Topic: Help...Please....electrical problems..no spark

[evilaaron]

Hello folks,

I am a fairly new builder and I'm working on a CB750 build.  I am putting together a CB750K that I had parts from other bikes.  Since I've never had it running I assume there are lots of problems.  My main problem is I can't get a spark from the plugs.

First off, does this sound correct for the ignition... When I turn the key one click the dash lights turn on and the headlight turns on.  When I turn it two clicks all lights turn off??? 

There are no controls on the handlebars so I am working with wires.  I tried grounding the green wire with red stripe to the bike.  The engine is turning and I can feel the generator spinning.  However, I can't get a spark from the plugs.  This is all on one turn of the ignition switch. When I turn the key two positions (ignition position), the grounding of the green with red stripe wire doesn't do anything.

I am looking at the coil and I don't have any volts coming from the yellow, blue, or black with white stripe wires at the coil.  When would this have volts on the wires?
 
On the bike there is a fuse box:
Top Fuse - 12 volts when ignition switch is turned two clicks only
Middle Fuse - no volts in any position
Bottom Fuse - 12 volts when ignition switch is turned one click only
Is there another main fuse somewhere?

I could really use some insight.....

thanks!

[grcamna2]

Your system is set up for the ignition key switch to work with it in the first click position & that is full on,the second click is for the parking light and only the taillight & meter lights will be on; Do you a have a very good Service manual with an excellent wiring diagram ?

[TwoTired]

I am a fairly new builder and I'm working on a CB750 build.  I am putting together a CB750K that I had parts from other bikes.  Since I've never had it running I assume there are lots of problems.  My main problem is I can't get a spark from the plugs.

There are several models and years of the CB750, and the wire diagram varies among them.  We could help better if we knew what year wire harness you are using.

First off, does this sound correct for the ignition... When I turn the key one click the dash lights turn on and the headlight turns on.  When I turn it two clicks all lights turn off??? 

Yes.  The last position is a park position where only the tail light stays on.

There are no controls on the handlebars so I am working with wires.  I tried grounding the green wire with red stripe to the bike.  The engine is turning and I can feel the generator spinning.  However, I can't get a spark from the plugs.  This is all on one turn of the ignition switch. When I turn the key two positions (ignition position), the grounding of the green with red stripe wire doesn't do anything.

The starter circuit and the ignition circuit follow different paths.
In general, the key switch connects the Black wires to the battery Red wires.  The Black wires do power the starter solenoid.  I guess on your model/year bike, the Green/red supplies the Batt neg return to activate the starter.
However, the coils are normally powered through the Run-Stop-Run switch on the bars, to the Black/White wires of the coils.  Then the coils connect to points via the Blue and Yellow wires.

I am looking at the coil and I don't have any volts coming from the yellow, blue, or black with white stripe wires at the coil.  When would this have volts on the wires?

See above...
 

On the bike there is a fuse box:
Top Fuse - 12 volts when ignition switch is turned two clicks only
Middle Fuse - no volts in any position
Bottom Fuse - 12 volts when ignition switch is turned one click only
Is there another main fuse somewhere?

The middle fuse may be a headlight fuse which would get power without the headlight switch on.
The other two are likely your Main fuse (15A) and a Stop/tail light fuse (5 or 7 amp).  Look closely, the block may have labels on it in raised letters.

Cheers,

[grcamna2]

Do you have a pair of handlebars w/ good switches on them? that is a good thing to have to complete your wiring tests.

[Hush]

Sounds like your "kill switch" is active (or disconnected which ever applies) and therefore it is stopping spark occurring.

[grcamna2]

Also there a number of grounding points for your harness/switches to have...it would be best to have everything all hooked up on the bike just like you were ready to go for a ride...electrically.

[evilaaron]

I appreciate the help folks.  I don't have the original handlebars with the wiring but I think I'm almost there. The wiring harness is from a 73 CB750.

I have the wires fairly organized.  All of the left control wires are separated from the right controls wires and I went through the wiring diagram and labeled all the wires.  I am only focused on the right control wires as the left don't appear to have any impact on starting the bike.  All of the wires are pretty organized and I performed some continuity tests and power tests.  Everything looks pretty good besides no spark.   

I have crossed the yellow with red stripe and the green with red stripe wires that were going to right control.  That allowed me to ground the other end of the green with red stripe wire and get the engine turning but no spark.

The only two wires I have left were the two black wires in the right control.  I believe one was going to the headlight and the other

What wires in the head light do I need to connect or ground to NOT activate the kill switch which may be preventing a spark? I assumed the kill switch kills power to the ignition coils. if it isn't pressed it shouldn't prevent a spark?

It also won't spark when I kick it...

[Spanner 1]

Two Black wires ?... yeah, one goes to the headlight switch and the other goes to the kill switch.
The kill sw. black must be connected along with it's partner black/white to send power to the coils.....
O.K... from the right hand switch = black/black/brown-white/yellow-brown/green-red...for power to coils black and black/white must be connected.. you can leave the rest unconnected if you like, won't stop the coils from getting power.

[Hush]

Good on ya Spanner, not being a 750 guy I wasn't sure of the colour codes but was pretty sure he has the kill switch active. Which in fact means the blk/w wire must be connected to the black wire, easy when you say it fast eh!
Report back Evilaaron as we like to know we got it right or if you need more help.

[evilaaron]

got it right! I got a spark now at the plugs.  Appreciate you guys!

Black with white stripe and the black wire together fixed the problem.  However, the engine didn't start yet.  Engine sounds really faint like it has very little power.  It wears down the battery really fast (even an additional jumpered battery was drained).    I think I have everything connected correctly but a bad wire/loose connection somewhere.