Bike wont start and backfires

[phatman1969]

Hi all. Hope everyone is doing well. I am back to square one on my 82 xs400 dohc. Last week i thought i had this thing figured out. I switched the spark plug wires around and she ran like a scalded dog. Two days later she wont start. Backfire, backfire, nothing. So today i switched the plug wires back and got it to start but only runs up to 4k rpm and then wont go any higher. You have to full throttle it to keep it at 4k rpm. I guess my question is regarding the coils. The coil on the left side in the downtube has orange and red leads. Which spark plug should that go to. This is the left side while sitting on the bike. My coils are good, as well as the ignitor box, and I have also changed my pick up coil as well. If i know that I have the correct coil on the correct plug maybe it will help me narrow this down. Thanks for the help.

 

[JARichmond]

If I understand correctly, you're looking for which coil goes to which spark plug, correct? On the stock setup the coil on the left side goes to the left plug. Of course, if your timing has been messed with, that may not be the case.
From my experience, an XS that has a dead cylinder will struggle to increase RPM up to 4-5k, sometimes higher/lower depending on your timing. It's all about friction.
I would suggest that you trace your leads if there is any doubt which coil is timed to ignite which plug and when. You can also have a look at your timing too, which seems to be at least part of the problem.

 

[phatman1969]

Thanks for the reply. How do I check the timing on this bike. It is electronic ignition. I didnt think I could mess with it. It ran again last night wide open, pulled through all the gears, rmp all the way to 9k. I pulled it in the garage last night and switched it off. NO changes, nothing moved and today she wont start. Frustrated.

 

[JARichmond]

Have a look at your spark plug tips. I have a constant issue where my plugs foul very easily (I need to go a step hotter) and so it will sometimes refuse to fire up even though nothing has changed since I last touched it.
You shouldn't have to alter your timing at all with electronic ignition, so I would check elsewhere.