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Offline oldfett

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Strange Electrical Problem
« on: November 22, 2009, 11:55:09 PM »
I've been overhauling alot of systems on my old 1975 CB750K daily rider and hit a snag. I finally got everything all buttoned up but I'm having probems with sparking. The timming is spot on statically. It has used CB900 coils with brand new Dyna 7mm copper wires and newer plugs that are properly gapped and not damaged or too fouled. I'm also running a Dyna S ignition. If I pull out the plugs and try to ground the plugs on a bolt I don't get a spark. However if I pull the plugs an 1/8th inch or so off the bolt I get a spark. I also tested the bike with my old CB750 coils (they have a bad 3rd wire that's why I replaced them) and I get the same problem. The coils test ok, the spark plug wires all have good contact, and I'm getting proper voltages when the coil should be firing and when it shouldn't.
Some of this is telling me that I have a ground somewhere I shouldn't or it is not grounding somewhere it should, however some other parts leave me stumped. The wiring harness on this bike is less than great and the previous owner attacked it all over with electrical tape and driveway splices. Does anything stand out for you guys on what I should be looking for?
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Offline Raul CB750K1

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Re: Strange Electrical Problem
« Reply #1 on: November 23, 2009, 12:47:54 AM »
I'm a bit lost after all the mods you mention, but if the coils you've used work as the original ones, you shouldn't remove all caps at once. That is, if you want to check spark #1, spark #4 should be connected because it is the return path for the spark.
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Offline oldfett

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Re: Strange Electrical Problem
« Reply #2 on: November 23, 2009, 01:02:43 AM »
Hmmmm. Well it does it the same if I take everything out and only have one spark plug hooked up, the other three wires dangling, or if I have all the wires hooked up, 3 spark plugs installed and 1 pulled to ground on the engine as a visual test. We just did the later of the two tests an hour or so ago to make sure the pickup on my timing light was working. It will flash properly if the spark plug is pulled and held an 1/8 of an inch or so away from a bolt but will not flash if installed in the engine or held against a bolt.
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Offline w1sa

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Re: Strange Electrical Problem
« Reply #3 on: November 23, 2009, 01:32:37 AM »
Quote from: oldfett on November 22, 2009, 11:55:09 PM
............. and newer plugs that are properly gapped and not damaged or too fouled..................

Are the centre electrode insulators clear of sooty deposits?
Does a brand new unused plug do the same thing?
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Offline Raul CB750K1

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Re: Strange Electrical Problem
« Reply #4 on: November 23, 2009, 01:51:16 AM »
I think I don't quite understand you. Fromwhat I understand, you don't say you don't have spark, but your strobe light won't flash like if there is spark. Is that what you mean?
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Offline oldfett

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Re: Strange Electrical Problem
« Reply #5 on: November 23, 2009, 08:56:11 AM »
I'm sorry I have a cold so my head is nothing but sludge so I know I am not being as clear as I could be. I can get a spark as long as the plug is NOT grounded, ie just a hair away from a ground. Such as if you hold it just a little bit off of a bolt on the engine it will have a nice blue spark. However if you touch the plug to the ground, such as install the plug or touch the plug to the bolt on the engine it will NOT spark. The strobe light seemed like an easy way to check if it was sparking again, so I hooked it up. If the plug is installed in the bike I get nothing, so I know its not sparking. But if I hold the plug just off of the engine bolt the strobe light works so I know my timing light works at least :D

All the connections are nice and clean, some of them are even fresh and new as I just installed them. I even tried installing brand new yellow and blue wires straight from the ignition to the coils and I get the same thing, just as a test. The coils and the ignition are also getting a solid 11.94volts just like they are supposed to, so I am baffled.
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Offline 750goes

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Re: Strange Electrical Problem
« Reply #6 on: November 23, 2009, 11:58:46 AM »
sounds like the power to the coils is being grounded ?? but I don't really know - just a guess.
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Offline oldfett

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Re: Strange Electrical Problem
« Reply #7 on: November 23, 2009, 01:40:17 PM »
I had to replace the gaskets on the left side of the engine as the previous owner just hacked it back together. I am almost wondering if one of the wires off of the rotor coil got pinched in one of the covers. Thats the only other thing I can think of.
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Offline TwoTired

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Re: Strange Electrical Problem
« Reply #8 on: November 23, 2009, 01:57:01 PM »
Adding extra gap forces the coils to create a higher voltage to jump the gap.
You really ought to have some resistance in the spark circuit, either resistance wire, spark plug cap resistors, or resistor spark plugs.

Again, their needs to be an air gap in the circuit to allow the voltage to build in the coil.  If the center electrode insulator of the spark plug has a conductive coating on it, then it shorts the air gap of the plug itself, forcing the spark voltage to only build when a gap is added between plug body and cylinder head.

BTW, "ground" is irrelevant in the spark secondary circuit.  The cylinder head is merely used as a conductor to connect the two paired spark plug bodies.

Some timing lights are sensitive to pulse polarity. I.E. the inductive clip has to be positioned upside down when transferring from, say, the #1 spark lead to the #4 spark lead as they are attached to opposite ends of the spark coil, and thus have opposite polarity.
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Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline oldfett

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Re: Strange Electrical Problem
« Reply #9 on: November 23, 2009, 02:29:36 PM »
Well the spark plugs are DR8EIX's, so that satisfies the need of having a resistor correct? The wires are Dyna's DW200's which are solid core copper. I really have no idea what to look for in solving this problem.  :(
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Narsist

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Re: Strange Electrical Problem
« Reply #10 on: November 23, 2009, 02:37:40 PM »
the same thing was happening to me not to long ago it turned out to be fouled plugs. I did not have any flash from my gun on 2-3 wires.1 you do have to have a plug in and in good working order to get a flash without a plug firing there will be no flash.  I know you said that yours are not but mine looked good also so i would go grab a cheap set of new plug and give it a try. Hope it helps.

later frank
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Offline TwoTired

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Re: Strange Electrical Problem
« Reply #11 on: November 23, 2009, 02:39:42 PM »
Are the spark plugs brand new, never used before?  Is the center electrode insulator clean?

Yes, you have resistor plugs, which is correct for the mods you've installed.  Are these iridium?  Have any standard plugs to try out?

Take some Sudafed for your "sludge".  Otrivin is even better, if you can find it.
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Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline oldfett

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Re: Strange Electrical Problem
« Reply #12 on: November 23, 2009, 03:03:17 PM »
No these aren't new plugs. There kinda dirty so I'll run to town and pick up a set of standard resistor type plugs to see if that works. I'll let you guys know in an hour!
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Narsist

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Re: Strange Electrical Problem
« Reply #13 on: November 23, 2009, 03:08:24 PM »
good luck it worked for me
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Offline oldfett

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Re: Strange Electrical Problem
« Reply #14 on: November 23, 2009, 04:15:25 PM »
Well I just got back from Autozone. I picked up a set of DR8EA plugs. Made sure the gaps were about right and installed them.


and.....


She runs like a top! I'm a little ticked because it was something so simple, however I am also relieved that it was something so simple. Either way She has never started so easy and idled so well when she was cold so all in all I think it was worth it! Thanks for all your help guys. Once again I am in debt to this site.
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Narsist

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Re: Strange Electrical Problem
« Reply #15 on: November 23, 2009, 07:07:37 PM »
Great Job!!!!
have fun and keep the rubber side down

Later frank
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