No spark

[mikeb]

Hello,
I have a 78 cb750k and can't get it to fire. I have 12v at the battery and read 9.5v - 10v at the coils and points. Could this small of a voltage drop cause the coils not to fire? Neither of the 4 plugs have spark. I'm using a new plug and holding it to the cylinder head to test.  Not sure where to go from here
Thanks

[rrgunslinger]

Sounds like you have a bad connection or ground somewhere.

Clean all your connections.

On Tue, May 29, 2012 at 9:22 AM, mikeb [via Honda CB750'S] <[hidden email]> wrote:

Hello,
I have a 78 cb750k and can't get it to fire. I have 12v at the battery and read 9.5v - 10v at the coils and points. Could this small of a voltage drop cause the coils not to fire? Neither of the 4 plugs have spark. I'm using a new plug and holding it to the cylinder head to test.  Not sure where to go from here
Thanks

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NAML

[TOOLS1]

I have to go with Gunslinger. You should have a lot more then 9.5 volts. I once had something like this happen, and it was a coil going bad, so check them also.
TOOLS

[mikeb]

Thanks for the response guys. I'll check it out and let you know what I find.

[mikeb]

Ok, I've ran a wire direct from the battery to the primary side of the coils and i saw one faint blue spark on one of the four plugs and that was it.  i'm using electric start vs. kick to test.  I disconnected the primary and secondary 12v wires on the coils and checked continuity like the manual says and it was good on both. I also unscrewed the plug wire caps and checked continuity between the two plug wires on each coil and got a resistance reading of .015 on both coils.  I'm not sure what that reading should be but since both have resistance at the same level I'm thinking it's very unlikely I have two bad coils.
I also have continuity on the blue and yellow wires from coils to points.  I Also have continuity between the point mount plate and negative battery post.
The single faint blue spark to me indicates a bad or intermittent connections somewhere?  But it has to be a problem that is affecting both circuits.  Is it possible to have four bad plug wire caps?
Points and condensers look like new.
This bike is all original (except for the 4 to 1 exhaust) with 12,000 miles but hasn't run since the late 80's.  
Any ideas? Thanks
 

[shinyribs]

There should be a big green ground wire that attaches to the coil where the front bolt mounts it to the frame. It's not real big,but larger than the others on the bike,and it should have a protective heat wrap on it(green also).Make sure that green ground wire is clean,connected and tight. Thats all I can think of.Sounds like you tested it pretty well.

[mikeb]

That connection looks good. I disconnected it and have continuity from the wire to ground and it tests good when I reinstalled it.  I'm starting to think I have two bad coils. The manual says to test continuity between the plug wires and I get the same resistance reading on both coils, maybe it should be 0?
When I ran the wire direct from the battery to the coils I read line voltage back from the points to the coils.  
I can feel current coming through the plug wire but it's not enough to produce an arc at the plug.  I should be able to see a definite blue spark and hear the click when it arcs right?.  I even removed a plug cap and stuck the plug directly into the wire just to make sure the caps aren't the problem.
The plug I'm using to test is brand new, never installed.

[mikeb]

Well I ended up buying two new coils after testing this thing nine ways to sunday.  Also bought new NGK Iridium plugs. I found that I can do a spark test against the shiny new metal bracket on the coils and I see a nice blue arc on the plug. I still don't see a spark when I hold the plug against the cylinder head, maybe it's too oxidized?  I read ground continuity to the cylinder heads.
Now it appears I have a fuel flow problem.
This bike has been setting since 1989 so I knew this process wouldn't be easy.  It shows 11k miles so hopefully it has a lot of life left in it.  I'm bound and determined to get it back on the road again!