Firing sporatically on 1 cylinder, just a bad coil??

[EasyE]

Have a 78 xs400, fires up, idles, and rides perfect, until I get around 20 miles and it starts acting up. Once coming to a stop the bike wants to shut off, can restart and then power drops like a rock bc its only running on the left cylinder. Next day the bike will fire both cylinders pumping and doing they're thing and same problems after riding. First thought is a bad coil?? but I found this pill under my right coil and just for education I was wondering what that was and its functionality.

I also have a wax coming down from my coils??

 

[drewpy]

what you talking about, Pill?

loads of wax means the coils are overheating so much it is melting the insulation inside the coils. could mean the contacts are not gapped/cleaned properly

 

[EasyE]

The brass cylinder under the removed right coil

 

[drewpy]

that's a condensor (actually its two connected together)
its connects between the coil and contacts and stops arcing of the points.
you don't want arcing as that's how arc welding works, which is no good if you want points to actually open.

 

[EasyE]

Took off ponts cover and they're new and no marks of arking, pull right plug (side that was firing and started her up no spark on plug but ran on left cylinder. Switched up coils and bike wouldn't start...coil problem. What's the functionality of the condenser??

 

[drewpy]

A condensor (actually its two connected together)
its connects between the coil and contacts and stops arcing of the points.
you don't want arcing as that's how arc welding works, which is no good if you want points to actually open.

 

[drewpy]

double post

 

[EasyE]

new coil didn't do it...is there a way to check the condenser???


points look good

remind you that the bike is only giving problems when riden a while and hot

 

[79xs]

For testing you can disconnect the condenser when the bike starts to act up. It's there to keep the points from arcing when opening. It's not good to run it without the condenser, but it will be fine for testing.


You made it sound like the problem followed the coil, but swapping coils made it stop running? Seems odd. What does the plug look like in the cylinder that always fails?

 

[Doubleath]

I have an 80 with electronic ignition and I'm having exactly the same problem. Starts and runs great till I get about 20 minutes into a ride. When i come to a stop, give it a few seconds and it starts to miss and stall. It will restart and I can take off running fine until the next stop.

I was thinking I have a cracked carb holder so I soaked them down with started fluid once the problem started but no change.

I found that I can disconnect the right cylinder plug wire and it will keep running with a very slight change in exhaust note. Tried the same trick on the left cylinder and it dies instantly.

I checked the wiring to the coils and found when removing the right cylinder plug wire from the coil that it was really corroded. I cleaned it up and reseated the plug wire but still not change.

After the above I swapped the left and right coils, still no change. My coils look fine, nothing leaking from them.

I checked the plugs and the right cylinder plug looks black (rich) and the left is gray/white (good).

I really don't understand why it will run right cylinder only but not left cylinder only. Just posting my experience and hope someone has dealt with this before.