FYI Thought was bad coil

On my way home I get within 1 mile and blahh it's only firing on 2 cylinders. Luckily i managed to limp in garage. Feel the pipes and 2 are cool, hmm has to be electrical. Figure might as well take both coils out and test. Put meter on good coil and within ohm range, test one I figured bad , yup nothing . Might as well buy 2, check prices yeowww. Take a look again check spec's just to make sure, Now both check out ok, hmm test bad one again nothing, now really confused.
Look inside spark plug cap and screw cap loose, take out resistor, spring and lil cap all rusty. Clean up with sand paper reinstall and make sure all tight.
YEA Baby she runs better then ever!
So before buy new coils check inside plug cap.

Thanks to all who post this forum, has helped time and time again

 

Remember, heat causes things to expand. It is quite common for electrical contact to be sound when the part is cool only to open up when the temperatures climb.
Time for new coils. Don't chuck the old ones, I'm looking for dead coils to experiment with. I'm trying to find an easy way to tear into these things and replace the secondary high tension leads (these are the part that fails most often). I pay postage.

 

Robert
I agree about expansion and electrical items do funny things at times but I don't feel there is anything wrong with coils as the screw cap was extremly loose.Thanks for input ,if they act up again I will replace ,they are 26 years old. I would then be happy to send them to you.

My job is electrical trouble shooter on printing presses. So to take coils apart and attach new secondary wires to windings Hmmm a noble cause and my hats off to you,if you succed. They are basically a step up transformer, only trouble spot would be varnish which insulates windings. If you could try and disturb as little varnish as possable to reattach and reinsulate it may work on some.
Have fun
Bill
82 Xj550 (restored 2006)
1929 Ford Model A (restoring, current project)
1973 Mercedes Benz 280SEL (restored 2005)
1978 Yamaha Phazer(snowmobile complete restored 04)

 

Thanks for the advice Bill. I've milled out the case on a few and have located the high tension lead spike. The factory soldered the conductor on to the spike and the coils are still sealed up nice. I've just got to find a material to pour back into the channel I've cut that will be both vibration and fuel resistant. So far the candidates are JB-Weld (yes, it will work but it ain't pretty), a liquid silicone (not cheap) and run-of-the-mill RTV (with dielectric properties I'm not familiar with). So far JB is my first choice.
I'm glad to hear you are just dealing with plug caps. After a while, the cap screws loosen up and you are back at square-one. This is not a major issue but can be a pain after a few cycles.

 

Here's a question for you electrical gurus: Why couldn't you use a pair of Standard Motor Product's part number DR-49 (or something similar) ignition coils mounted below the air intake boots to fire the plugs? Here's a picture:



You'd have to run leads from there back up to the plugs, but there should be sufficient room for that.

 

You can use any sort of coils you want as long as the windings have the same
resistance as the stock ones of about 2.5-3 ohms so you don't burn out the ignition box. I've used stock Harley coils with good results. They are cheap on ebay and provide a better spark than the weak Yamaha coils.

 

Just to clarify, you're talking about the low-side (or primary side) of the coil right? I'll check one when I get to work to see what it Ohms out at.

 

The 2.3 ohms refers to the primary side (low-side).