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Author Topic: This is a new one.  (Read 1621 times)

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Offline tomsweb1

  • '78 CB750A
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This is a new one.
« on: August 25, 2009, 06:38:20 PM »
This is WEIRD. Can anyone one tell me why this is happening?

spent the last couple days  Trying to figure out why 3 out of 4 cyls. were not running. Plugs were fouled. This is normal, if you have read my other posts about my old girl running rich. Now, cleaned 'em all up and.... no difference! Of course, #4 was still running, trying to get the others to wake up. Tried witching plugs etc. etc. Turns out ground wire to Coil 2/3 had become disconnected. Ok, now we have a running bike! *happy dance* But wait. #1 is still not running.

 I pulled the plug. Wet with gas. I run the bike with the plug up against the jug. No spark. That explains that. But wait... As I pull the plug AWAY from the jug... Spark between the electrodes, Then through a very significant gap in the air to the motor case. WTF!?! I Touch the plug to gorund again. NOTHING. I repeat this many times to make sure I'm not dreaming. Yup. The plug sparks away just fine, as long as she's not touching the case....

Anyone else seen this before? Know why it's happening? Why only cyl #1? Know how it can be fixed?
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Present: 1978 CB750A, 1978 CB400A, 1983 Nighthawk 550, 1984 CM250C

Past: 1977 XL100, 1982 KZ1300

Offline Steve F

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Re: This is a new one.
« Reply #1 on: August 25, 2009, 06:43:16 PM »
It could be that the plug boot/cap has a crack in it and when you put it on the plug, the sparks jumps through the crack and won't go throught the plug.
Steve F
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Offline bistromath

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Re: This is a new one.
« Reply #2 on: August 25, 2009, 06:43:53 PM »
Plug fouled with gas, oil, or carbon.
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Offline tomsweb1

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Re: This is a new one.
« Reply #3 on: August 25, 2009, 07:09:57 PM »
New caps, wires seem ok. I will take a closer look.

Quote from: bistromath on August 25, 2009, 06:43:53 PM
Plug fouled with gas, oil, or carbon.

Could that be all? I cleaned the electrodes. One would think it would more easily jump between the electrodes when grounded, rather than and inch or two away...
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Present: 1978 CB750A, 1978 CB400A, 1983 Nighthawk 550, 1984 CM250C

Past: 1977 XL100, 1982 KZ1300

Offline Spanner 1

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Re: This is a new one.
« Reply #4 on: August 25, 2009, 07:29:00 PM »
Common problem, I'd say.   When the plug got fouled it was now shorted-out...i.e. the center electrode shorted to the plug barrel and with the plug sitting on the head = no spark.
Then lift plug away from head and you have created a spark gap to the head ( ground )....
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Offline tomsweb1

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Re: This is a new one.
« Reply #5 on: August 25, 2009, 07:52:54 PM »
makes sense except for one thing... It sparks between the electrodes as well as to the case, meaning it's only doing what it's supposed to when it's not touching the case.  ???

I get what you're saying, though, and would totally make sense if it weren't also jumping through the electrodes
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Present: 1978 CB750A, 1978 CB400A, 1983 Nighthawk 550, 1984 CM250C

Past: 1977 XL100, 1982 KZ1300

Offline IHWillys

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Re: This is a new one.
« Reply #6 on: August 26, 2009, 07:43:46 AM »
For kicks you might like to try this.  Install the plug in question.  Start the bike.  Now carefully pull the cap off the missing plug, but not completely, just far enough to force an arc from the cap to the plug.  See if that gets the plug to "clean up" and start firing like it should.  If it does, then ease the cap back on and it may continue to fire.  Of course, all this is of no help in actually fixing the problem with that cylinder.

I had an '82 Virago that did this on the front cylinder at about 35K miles.  Due to the engine config it was very easy for me to do this procedure while sitting on the bike, thus I rode it like this for quite some time.  It did this if it was cold(bike and ambient temp) and normally just a few seconds(longer the colder it was) of running with the cap dislodged allowed the cap to be placed back on the plug securely and it continued to fire.  It was a bit of a rat bike.

Ken
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Offline tomsweb1

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Re: This is a new one.
« Reply #7 on: August 26, 2009, 02:55:06 PM »
that actually makes sense, I will give it a shot!
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Present: 1978 CB750A, 1978 CB400A, 1983 Nighthawk 550, 1984 CM250C

Past: 1977 XL100, 1982 KZ1300
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