Hi

I have a 1981 XS400 with the electronic ignition. I want to get new plug wires as the old ones look like they are original. I don't want to buy factory ones as they are expensive. I found a web site that has bulk wire and NGK plug ends. The only problem is the ends have different resistor ratings. I don't know what is the correct resistance for the stock wires. If my bike had points I wouldn't worry about it but I don't want to mess with the TCI unit. Does anyone have any ideas?

Steel core spark plug wire will do, the resistance is in the coil windings, not the plug wires themselves.

Steel core spark plug wire will do, the resistance is in the coil windings, not the plug wires themselves.

Thanks for the reply duaneage.

What I am talking about is the resistance in the plug boots (cap). In my old bike with a 6V system and magnito , it is perferable to have no resistance in the plug/wire/cap combo to get the most voltage to the plug. The downside to this is RFI noise, heard as a crackle on your car radio. The manufactures started to use resistor caps and spark plugs to eliminate this. I didn't know if the RFI would affect the TCI (black box) and if it needed a specific resistance in the caps to operate properly.

in the yamaha service manual cap resistance is 10K ohms

Copper or solid plug wires are the most reliable, an NGK resistor cap will be fine.

It is maybe an odd thought, but the resistor actually makes the engine run better. Why? The voltage has to build up more before it can jump across the resistance, giving a much cleaner, higher voltage spark. You'd think it would be the other way round, but will high voltages and high resistance odd things happen! Without the resitor the spark with be more spidery and lower voltage, although you'd need test gear to see the difference.

Hi.

Sorry I have been away from this site for a while.

If anyone would like more info on this, I came across a couple of web sites that help shed some light on the topic.

The first one is a site for ultra light aircraft. They take their ignition system very seriously. He does a couple of tests to see how resistance affects the spark.

http://www.ultralightnews.ca/articles/resistorcapsandplugs.htm

The second one I believe is a site for BMW F series bikes.You’l have to ignore all the info about rebuilding the resistor caps because ours are not rebuildable. There is some good info here also.

http://faq.f650.com/FAQs/PlugCapsnCoils.htm

The short story is that I feel perfectly fine putting 5K ohm resistor caps on my bike. I don’t believe that there will be any downside to the decreased resistance.The 5K caps are also much more easy to find.

Don't forget aero engines use self generating magnetos ( well every one i worked on over the past 20 years did), the xs a battery and coil ignition, totally different set ups. I'd go for what the manual says for the engine.