Good morning all!
When I get into a new activity or return to one as I have returned to motorcycles after a 25 year absence, I always gather as much literature as possible on the new topic and read it. That is the best way to find out what is going on, who are the best suppliers of parts etc. and also, to make some new friends. With my return to motorcycles and particularly, old ones like my XS650s and even the 2007 Honda ST1300, I have subscribed to a couple of UK magazines:
Practical Sportsbikes and
Classic Motorcycle Mechanics. By the way, my local Chapters (like the US Barnes & Noble) stocks both of these so you can sample them without committing to a subscription. I also got the US-based
Cafe Racer magazine which is also good.
These are all truly excellent publications and really informative because they actually show you how to
DO something on your bike as opposed to simply how to
BUY something for it which is what most of the "mainstream" bike magazines put-out these days - along with a lot of useless sh!t about riding the latest big-buck Harley / Indian / Victory / Ducati / BMW etc. etc. through "Wine Country". Sorry, journos, but not everyone is a California-based zillionaire whose business associates provide them with free bikes and hotel rooms and pays them to swan around writing about their adventures while wearing fancy biker gear and eating gluten-free, fair trade, free-range avocado-coriander-caviar-curry tacos.....
OK - I
wish I was one of those people - but I'm not. Rant mode disengaged now. Anyhow....
....on the matter of ignition coils, I have read quite a bit on this Forum about how the old Yamaha OEM XS650 coils were weak and get worse over time and while the initially low kV rating of 1970s vintage coils did make sense, I did wonder why they deteriorated over time. It seemed to me that since an ignition coil is simply...a couple of coils of wire wrapped around an iron core and that the insulation would either be good or bad and that the wire would either be contiguous (not broken) or it would be broken. If the insulation is bad or if the wire is broken - you have a dead coil and can simply toss it because these things are sealed (potted) in epoxy and cannot be fixed. I suppose the insulation might be ok cold but begin to break down when it heats up - but whatever.
To rescue was the recent June 2017 issue of
Practical Sportbikes magazine with an article on testing and refurbishing old ignition coils. The article essentially said that if the coil measures properly with an ohm-meter (ie. the primary side and the secondary side are within spec) - and this is a very simple yes/no measurement, then you can refurb the unit by simply cleaning the contacts really well with some steel wool or sandpaper and contact cleaner.
Due to copyright concerns, I can't reproduce the article here but the step-by-step gist of it follows:
- CLEAN the terminals - each coil has a primary (low voltage) side and a secondary (high voltage) side.
The
primary side is what connects to your bike's 12volt wiring harness and the two connections may both be wires with bullet connectors or one of the connections may be a part of the iron core of the coil sticking out the end of the unit. A bolt will go through that hole and act as a ground connection to the frame of the bike.
The
secondary side of the coil is where the big voltage spark comes from. On our OEM XS650 coils I think it is around 15,000 volts but modern systems produce as much as 80-90,000 volts or 90 kV. There will only be one terminal for the secondary side as the spark plug screwed into the cylinder head provides the ground connection for this portion of the circuit. The secondary coil terminal has a large diameter plastic "nut" through which the spark plug wire goes. You can unscrew that nut and inspect the actual connector which is shrouded in a little plastic housing. Down in there is a thing that looks like a wood screw - THAT is the connector. To make the secondary connection, you simply screw the spark plug high tension (tension is another word for voltage) wire onto that "wood screw" thingy and slide the plastic nut back on and tighten it - with a little dab of silicone dielectric ignition system sealer grease to keep the water out.
You MUST have good connections on both the secondary and primary sides of the coil for it to work properly - and apparently, rust and crap on one or more of those three connections is the almost always the culprit when a coil is said to be "weak" or dead. The actual failure of a coil is not unknown, but it is rather rare, apparently. The magazine suggests a wire brush and a can of brake or contact cleaner to polish up the connections.
OK - so now you have cleaned the connections - let's test the coil!
TESTING an IGNITION COIL:
- PRIMARY (low voltage coil - the 12volt part of the coil): this portion of the coil has about 300 turns of heavy gauge wire wrapped around an iron core. The resistance of this relatively short piece of large diameter wire should be quite low. Put your ohm-meter on the lowest scale and measure the resistance tween the two 12v terminals (one of which may be the mounting pad / bolt hole of the coil itself) - it should measure around 1-3 ohms (the value given in my Clymers manual is 2.25-2.75 ohms).
- NOTE: measuring small resistance values is not easy, so when checking a primary side coil, what you are looking for is a small value. A large value - like 100s or 1000s of ohms is bad.
- SECONDARY (high voltage - spark plug lead side): this side will have around 20,000 turns of small diameter wire and resistance will be quite high - likely around 10-15,000 ohms. Put your ohm-meter on the 20k ohm scale and measure between the "wood screw" and the ground terminal of the coil (which, again, may be a mounting bolt hole through the body of the unit) - or if the coil has two outputs, simply measure between the two "wood screws". You want to see a resistance of around 10-15,000 ohms here - if not, you either have a terminal which needs more cleaning - or a bad coil (which cannot be repaired).
Anyhow - I'll bet MikesXS/XS650 Direct has made a lot of money selling coils to folks who could still be using their old ones if they had simply cleaned and tested them. This is a very simple task - likely less than 30 minutes start to finish.
Don't forget to put a little dab or dielectric silicone grease on the secondary coil connection (the spark plug wire) to keep water away from the signal. If water gets in there - your spark plug may not fire consistently and the engine will "miss".
Once you've done all of that,
wah-la - your coil should work like new again - or truly be dead - but at least you will know for sure - prior to spending a bunch of hard earned cash on a new one.
Pete
I have 4 coils , all 78, 4 plug leads, and many plug caps.. So late after work I made it home to take some readings. Did notice after unscrewing the black plastic nuts (cool) that two of my coils were sparkling clean ! One dirty, one green contaminated. Plugs leads reflected all this too.. So on 20k I started probing, and again, again... wth , my ohm meter will not repeat to save its own life ! So I just gladly poured my beer on it and declared it Dead ! That is great news and was well worth my time because now I believe my testing of other components (rotor, rectifier) was incorrect ! New excuse to buy a better one at say Ace Hardware ? Recommendations ? I really need to figure all this ! Thx, MaxPete 
