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Discussion Starter · #1 ·
Hey all,

I've been snooping for a while (thanks for all the great info!) but I've got something I just can't quite find an answer, and I'm on a shorter time frame.

I've had a 79 CX500C for about five years now. I've left it basically stock, except for replacing the needed consumables. Last summer I rebuilt the carbs using a Randakk's kit and Larry's excellent book (although I didn't have an ultrasonic cleaner and it was my first carb rebuild). It ran great all through the fall and into this summer.

Yesterday while riding around town on a pretty hot day, it developed a noticeable stutter at 5k rpm. Listening to it more closely, it sounds like just the left side is cutting out, and the sound of unburned fuel popping in the exhaust sort of confirms that. If I relax the throttle a bit and increase steadily, it'll ease through the stumble and accelerate, but if I open it up above 5k it stumbles again.

I thought stator before realizing it was just one side, but all the resistances check out (a few ohms above the spec for the Haynes manual check). I haven't done a voltage check, but I assume it would be fine. I'm running the stock (and original) CDI and coils, but don't know a lot about checking those and don't have access to known good ones to swap out. I haven't pulled the plug yet, as I'm in class now and noticed the one side issue on the way up here.

I'm just wondering what the checklist of things are that could cause these symptoms? I'm moving at the end of July, and either I find a way to fix and keep it, or I can't justify trailering it across country and sell it.

If it's simple and I'm just ignorant, I'll be thrilled.

Thanks in advance for any help!

Kyle
 

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Check your coils. Seems to be the issue a lot of times with this sort of thing.
 

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Discussion Starter · #5 ·
Update:

I pulled the plugs and they were bad, but not awful. I'll replace those.

I took off the coils to check resistances, but the left side resistor (original coils based on the pink/yellow wires) was completely corroded and fell apart when I got it out. On the right side, the plug wire came apart (detached from the coil) when I got it off. It was completely corroded at that attachment point.

So what are some good options for coils? I'll need new caps as well...
 

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Discussion Starter · #6 ·
Better do the stator test anyways
You may have a high speed pickup fail or a bad advance pulser
Should I do the stator voltage test? The resistance test showed just a few ohms high on all the values. And can I do that without working coils?
 

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I bought these: Ignition Coil Yamaha ATV Badger 80 YFM80 New 1989 2001 | eBay

They worked just fine for me. I know some others here on the forums run these as well. You will have to get creative to mount them since they are shorter than OEM... but ot's not difficult. I cut a couple small pieces of aluminum brackets and drilled a couple of holes to mount the coil to the frame. Easy install for sure.
 

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Discussion Starter · #9 ·
Update: the coils (with the caps removed) are reading just over 8 k ohms. However, both caps, even with the spring/rod/resistor out are reading as opens. Has this happened to anyone else before? Could it be as easy as new caps?
 

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I had an open cap, but the plug still sparked. Replaced both & did the brass rod change, and the bike ran better for a little while.
 

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Discussion Starter · #12 ·
Can I use some NGK straight caps with resistors just to do some testing and diagnosis before I buy the $40 ones from wemoto?
 

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Can I use some NGK straight caps with resistors just to do some testing and diagnosis before I buy the $40 ones from wemoto?
Sure, but don't spend the $40. Just replace the resistors with a similar length & diameter chunk of metal when you switch to resistor plugs. Just do it for your next plug change

Folks here like to use brass welding rod, but I just bought some brass screws at home depot & ground down the heads a bit to fit the openings but catch the springs.
 

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I'd replace them unless you can clean up the inside enough to get 0 resistance. The brass strap inside of them can burn or corrode out with age and you'll never get a circuit through them. Also, the insulation breaks down with age and they leak voltage.
If you are sus on the caps swap them side to side to see if the fault follows.
 
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