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1982 R100 After starting and riding for a few kms it misses and violently backfires spits and carries on and does not want to start again but when it does it usually goes for the rest of the day.
it has done this every time the last five outings.Don't think it is fuel. May be carbie?
Is there something strange that can happen with the ignition system? it is one of the early non points systems.
I have had the bike for 28 years or more and have never touched the ignition system except for plugs.

I had a similar problem on a couple of bikes with aftermarket plug wires. On one, they were too short and were not held by the rubber thingie under the tank, so wind, vibration and time made the wires slowly pull out of the coils. On another the wires were too long and there was no rubber thingie to hold them. Same symptom, cylinder would misfire, unburned fuel would ignite, big bang, run like shit, but then it would start and run again. Solution has proper length wires routed through the rubber thingie and a dab of Goop to hold the coil ends. That said, an electronic ignition with nowhere for the spark to go can get fried. Not sure if that's what's happening here. Dontcha just hate intermittent electrical faults!?

Had the same symptoms on my G/S, ended up being the ignition switch - the black plastic back of the switch is crimped in and after 300,000 hard kms it had just worn loose,and I was getting intermittent power.

Turning the switch off and on invariably restored the connection, a new switch fixed the problem.

Yes, it does sound like there is something happening with the ignition system. Whether it's strange or not will take some time to tell. Since you've not done anything other than change the plugs, there are a lot of possibilities, most of which will not fail until you are at least 100 miles from home and at the bottom of a hill with no safe place to leave your bike.

FIRST, check the chassis ground where all the brown wires are connected to the frame. One of those goes to the ignition control unit and if that connection is rusty, corroded or just plain loose the Ignition control module will act-up.

The 2nd suspicious item is the electronic control module. If it's anything like an '85, it needs heat-sink paste where it's mounted. The symptoms you describe sound like the old paste has hardened and is no longer doing it's work. See if that is a possibility before you go after the coil, wires, etc.
Heat-sink paste can be gotten from auto-parts stores as long as you are quite PLAIN about what you want. You do NOT want dialectric grease. I got a tube of heat sink-paste from a HVAC shop. It was about $12 for a 5 oz tube. Two heat sink pastes listed in Clymer's are GE Z5, and Dow Corning #340. Do not continue to operate the bike without checking for this!! A poor heat-sink "connection" will soon spoil the ignition module. It's co$$$tly.

jean wrote:The symptoms you describe sound like the old paste has hardened and is no longer doing it's work.

If it was the ICU would the symptoms persist when the bike was warm? There was the issue in the mid '80's where bikes might start on one cylinder, then eventually the other would kick in. That was often due to the ICU, but this sounds different.

If it is the ICU/ICM and it's toast, the one with the turquoise lettering covers all models. There are two sorts of those. One requires heat sink paste, the other latter variety doesn't. That one has rivets instead of the two bolts to attach it to the base plate. Either will do. Rick from M. Electrik and John from EME will also have an alternative that does the job for cheaper than oem I think.

viewtopic.php?f=1&t=5438&p=62726&hilit=icu#p62726

It can be intermittent if there's a bit of paste left that still conducts heat. HOWEVER, continued operation will soon cook the module. I'm not acquainted with the different mounting schemes and whether or not paste is needed on some...it's just an observation from experience with a 1985 module. Fresh paste solved the problem, which was a lot like that described.
THIS module doesn't seem to be TOAST...yet.