Author Topic: Spark at 1 & 2, not at 3 & 4. What?? (Read 1200 times)

Magpie · « **on:** March 03, 2025, 12:56:22 PM »

A 1973 K3, new coils, I have spark to 1 and 2, not to 3 and 4. 1 and 4 run on the same coil, 2 and 3 on the other one. New wiring harness. Connections all good. I don't get it. Would bad condensers cause this? I'm at a loss. Thanks for any suggestions you can offer. Cliff.

BenelliSEI · « **Reply #1 on:** March 03, 2025, 12:57:41 PM »

Cliff….. That is strange. Did you replace the caps? Try swapping them around and see if the problem moves with them?

Magpie · « **Reply #2 on:** March 03, 2025, 01:18:03 PM »

New caps. I put the timing light on 1 and 2 and get it to flash, connect it to 3 ad 4, no flash- so no electric charge coming down the wire to the plugs. Very strange.

scottly · « **Reply #3 on:** March 03, 2025, 01:54:57 PM »

Try flipping the timing light pick-up clamp over; the sparks for 3&4 are reverse polarity from 1&2.

newday777 · « **Reply #4 on:** March 03, 2025, 03:47:05 PM »

Quote from: Magpie on March 03, 2025, 01:18:03 PM

New caps. I put the timing light on 1 and 2 and get it to flash, connect it to 3 ad 4, no flash- so no electric charge coming down the wire to the plugs. Very strange.

Did you check that the cam is not broken? Turning on both sides looking at it through the adjuster caps?

BenelliSEI · « **Reply #5 on:** March 03, 2025, 03:48:46 PM »

Don’t trust the timing light. Mine does that all the time. Stick a plug in the cap and ground it to see if it sparks.

scottly · « **Reply #6 on:** March 03, 2025, 03:52:34 PM »

The pick-ups can be polarity sensitive: mine has an arrow that is supposed to point towards the spark plug. To test 3&4, the arrow points away from the plug.

BenelliSEI · « **Reply #7 on:** March 03, 2025, 04:00:03 PM »

Quote from: scottly on March 03, 2025, 03:52:34 PM

The pick-ups can be polarity sensitive: mine has an arrow that is supposed to point towards the spark plug. To test 3&4, the arrow points away from the plug.

Mine is the same!

HondaMan · « **Reply #8 on:** March 03, 2025, 05:38:25 PM »

I once had the reverse: weak/intermittent spark at 1&2, but 3&4 ran OK. Mine turned out to be a rusty ground for the coil bracket where it bolted to the frame for the coils. Can't say quite WHY it did that, other than to note that 1&3 and 2&4 sparks are the same polarity, so I supposed the crust was trying to act diode-ish? I just fixed it and called the owner, that was all it needed in the end of a long search.

Magpie · « **Reply #9 on:** March 03, 2025, 07:30:26 PM »

Hi!
Checked the cam operation, the valves are moving as the engine turns over.
I have the polarity correct on the timing light.
Checked for spark with a spark plug, no spark on 4, very weak spark on 3.
Put lead 4 to plug 1 and lead 1 to 4, 4 fired but not 1.
Fresh end on 4 to plug cap.
Thanks Hondaman, I'll check the grounds and report back.
Thanks to all of you!
Cliff.

Magpie · « **Reply #10 on:** March 03, 2025, 08:37:49 PM »

Ground spots down to bare metal, cleaned up the spots on the coil mounts too, still no joy. These are after market coils. I think I'll have to pull a set of coils off another bike that is running well and put them on the K3 to see what happens. Very strange problem.

BenelliSEI · « **Reply #11 on:** March 04, 2025, 06:46:43 AM »

Cliff….. using a new set of NGK plugs? Try four different ones.

Magpie · « **Reply #12 on:** March 04, 2025, 08:44:10 AM »

Brand new plug in #4, still no joy. It's almost as if half the coils are not working. I'll try a set from another bike today and report back. Cliff.

Magpie · « **Reply #13 on:** March 04, 2025, 12:01:49 PM »

Coils! I should have done this sooner instead of bothering you guys. I took a set off my 78 Supersport and installed them on the K3 -success! Now it's running like it's supposed to.

Cliff.

denward17 · « **Reply #14 on:** March 04, 2025, 12:09:55 PM »

Where did you get the new coils?

HondaMan · « **Reply #15 on:** March 04, 2025, 12:26:38 PM »

Quote from: denward17 on March 04, 2025, 12:09:55 PM

Where did you get the new coils?

Does anyone want to take bets on Cliff saying. "4 into 1.com?".

Magpie · « **Reply #16 on:** March 04, 2025, 12:50:19 PM »

You win Hondaman! 4 into 1 it is.
I think I'll send them an email about it.
https://4into1.com/ignition-coils-and-caps-honda-cb750-sohc/?srsltid=AfmBOopqeT7QEyhJiwuiU8nOuD-0XRjA9jCh1Yu3LLaAGN0XMyy_DfWC
Cliff.

Don R · « **Reply #17 on:** March 04, 2025, 01:21:21 PM »

Hey doc, it hurts when I do that. So, don't do that.

BenelliSEI · « **Reply #18 on:** March 04, 2025, 04:29:59 PM »

…..Brutal! Well done Cliff. Those are the ones that can make you crazy!

Tracksnblades1 · « **Reply #19 on:** March 04, 2025, 05:34:22 PM »

Have you checked your voltage at the points, engine running while you’re experiencing the mix firing..
It interesting that it’s misfiring on one of the plugs that the circuit is dependent on..🤔

My old F1 coils, one will fire down to 5.5 or so volts but the other signs off at 6.5volts..
It had high resistance in the point wire connections..

Medyo Bastos · « **Reply #20 on:** March 04, 2025, 05:41:43 PM »

Caps or plug wires?

Sent from my iPhone using Tapatalk

Tracksnblades1 · « **Reply #21 on:** March 04, 2025, 05:58:14 PM »

I may check if the fuel line is feeding 3-4 adequately or check the fuel bowl levels after the bike starts misfiring.
unless you know that those plugs for sure aren’t firing, but they obviously are completing the circuit for 1$2..
If you remove the wire far enough away from 4 you should lose 1 also…The same with number two’s running pair..

scottly · « **Reply #22 on:** March 04, 2025, 06:08:38 PM »

The problem was fixed when the new, but defective coils were replaced.

RAFster122s · « **Reply #23 on:** March 05, 2025, 05:36:46 AM »

Parts N more has new coils at affordable prices and they work well.

Magpie · « **Reply #24 on:** March 05, 2025, 08:54:56 AM »

Thanks guys! Now to get the head light to work. I bought the bike from a friend who took it all apart, had another friend do a bunch of motor work, I painted the motor several years ago, gathered up new parts and then lost interest. It sat in pieces for many years. It's a headache dealing with aftermarket wiring harnesses. No power to the light, everything else has power. Thankfully I have a good, in colour, wiring diagram.
RAFster122s, I often order from Parts N More and vintagecb750. I used 4 into 1 on recommendation from a friend. I'll let him know my experience.
Cliff.

HondaMan · « **Reply #25 on:** March 05, 2025, 09:50:43 AM »

Cliff: the K3 and later 750s have a START button that turns OFF the headlight when pressing for START. The housings on those switches distort if halogen headlights got installed, and the failure is on the "unpressed" side's contact for the headlight. Either the switch can be replaced, or the headlight can be powered by jumping into a BLACK wire to power the Hi-Lo switch on the left-side handlebar.

Or, I can make up a Start Button Saver relay kit for you, but postage would be nasty. I could send you a schematic if you want to make your own? It's just a relay.

Magpie · « **Reply #26 on:** March 05, 2025, 03:13:25 PM »

Thanks Hondaman!
The strangest thing happened when I connected a black wire to one of the blue wires, it seemed to reset the headlight system. I disconnected the black wire, hooked up the blues as they should be and the On/Off switch works as does the Hi/Lo. Hit the starter button and the headlight goes out. It's a miracle!!

We'll see how long that lasts. I did spray some contact cleaner into the On/Off switch last night.
It is a Halogen headlight to.
Hondaman does it again.
Cheers,
Cliff.

HondaMan · « **Reply #27 on:** March 05, 2025, 09:05:42 PM »

If that headlight is more than 50 watts (many of them are 55/60W for this bike) then it will cook the switch again and make it act up all over.

Sort of like the neighbor's 2-year-old here....

newday777 · « **Reply #28 on:** March 06, 2025, 03:34:35 AM »

Quote from: Magpie on March 05, 2025, 03:13:25 PM

Thanks Hondaman!
The strangest thing happened when I connected a black wire to one of the blue wires, it seemed to reset the headlight system. I disconnected the black wire, hooked up the blues as they should be and the On/Off switch works as does the Hi/Lo. Hit the starter button and the headlight goes out. It's a miracle!!
We'll see how long that lasts. I did spray some contact cleaner into the On/Off switch last night.
It is a Halogen headlight to.
Hondaman does it again.
Cheers,
Cliff.

The contact cleaner did it's job. The switch was covered with black carbon from the arching of the high loads. Spraying in the contact cleaner and working the switch cleans off the carbon, temporarily, and as Mark says, the neighbors 2-year old will return again when you need the light.
Get Hondaman's switch saver relay he said that he would make up or make your own relay to take the load away from the headlight switch. What the relay does is uses the high/low switch as a trigger of lower current 12v( so the switch doesn't burn) and the relay which is rated for higher amps and the bigger wire from the battery that you will add handles the load of the higher wattage headlight bulb, or you can also just change out the headlight bulb(H4?) to a LED bulb(draws lower amps so the switch doesn't burn).

Magpie · « **Reply #29 on:** March 06, 2025, 09:06:43 AM »

I changed out the Halogen headlight to a standard sealed beam.Hopefully that will help. I'll look for a LED unit.
Thanks for all your help!
Cliff.

newday777 · « **Reply #30 on:** March 06, 2025, 10:29:37 AM »

Quote from: Magpie on March 06, 2025, 09:06:43 AM

I changed out the Halogen headlight to a standard sealed beam.Hopefully that will help. I'll look for a LED unit.
Thanks for all your help!
Cliff.

If you have a H4 headlight?

.
Get this LED bulb from Electrical Connection it draws only 24 watts. You will be amazed at the output of these. And they last, I have their bulbs in my GL1800 for 10 years now.

https://electricalconnection.com/index.php/product/led-headlight-bulb-h4/

HondaMan · « **Reply #31 on:** March 07, 2025, 08:22:39 PM »

Thanks, Stu!
I do indeed have an a track record: I'll take yours! I didn't want one with a fan inside the Vetter, too much complexity for a headlight (IMHO). The other H4 LEDs I have seen locally have been uninspiring at best, mostly for lack of focus: they look like a shotgun pattern across the pavement.

Denver, CO is not known for stocking anything of special-purpose types: EVERYTHING here is always generic, and as cheap as possible (but not the price). That's why all day, 7 days a week, there are multiple Amazon, Fedex, UPS, DHL, and various specialty-delivery companies delivering 'stuff' everywhere in my neighborhoods. It's become amazing: we'll go out and walk the dogs for 1-2 hours and will see every brand of delivery company at least once, and usually twice in different vehicles, on a walk: I've seen up to 4 different Amazon trucks (one was electric! It got towed away about 2 months ago, though) in one walk. It's always the 'special' parts/things/widgets, too, that are coming when we talk with the neighbors: everyone here is frustrated with the lack of product selection (for ANYTHING) around here in the last 15+ years. I have been looking around here for just such an LED for more than that long, and not single vendor carries anything like it, but for the occasional Sylvania "replacement LED" headlights that can't make it half a block.

PeWe · « **Reply #32 on:** March 08, 2025, 12:59:40 AM »

I had to visit CMSNL to verify the headlight on my K6. It had done 2 seasons and +17.000km when I bought it. So not sure about stock light .
It had H4 when I bought the bike.
CMSNL show a H4 socket, 3 poles, HI, LO and ground
But a strange design of the bulb

https://www.cmsnl.com/honda-cb750k6-four-european-direct-sales_model14384/partslist/F01-1.html

The US version a sealed beam variant.

I did not have any issues with handle bar switches. Only the wirings when I replaced handle bar with wiring outside.
I did not grind deep enough in the switch housings so a wire could be squeezed and some shortages until I did it better

So my bike's switches survived until 96.000km when I restored the bike with new, not sun bleached switches.
Law here about LO during daylight. So constantly on.

It got relays, not only for less amps to the switches, better light and aftermarket ign switch that could have weak connectors.

HondaMan · « **Reply #33 on:** March 08, 2025, 12:02:57 PM »

Quote from: PeWe on March 08, 2025, 12:59:40 AM

It got relays, not only for less amps to the switches, better light and aftermarket ign switch that could have weak connectors.

Yeah, Per: those aftermarket switches got me in 2006 when the wires melted right off the back of the switch (Red and Black) on the way home from work one day! When I took it apart, and the old OEM switch (the key wouldn't stay in that one anymore, kept falling out) I found the contacts inside the OEM switch were more than 2x larger than in the new EMGO keyswitch. Then I noticed the wires inside its black sheath of the EMGO switch were only 20 AWG size (actually were slightly less than that) while the 750 switch had 14 AWG size (slightly thicker, a JIS size, probably). That's why they melted off: the 750 can pull up to 16 amps through that switch with the headlight on HIGH and the brakes on.

That's when I first made the Keyswitch Saver Relay kits, along with the new version of the Transistor Ignition.

PeWe · « **Reply #34 on:** March 08, 2025, 01:12:12 PM »

Mark, it was thanks to your posts about the weak ign switch I added relays. Headlight relays was part of it too.

Magpie · « **Reply #35 on:** March 20, 2025, 05:19:31 PM »

Okay,now I feel really stupid! It was spark plugs all the time. Replaced them and it runs like a top.

BenelliSEI · « **Reply #36 on:** March 20, 2025, 08:02:13 PM »

“Cliff….. using a new set of NGK plugs? Try four different ones”

NGK plugs used to be virtually infallible. Not so much anymore. Always start with the simplest solution.

Good to hear you solved it!

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Author Topic: Spark at 1 & 2, not at 3 & 4. What?? (Read 1200 times)