Author Topic: Spark at 1 & 2, not at 3 & 4. What?? (Read 1122 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.

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Author Topic: Spark at 1 & 2, not at 3 & 4. What?? (Read 1122 times)