Author Topic: No spark  (Read 702 times)

Offline PHILIP2908

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No spark
« on: April 21, 2019, 12:32:55 PM »
Hello all,
I did a search for this but had no success; the K2 now has its UK reg and thought I'd have a little run down the road to see if all was OK and the b**** won't start.
Its been idle for a couple of months as life got in the way but nothing has been touched since it was running fine.
There's no spark at the plugs 1/4 or 2/3 - so I thought dirty points (something that always stopped my CD175 sloper) so cleaned and gapped them, checked the static timing and nothing.
Strange thing is that with the ignition on, opening the points with my finger, there is a spark there but not further down the line at the plug.
I don't get it (and before you ask the kill switch is at 'run').
Thanks in advance all.
Regards
Phil
« Last Edit: May 22, 2019, 10:11:10 PM by PHILIP2908 »
1974 Honda XL250 K0 - Silver - owned for 32 years. Not running at the moment
1972 Honda CB750 K2 - Now candy apple red - Ex USA, a DK special - needs a lot more work than I thought - very quickly becoming a DS special...

Offline Rob62

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Re: No spark
« Reply #1 on: April 21, 2019, 12:37:13 PM »
Is it a big spark at the points? Have u got some spare condensers? Try those!  If not have you checked the battery voltage?.... and most of all, is there any fuel getting through?

Offline Johnwebley

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Re: No spark
« Reply #2 on: April 21, 2019, 12:38:41 PM »
Remove plug caps .check for circuit.
Maybe HT side of coils duff.
Also check caps for resistance

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Offline Bryanj

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Re: No spark
« Reply #3 on: April 21, 2019, 04:15:47 PM »
Be very unusual for both coils to go duff at same time, also condensers. More likely gone off ethanol fuel

Offline sye

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Re: No spark
« Reply #4 on: April 21, 2019, 07:32:51 PM »
Kill switch?

Offline Erny

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Re: No spark
« Reply #5 on: April 21, 2019, 11:31:06 PM »
Check if you have 12V on coils (Bk/W wire). If not look at headlight/kill switch or in wiring loom (I had corroded junction of Bk wires inside cabling loom, causing that one Bk wire had no power)

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Offline PHILIP2908

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Re: No spark
« Reply #6 on: May 22, 2019, 10:09:28 PM »
Right crew. Its been over a month and the CB750 is still lifeless. My job and family has got in the way so I haven't had the time to do much about it but believe me I'm getting close to pushing it off a cliff.
I've been tinkering and fiddling with contact breaker point powered vehicles from Hillman Imps to Morgans and C100s, C72s, CD175s etc and I well know that dirty or mis-set points will not allow the vehicle to run. The problem continues with the CB750 which is a USA import K3. There is a spark at the points but none at the plugs.
I've just had a flick though Mark Paris's book and its reminded me of the 'logic gate'. I don't know why its there or what it does (bit like an appendix). But it's there as a 1 inch square cube that is sealed and cannot be opened. Wires going in are:
-Light green/red
-Black
-Green
-Yellow/red
So what does all that lot do?? Is there a way of by-passing it or chopping it out?
The bike has not got side-stand or clutch switch.
I'd be grateful for any help otherwise I'll be down the white cliffs of Dover fairly soon.
Thanks in advance for any ideas...
1974 Honda XL250 K0 - Silver - owned for 32 years. Not running at the moment
1972 Honda CB750 K2 - Now candy apple red - Ex USA, a DK special - needs a lot more work than I thought - very quickly becoming a DS special...

Offline K2-K6

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Re: No spark
« Reply #7 on: May 22, 2019, 10:18:16 PM »
Have you tried direct connection from battery + to coil + terminal (classic hotwire) to see if it'll spark?

Offline Lobo

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Re: No spark
« Reply #8 on: May 22, 2019, 11:55:33 PM »
Phil... working from an old very grainy & cluttered wiring diagram, those connections to your ‘logic gate’ (in order) are NEUTRAL, IGNITION LIVE, EARTH and STARTER SOLENOID switched Earth. (ie as you press the starter button the YELLOW / RED is earthed at the handlebar switch)
... so my guess is that the STARTER MOTOR Engine start logic is dependant upon the Ignition ON & bike in NEUTRAL (ie you could bump start it in gear - very necessary)

But, you mention sparking at the points, which tells us the low voltage circuit of the coils are energised, and defacto the HT circuits are good to go.... ie I see little point in by-passing that logic switch.

I know it can be irritating when folk suggest the obvious, and sorry, but are you cranking via the starter motor? This is a massive drain (80-100A) and will likely kill off any spark if the battery is a bit weak. (And more especially so if you have an electronic ignition, as these are voltage critical.)

Even if you are indeed kicking it over, significant battery drain is already taking place (coils, voltage regulator ... lights (?)) which will sap energy to the HT circuit.

Staying with the battery, how old is is... often enough they appear fine, and only fail (to deliver significant current) under load. Have you any other battery you can swap-out?

As mentioned before... fuel... carb drains unscrewed to check fuel actually  in ‘em?

And sheesh... bloody Hillman Imps eh? Those were the days... every weekend in breakers yards climbing over / into vehicles; trying to fettle air locks in the wretched heating system... um everything.

Good luck,
Simon.
« Last Edit: May 23, 2019, 01:32:02 AM by Lobo »

 

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