Topic: Coil troubleshoot

[Ernest]

I just finished replacing a motor in my 750.  It's a K3 motor in a 74 cb750. 

The bike did run before but the motor had no compression on cylinder 3 so I replaced it and plan on rebuilding this winter.
Now with the new motor in I get no spark on 1 and 4 plugs.  Pretty sure I wired it back correctly.

I used a voltmeter to check the wires going to the coil and my results are:

The battery was getting low from trying to start it but the battery tested at around 11

the black/white wires tested 10.6 something, the  solid dark blue  10.6, the solid yellow .34

Not sure why I'm get low reading on solid yellow wire but solid yellow is on 2 an 3 coil, It works.

Any idea what's going on?

Have any better test to see if my coil is actually any good?  It seemed to work on my old motor

[Bodi]

The blue and yellow wires go to the points, which do connect to ground - zero volts. The yellow wire is showing closed points, the bloe wire is showing either open points or a problem.  Rotate the engine through two turns and watch that voltage on the blue wire, it should be very low for much of the time.
If you changed the engine, you probably have the points plate from the "new" engine, I think there's a problem there or with the points wiring.

[chung]

An instructor told me once that a number means nothing if you don't know what the number represents IE: girls, volts, amps, beers, etc
The resistance on a coil should be somewhere between 3 and 6 ohms. (ballpark figure) when checked across the primary leads.
 I would set the points first and then plug everything back in as if it were a running machine. Then turn the Ign on and check for fire by rotating the engine and checking a plug. If you only get fire in 1/4 or 2/3 then switch the yellow and blue wires. Repeat the check for fire at a plug. If the dead side changes to the other side then the problem is most likely the coil. Change the wires at the coil, not at the points to rule out the wiring.
Hope this helps

[bryanj]

Check that either the points are opening or you have acidentaly disconnected one of the wires, usually at the rubber boot by the rear brakelight switch

[Ernest]

I have swapped blue and yellow wires to the opposite coil and the problem follows the blue wire.  Whatever coil the blue wire is on doesn't wan't to spark correctly.  I got the bike running, had my timing light on plug wire 4 and the timing light would only blink intermittently about once every 2 or 3 seconds.  It runs bad and it seems like that blue wire has a short somewhere but my wireing and the points looks good.  I can't figure out what's causing it.

[750goes]

check the black/white wires also running to the coils for good voltage.... you may find that their contacts are not real good either.

The black/white wires I think should have 12V going into them,,, so check that they have voltage up to the coils.

check the black/white wires for CONTINUITY (that will tell if you have a good wire or not)

also the blue and yellow wires from the coils to the points plate - you may have a pinched wire...

[mrbreeze]

Damn...Chung is back!!!!! Dude ,you were on here when I first started checkin' out this site. Good info....keep it coming!!!

[HondaMan]

An instructor told me once that a number means nothing if you don't know what the number represents IE: girls, volts, amps, beers, etc
The resistance on a coil should be somewhere between 3 and 6 ohms. (ballpark figure) when checked across the primary leads.
 I would set the points first and then plug everything back in as if it were a running machine. Then turn the Ign on and check for fire by rotating the engine and checking a plug. If you only get fire in 1/4 or 2/3 then switch the yellow and blue wires. Repeat the check for fire at a plug. If the dead side changes to the other side then the problem is most likely the coil. Change the wires at the coil, not at the points to rule out the wiring.
Hope this helps

Calling Chung...Mike (MRieck) says I should contact you about the unused Dyna III box he sent you. I have a plate, need a box. Still have it?

[dt77]

Ernest,

I know I'm a bit tardy on the post, but did you find a resolurtion to you problem?  I'm having the same issue:  no spark on the 1&4 plugs (the blue wire). 

Cheers!!

[HondaMan]

It sounded like a loose wire between the coil and the points. There are 2 connections along that route: the blue near the coil goes to the blue near the back of the engine and the blue at the points. Same for the yellow circuit.

If the wire is loose or has a bad connection in those little "bullet" plugs, the voltage will stay near 11 volts up by the coils, all the time. This is pretty common: clean the connectors and squeeze the female ends down a little to improve their grip on the male plug end. You can clean them with Q-Tips and alcohol (I like Jim Beam) or brake cleaner.

[medic09]

It sounded like a loose wire between the coil and the points. There are 2 connections along that route: the blue near the coil goes to the blue near the back of the engine and the blue at the points. Same for the yellow circuit.

If the wire is loose or has a bad connection in those little "bullet" plugs, the voltage will stay near 11 volts up by the coils, all the time. This is pretty common: clean the connectors and squeeze the female ends down a little to improve their grip on the male plug end. You can clean them with Q-Tips and alcohol (I like Jim Beam) or brake cleaner.

A shot for me, a shot for the bike; a shot for the bike, two shots for me, eh HondaMan?   

[HondaMan]

Yeah, it runs smoother with a little alcohol in the tank. 

I'm MUCH stronger after one or two, aren't you?

[Ernest]

Ernest,

I know I'm a bit tardy on the post, but did you find a resolurtion to you problem?  I'm having the same issue:  no spark on the 1&4 plugs (the blue wire). 

Cheers!!

Yes! I did.  This was awhile back.  I got to know my electrical very well and pretty much rewired the whole bike with all new fresh connections. 

There is a animated wiring schematic where you can turn on and off circuits to simplify the diagram.  I believe there is a link to it in the FAQ but if someone knows what I'm referring to could you please post a link.

good luck,

ernest

[Bob Wessner]

http://oldmanhonda.com/MC/wiring.html