boots.I have a pod style filter and never had issues with it except in extremely gust (40+mph) winds.No head head gasket issues,holds compression hot or cold.
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...
I had done all the maintenance on my bike and about 300 to 500 miles later,I get this.So I tear the bike down and check everything.Everything checks out.So I strip down another bike I'm working on and use all the ignition parts on mine except the coils which have mysteriously went missing.I figure $10 to $15 for new plug wires and ends on a bike built in '89 isn't to much to spend.Napa has bulk 7mm plug wire for 69c a foot.I need two.They can also get plug caps in 1k,5k,and 12k resistance.