Topic: the 1-4 coil mystery??

[saw750]

first let me make clear that issue described below is only happening with 1-4 coil (1976 K6)

when i run the starter i have no spark from the 1 or 4 plug.

i checked the points. the gap was correct and the points were in good condition.
have power going to the coil and brand spank me new plugs

sooooo i decided to bridge the points for the 1-4 annnnd bingo! great spark!

kicked the bike over manually and again great spark!

ran the starter and NO SPARK ..what?

went thru everything once again and sure enough..no spark (when i ran starter)

the 2-3 plugs/coil are producing strong spark regardless of how i start the bike.

any ideas?

why would running the starter cause no spark? is it causing a voltage drop that is preventing
the coil from functioning?

condenser? (tomorrow i am going to pull condenser for the 1-4 and retest)

thanks in advance - saw

[Spanner 1]

My guess would be the + voltage at the 3-way black/white coil connector under the tank has a loose 1-4 connection...... or generally poor connections in the ignition circuit ( inc. kill switch wiring ) is causing enough voltage drop when starter engaged to cause one coil to 'drop out '. When the voltage goes low, one coil/plugs/ points would be 'favored' if it's has a lower resistance path than it's twin and may take all the available energy.... IMO.

[Frostyboy]

My guess would be the + voltage at the 3-way black/white coil connector under the tank has a loose 1-4 connection...... or generally poor connections in the ignition circuit ( inc. kill switch wiring ) is causing enough voltage drop when starter engaged to cause one coil to 'drop out '. When the voltage goes low, one coil/plugs/ points would be 'favored' if it's has a lower resistance path than it's twin and may take all the available energy.... IMO.

+1.
What if you were to disconnect the low tension wires from the solenoid, hook up a 12v supply straight from the battery to one wire & touch the other to ground to fire the s/motor.
Check for spark on 1 & 4. If spark is ok then that would back up this theory. There's been lots of stories here about poor connections, starting at the fuse box & all the way through the loom.
Good luck.

[saw750]

good answers! thank you!

would the starter button have anything to do with this issue?

my guess is no but then again you never know.lol

the stock starter button fell apart so i replaced it with dual post momentary switch - normally open from radio shack. i combined black and black/red to one post and yellow/red to the other. i came up with solution via the wiring diagram and lil trial and error.

it does work but i am curious if this might not be an issue?

the K6 has its own specific wiring diagram. here is the link (but it doesnt appear to be working)


http://data.sohc4.net/wiring%20diagrams/cb750k6.pdf

 

[TwoTired]

Placing a load on the battery reduces the voltage.  Loading it more reduces the voltage more.  Your switch bypass allows the haeadlight load to add to the starter load and drag the voltage lower than what was necessary.

The voltage output potential of the coils is proportional to the voltage it receives from the battery.

If the spark gap is increased, more voltage is required for spark.  Could be you have some spark plug boots where the resistors have opened to cause an additional gap.  Then the low voltage due to headlight (I can also imagine and brighter, higher wattage headlight installed) and the load from the starter motor, dip the voltage during start to a level below what is needed to bridge all the gaps in the coil output circuit.

There are some other possibilities.
The condenser for the 1-4 may be leaky, and this effects the voltage input to the coils and how briskly they can fire the spark plugs.
Further, point contact deterioration may be adding resistance to the 1-4 primary circuit, which in turn, effects how much spark voltage can be developed by the coil for the existing spark gap.

Lastly, when was the last routine 3000 mile tuneup done to the bike?

Cheers,

[Hush]

What Mr Bones said sounds right mate, try disconnecting the head light and firing up from the starter motor, if that works then TT's theory is proven.

[saw750]

lol "Lastly, when was the last routine 3000 mile tuneup done to the bike?"
possibly in 1980 lol

this bike was a barn find with 8,000 miles. it was complete and hadnt been molested. however it had sat since 1980 until March of this year.

Despite the pigeon pooh, the po' failure to drain the fuel before he stored the bike and that starter button..it is in remarkable condition.

Carbs and tank are now spotless. Changed the oil (dropped the pan etc) and now I've turned my attention to spark.

Everything is stock (sans the starter button).

I am going to:

• pull the headlight.

then

• make sure I am getting a good connection at the 3-way black/white coil connector - spanner1

if the condition isnt resolved...

• pull the condensor

if that doesnt fix it...

• i am going to take a closer look at the run/stop switch

[saw750]

well so much for the obvious..lol

pulled the headlignt - no spark

cleaned the the 3-way black/white coil connector - no spark

freed the condenser - no spark

pulled the connections from the 1-4 coil and ran a jumper to the battery - spark

the only thing i havnt done on my "to do" list is pull apart that stop/run switch.
its definitely making a connection/functional and the wires etc looked good
but i will tear it down regardless

any other ideas??

saw

[crazypj]

I would say that bridging the points gives spark, turning motor doesn't, points are at fault.
 Try cleaning and re-gapping them then make sure they break at II T1.4

[CB750F2]

Saw, you say that you "freed the condenser". A serviceable condenser connected is required for good spark and to minimise pitting of points. I would also check the spark plug boots on plugs 1 & 4. One or both maybe open circuited. You could swap them with the ones from 2 & 3 - do one at a time. I would also check the  battery voltage when cranking with the starter motor. Re-read Lloyd's post - you have not done all that he has suggested. Pat

[saw750]

points are clean and gapped (i will give them another cleaning). plug boots have been checked

when i say i "freed the condenser" i removed the retaining screw and pulled it away from the base plate.

yes thank you pat. i missed a post or two ie frostyboy

[CB750Stan]

Hi

I had a similar problem on my bike.

On the coils fitted to your bike the Black and Blue/Yellow wires are soldered on to the ends of each coil. I can't remember whether the Blue or Yellow contact breaker wire goes to which coil, but on my bike the solder had worked loose causing the wire not to make contact.

Re-soldered wire. Job done. Spark re-ignited.

I'm not saying this is going to be the same on your bike, but it may be worth a look!

Good luck.

Stan

[saw750]

here is the wiring diagram for the 1976 CB750 K6

[Spanner 1]

Are you sure the 1-4 wire at the point ( Blue ) is connected correctly and not grounded... ??

[saw750]

reasonably certain. i dont have it front of me but unless someone (ie previous owner) had messed with it...it should be correct. again, the bike appears to unmolested as if it just came off the showroom floor (well minus the bird pooh lol) i will of course check.

[CB750F2]

The fact that the motor starts and runs normally when you use the kick starter means that the wiring is correct. You need to test and then reconnect the condensor that you disconnected - no condensor means very poor spark. I believe that Spanner's first post is on the right track. Battery voltage will drop quite a lot when using the starter motor and if there are any poor connections in the primary or secondary circuits of the coils that particular coil will not fire. I have seen bikes cranking over via the starter motor and not firing at all because the battery was in poor shape and then start first kick with the kick starter. Pat

[Spanner 1]

With respect cb750f2, OP never said that the bike runs normally or at all!.... I think it's just a crappy battery at this point esp. since his headlight is always 'on' while trying to start with the button...

[CB750F2]

Ok my mistake but Saw did say that he has spark when using the kick starter and no spark when using the starter  motor, so I believe the last half of my reply is still relevent.
Pat

[Spanner 1]

Googled Ayr Pat, wow, your well up there into the Tropics, yes.... my bestest friend since 1st. grade is in Alice NT.... we talk 1 or 2 times a month.....
O.K...now back to the prob.

[saw750]

ah yes i failed to mention that its a brand new battery lol - properly brought into service, maintained and voltage checked : )

[CB750F2]

Ok. I would check all connections from the primary winding of 1-4 coil through the blue wire to the points and then to frame. You have already checked the connection where the blue wire to the primary joins the black/white wire coming from the ignition switch. There is another connection which joins the blue wire coming from the coil to the blue wire going to 1-4 points. You can find this connection on the RH frame just above the rear brake switch. Check that these conections are clean and tight. Then I would disconnect the blue wire at 1-4 points, turn the motor until points are closed and then use a multi-meter to check the resistance between the terminal that the blue wire was conected to and ground. There should be almost zero resistance. If you do not have a multi-meter open the points and make sure they are clean. A high resistance reading by the multi-meter indicates dirty points or a poor connection between the base of the points and the points backing plate. Now that you are satisfied that your points are clean reconnect the blue wire to the points. I would also disconnect the condensor and have it tested before reconnecting or replaced with a known good one.  If, at this point, you have found and repaired dirty connections or replaced a faulty condensor I would try turning over the motor with the starter motor to see if you have spark. If you still have no spark I would turn the motor until 1-4 points are closed, turn on the ignition and then measure the voltage on the blue wire where it joins the black/white wire from the ignition switch. I would expect the voltage reading to be 0.5 to 1.0 volts lower than the reading you would get if measuring from the battery +ve post to ground. Do the same voltage measurement at the yellow wire. If you find that the voltage is low at these points run a temporary wire form battery +ve to the junction where the black/white wire meets the blus and yellow wires and then try with the starter motor. See how you go with this lot and then feedback to us. Pat

[saw750]

good stuff pat thanks! i will get on this asap.

[saw750]

here are the results:

1. secondary blue connection RH frame: clean and tight (for future generations this connection is secured to the frame - slightly behind the oil bag)

2. disconnect blue wire at point terminal and test for resistance: zero resistance

3. test/replace condenser & clean points: the points are properly gapped and cleaned to perfection. i switched the condenser from the functioning 2-3 points to the 1-4 point and visa versa . 1-4 plugs no spark. 2-3 sparked. returned condensers to their original positions and again 1-4 no spark, 2-3 sparked.

4. connections at coil - blue and yellow voltage test:
- battery: 12.54 volts
- yellow: 11.42 volts
- blue: 11.48 volts

5. scratch head: head in process of being scratched.

i have not performed the solenoid test

[Bill/BentON Racing]

With respect cb750f2, OP never said that the bike runs normally or at all!.... I think it's just a crappy battery at this point esp. since his headlight is always 'on' while trying to start with the button...

Stock wiring RH switch headlight power is cut so more power is available to battery.I suspect RH switch/secondary wiring.These switches wear our,poor hardware,etc.
If you turn on key and bridge solenoid you might have spark.Good Luck,Bill

[OLDHEAD]

sooooo i decided to bridge the points for the 1-4 annnnd bingo! great spark!

excuse my ignorance what does it mean bridge the points