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Author Topic: 72 cb500 - Cylinders 3 and 4 not firing  (Read 2861 times)

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Offline RichJr

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72 cb500 - Cylinders 3 and 4 not firing
« on: April 22, 2011, 08:19:23 PM »
Hi All,

My issue is with my 72 cb500.  Starts up 'fine' but when put into gear, it sputters and comes close to dying or just dies all together.  I can sometimes get it rolling (all but very poorly) if I let the clutch out very slowly and inch along until it catches, at which point it will not go over 20 mph and wants to die at every intersection.

I just had the carbs cleaned, points/condensor and coils checked and they're good.  Noticed that when/If I do get her running, cylinder #4 is not firing.  I replaced the plug wire (spliced from existing wire from coil) on #4 but it had no affect. 

Any ideas?
« Last Edit: April 23, 2011, 10:58:52 AM by RichJr »
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Offline RichJr

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Re: 72 cb500 - Starts, then sputters and eventually dies in gear
« Reply #1 on: April 23, 2011, 10:57:24 AM »
Allright, after a little more investigation, realized cylinders 3 and 4 (sitting on bike, from left to right) were not firing due to the exhaust being cool to the touch (1 and 2 were hot).  Since they are running off different coils, I'm assuming the coils aren't the issue.  They are both however fed from the same fuel line, which has a fuel filter on it (?).

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Offline bryanj

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Re: 72 cb500 - Cylinders 3 and 4 not firing
« Reply #2 on: April 23, 2011, 11:14:29 AM »
May sound silly but how good is your battery, a bad battery or one with low charge will not have enough "Juice" to fire the coils properly
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Offline RichJr

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Re: 72 cb500 - Cylinders 3 and 4 not firing
« Reply #3 on: April 23, 2011, 11:32:34 AM »
The battery is good, as it was checked out by the mechanic who cleaned the carbs, etc...

Looking at the coils, everything looks connected properly and grounded.  What could cause only 3 and 4 to not fire?  I'm rather new at motorcycle maintenance but the only thing I can see that correlates cylinders 3 and 4 is the fuel line.  Correct me if I'm wrong, but don't 1 and 4 pull from coil A and 2 and 3 pull from coil B?
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Offline CBGhia

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Re: 72 cb500 - Cylinders 3 and 4 not firing
« Reply #4 on: April 23, 2011, 11:47:29 AM »
I bet that you either have a clogged line or petcock.  Pull the tank; put 2 fuel lines on it going in to different jars. Then turn on the petcock and see if one side is flowing too slowly.   If they flow fine there, put new fuel line going to the carbs.
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if you dont trial spin the camshaft in the head and cover you are a novice,with no natural mechanical appitude,destined for destruction.
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Offline dave500

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Re: 72 cb500 - Cylinders 3 and 4 not firing
« Reply #5 on: April 23, 2011, 02:06:14 PM »
if you have acess to a multimeter check the plug cap resistance,if you can rig up a temporary wire to bypass those two caps try that.
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Offline mjstone

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Re: 72 cb500 - Cylinders 3 and 4 not firing
« Reply #6 on: April 23, 2011, 04:05:57 PM »
Fire it up without the air box on and spray some ether into the #4 carb and see if it picks up. If it does than its a fuel problem, if not its something else.

MJ
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1973 CB500Four (Oliver)

Offline RichJr

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Re: 72 cb500 - Cylinders 3 and 4 not firing
« Reply #7 on: April 23, 2011, 06:19:11 PM »
Thanks Gents for taking the time to answer, this is one hell of a community!

Replaced the fuel line running to #3 and 4 and she fired up with all cylinders.  It's still missing occasionally, so there might be a coil issue.  Also, to CBGhia's point, I still might have some blockage in the petcock.

Thanks again!
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