Topic: chasing electrical issue CB750 - '73

[dylboss]

So I recently aquired a '73 750 and been diving into getting it running.  Everything looks good, except I cannot get spark to any of the plugs.  All other electricals work.  Using a simple voltage light, I'm getting power to both coils from the kill switch, and I also tested both points when they were in the open state.  i have power to both. 

I've been told that it would be extremely rare and unlikely that both coils would be junk at the same time.  I'm running out of ideas here.  Anybody?

[Steve_K]

Test the plug caps for resistance, should be between 5 & 7 K .  Cut a 1/4 to 3/8 inch off the coil wires.  Manually open and close points to make it spark.  Recheck wiring and not to be a smart a$$ check the killcheck  switch if it is on.  I know you said there was power to the coils, check  if it is the full 12 volts.  Recheck grounds and have a cold one while thinking about her.
Steve

[dylboss]

Test the plug caps for resistance, should be between 5 & 7 K .  Cut a 1/4 to 3/8 inch off the coil wires.  Manually open and close points to make it spark.  Recheck wiring and not to be a smart a$$ check the killcheck  switch if it is on.  I know you said there was power to the coils, check  if it is the full 12 volts.  Recheck grounds and have a cold one while thinking about her.
Steve

Sorry, I'm not really an electrical guy.  What tool do I need to check for resistance?  I guess the most confusing part of this is that all 4 plugs are not getting spark.  Seems strange that all the plug caps would be bad.  I did cut one of them down and reattached the cap, with no success.

[Spanner 1]

Shine, I mean shine the point faces and try again.....

[dylboss]

Shine, I mean shine the point faces and try again.....

Thanks, but given that I'm getting power at the points does that even matter at this point (no pun intended)?

[Spanner 1]

Power at the points = great. But that only tells you that you have power at the points.
What also needs to happen is that the points close and ground the coil primary winding. If the point faces are covered in a film of corrosion ( will 'look' O.K. ), then the coil primaries cannot ground-out, saturate, and be ready to suddenly lose their energy when the points open which induces a high tension voltage in the coil secondary winding which then flashes across the plug ( spark )... So point faces need to be clean and shiny, point gaps need to be set and points need to open @ the 'F' mark.....

[jason41987]

well, in order to get spark you need two things.. power, and timing... my bike had a similar issue, no spark, tested the resistance of the plug caps, and one was something like 4 million ohms, the rest didnt even register.. so my spark plug caps were all  junk, and the culprit..

touching your spark plug wires to a multimeter should get around 15,000 ohms i believe?.. if youre getting around that, coils are fine, check the caps, if you get around 5,000 ohms for newer caps or 10,000 ohms for older then they should be good, at this point id check the wiring leading to these coils for continuity, could have a broken wire somewhere... if not, its also possible somethings not grounded properly.. lastly, you could have a problem with the points or condensers which can also be checked and verified... theres a list of things to do, and i hope you have a multimeter

[dylboss]

Power at the points = great. But that only tells you that you have power at the points.
What also needs to happen is that the points close and ground the coil primary winding. If the point faces are covered in a film of corrosion ( will 'look' O.K. ), then the coil primaries cannot ground-out, saturate, and be ready to suddenly lose their energy when the points open which induces a high tension voltage in the coil secondary winding which then flashes across the plug ( spark )... So point faces need to be clean and shiny, point gaps need to be set and points need to open @ the 'F' mark.....

thanks for that explanation.  I did sand them down, and set the point gaps.  I'll revisit this.

[HondaMan]

Shine, I mean shine the point faces and try again.....

Thanks, but given that I'm getting power at the points does that even matter at this point (no pun intended)?

In case you don't know about points: here's how they points work:
They get [about] 12 volts thru the coils when they are open, and when they close, the current charges the coils. The Black/White wire feeds the "high" side of the coil(s), and the Blue (1-4) and Yellow (2-3) wires go to the points to get switched ON/OFF by the crankshaft. They essentially ground out the [blue or yellow] wire on the "low", or "switched" side of the coils, when closed. This charges the coil.

When the points open, the coil's magnetic field (which was built up by the current passing thru it) collapses, making the spark. If the points can't ground very well, this makes for poor (or no) spark, because the coil won't charge.

So, with the same meter you've been using: connect it with one side to the engine block, and the other to the Blue (1-4 coil) or Yellow (2-3) wire at the points, and see if it switches between 12-ish volts and ground (0 volts) when open or closed, respectively.

If it does, then the coil will make some sort of spark. If it does not switch to 0 volts, the points are not conducting, and need to be polished up good to make them work. That's the job of the points file.

Now...there are a few things that might not let the points ground. Two of those things are:
1. The bike's GREEN ground wire is not [properly] grounded to the frame. On the K3, it will be grounded in either: the seat latch bracket, under the seat, or: the front coil bracket, up under the gas tank. The K3 came both ways. If this Green wire is not grounded, it is real difficult to make spark.
2. The big, Black battery ground wire from the battery's (-) terminal must be connected directly to the engine. On the K3, this normally would be on the left top rear engine mount, in between the frame and engine bolster. If this is missing, again it is difficult to make a spark, since there is no ground for the sparkplugs.

Your friend is right: a double-coil failure at once would be one for the books!

You can check the coils, though, if you can figure out how to measure ohms (resistance) with your meter. If you figure that out, write back and someone will walk you thru it. The primary side of those coils should be about 4.6 ohms, the secondary (spark) side, with the plug caps removed, should be a few thousand ohms, tops.

[dylboss]

Shine, I mean shine the point faces and try again.....

Thanks, but given that I'm getting power at the points does that even matter at this point (no pun intended)?

In case you don't know about points: here's how they points work:
They get [about] 12 volts thru the coils when they are open, and when they close, the current charges the coils. The Black/White wire feeds the "high" side of the coil(s), and the Blue (1-4) and Yellow (2-3) wires go to the points to get switched ON/OFF by the crankshaft. They essentially ground out the [blue or yellow] wire on the "low", or "switched" side of the coils, when closed. This charges the coil.

When the points open, the coil's magnetic field (which was built up by the current passing thru it) collapses, making the spark. If the points can't ground very well, this makes for poor (or no) spark, because the coil won't charge.

So, with the same meter you've been using: connect it with one side to the engine block, and the other to the Blue (1-4 coil) or Yellow (2-3) wire at the points, and see if it switches between 12-ish volts and ground (0 volts) when open or closed, respectively.

If it does, then the coil will make some sort of spark. If it does not switch to 0 volts, the points are not conducting, and need to be polished up good to make them work. That's the job of the points file.

Now...there are a few things that might not let the points ground. Two of those things are:
1. The bike's GREEN ground wire is not [properly] grounded to the frame. On the K3, it will be grounded in either: the seat latch bracket, under the seat, or: the front coil bracket, up under the gas tank. The K3 came both ways. If this Green wire is not grounded, it is real difficult to make spark.
2. The big, Black battery ground wire from the battery's (-) terminal must be connected directly to the engine. On the K3, this normally would be on the left top rear engine mount, in between the frame and engine bolster. If this is missing, again it is difficult to make a spark, since there is no ground for the sparkplugs.

Your friend is right: a double-coil failure at once would be one for the books!

You can check the coils, though, if you can figure out how to measure ohms (resistance) with your meter. If you figure that out, write back and someone will walk you thru it. The primary side of those coils should be about 4.6 ohms, the secondary (spark) side, with the plug caps removed, should be a few thousand ohms, tops.

Hondaman,

thanks so much for this info!  I will test all of this out tomorrow and post the results.

[GR8WITE]

anybody ever figure out what was causing this issue? Same exact problem on my bike.. power everywhere except to the plug wires. Came out of nowhere, the bike was running fine.

[Vinhead1957]

There are two weak spots in the 73.  Trace 12V when the key is on.  The points should have 12v when they are open. 0 when they are closed.  The blk/wht wire from the headlight bucket gives power to the coils when the key is on and the kill switch is on.    The bucket is the first place. The second is the safety switch. It can be bypassed by jumping the green  and grn/red wire in the connector.

[HondaMan]

anybody ever figure out what was causing this issue? Same exact problem on my bike.. power everywhere except to the plug wires. Came out of nowhere, the bike was running fine.

Kill switch on the handlebars?