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#2 amd #3 cylinder stopped firing

  • Thread starter Thread starter brand new bike zealot
  • Start date Start date
B

brand new bike zealot

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I have a 78 550 that has been having some hot idling issues for a while. Runs great until it gets really warm (stop and go traffic in summer) then the idle drops below 1000 and it bogs a little unless you ease intot the throttle. I replaced the plugs (looked blackened, and at were at least 15 years old) and it ran a lot better but still did the same thing but to a much lesser degree. After talking to a motorcycle friend, the conclusion that the ignition system is the likely culprit, not carbs as I previously thought. So I went to my shop and lightly oiled the felt, and cleaned the points with some laquer thinner and the warmed engine now only runs on #1 and #4. Dang.

So. Long story short, to check for spark, do I take out the plug, connect it to the lead and hold it to the engine with my hand? I don't want to get zapped...

Having 2 cylinders dead suggests to me half the system is dead, so checking coils is in order. What is primary resistance? I understand checking across 2 plug leads for around 30k ohms.

The points are pitted, but having never seen new ones, I can't really say how worn they are. They don't look too bad... The carbon is a little mucky at the armature, but I cleaned it up and stil no fire.

I have the clymers, and will be reading it, but first hand knowledge and advice is much appreciated and easier to understand sometimes
 
From what I understand, 1 and 4 are paired, and 2 and 3 are paired so you may have a coil that is failing under high temperatures. I'm not sure on how to check it though...
 
You might want to clean that set of points with something like a piece of fine sandpaper doubled up and pulled back and forth. The may have a built up layer of oxidation on them impeding current flow. Also, are they opening and closing like the other set? While running I can see both set of points on my 850 sparking while it idles.

Pat
 
I was looking for the points spark, but couldn't it on either really, as I was outside. Should this be visible in sunlight, or only when things are a little darker.

Does the coil come before the points or after? If bad spark at points, does that guarantee bad coil?
 
The condensor is what is supposed to limit the amount of sparking across the points to help them live longer. When the points separate, it breaks the current to an electromagnet in the coil. This degredation of the magnetic field induces a high voltage current in the core of the coil which is what creates the spark.
 
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