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Author Topic: Advice needed - random ignition problem  (Read 347 times)

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Offline Henning

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Advice needed - random ignition problem
« on: September 13, 2019, 09:15:47 am »
I need some advice on sorting an ignition problem. The bike sometimes stops firing on two cylinders for about 1 – 5 seconds then recovers and runs normally. I have a 12 mile commute and this may happen once or not at all during the ride. No real pattern except it seems to happen most often after 4 – 5 miles. Recently I put on 350 miles during a weekend and it lost two cylinders twice in the first ten miles and not at all for the rest of the 350 miles.

Question is, how the heck do you sort a problem like this? Is it possible to put a pickup on the spark plug leads to monitor the spark (would help to know which cylinders are the problem ones)? I really don’t want to just replace stuff until the problem goes away. I’ve been through the wiring and all looks good.

So, it’s a 750 K1, stock except for the Dyna ignition and coils. The kill switch powers a relay which in turn powers the coils and Dyna via a fused wire. Bike starts easily and revs willingly. Battery is a one year old Yuasa lead acid.

Thanks in advance. Pic is an under-tank look. The orange bit is the ignition relay.

71 or thereabouts 750 K1 - this one should have been put down

Offline Jerry Rxman Griffin aka MuthaF'er

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Re: Advice needed - random ignition problem
« Reply #1 on: September 13, 2019, 09:47:37 am »
Determine which 2 cylinders first. I expect it to be pairs 1 - 4 or 2 - 3. If it's another combination ie 1 - 2 or 3 - 4 that's another ballgame. All else fails and you can't determine what the issue is you may need to turn to Dyna for diagnosis.
As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)

Offline Spanner 1

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Re: Advice needed - random ignition problem
« Reply #2 on: September 13, 2019, 07:53:09 pm »
Dyna's wont work with the stock regulator IMHO. The stock regulator 40 yrs on will allow up to 15v before switching to 1/2 power battery charging. The Dyna will shut down if it gets over about 14.2v. Usually drops out 2 cyls first. Stock points don't care about 15v ! Answer is aftermarket solid state rectifier/ regulator properly wired ;)
« Last Edit: September 13, 2019, 07:55:52 pm by Spanner 1 »
If your sure it's a carb problem; it's ignition,
If your sure it's an ignition problem; it's carbs....

Offline Spanner 1

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Re: Advice needed - random ignition problem
« Reply #3 on: September 13, 2019, 08:16:04 pm »
BTW... if your battery + terminal tends to 'furr up ' with white powder, then your charging system is producing too much voltage to the battery ( considering the connection was clean ) .
If your sure it's a carb problem; it's ignition,
If your sure it's an ignition problem; it's carbs....

Offline Henning

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Re: Advice needed - random ignition problem
« Reply #4 on: September 13, 2019, 11:56:38 pm »
Thanks for the replies. The Dyna ignition and coils have been on the bike for over 12 years / 30k miles trouble free; this is a recent problem in the last three months. The battery keeps its charge and I'm not blowing bulbs (overvoltage) so I'm assuming the regulator is working correctly. I confess I've never actually adjusted or measured the output.

The only recent change is a set of new plugs, about six months ago. I suppose the first move is to put in a new set; just because they're new doesen't mean they're good.

Still love to hear if anyone has a suggestion for monitoring the spark via some sort of pickup on the plug leads.
71 or thereabouts 750 K1 - this one should have been put down

Offline PeWe

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Re: Advice needed - random ignition problem
« Reply #5 on: September 15, 2019, 04:05:04 am »
Check all connections, maybe corroded pin/socket, bad crimp somewhere.
Try another ignition if you have. Like stock points that need 5 ohm coils. Maybe your ignition use the Dyna green 3 ohm coils?
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Offline rotortiller

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Re: Advice needed - random ignition problem
« Reply #6 on: September 15, 2019, 05:03:56 am »
I'd feel which pipes are cooler, then I'd stick an inductive timing light on the sucker to verify miss firing. I would troubleshoot further through component swapping/shifting if possible with pickups and coils.

Offline Spanner 1

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Re: Advice needed - random ignition problem
« Reply #7 on: September 15, 2019, 08:07:38 pm »
Carry a voltmeter and when the bike is acting up measure the voltage across the battery with the motor revved to 5,000 rpms. If its making more than 14.5v then that may be causing the Dyna to shut down or drop two cylinders.  15v wont blow your bulbs btw.
If your sure it's a carb problem; it's ignition,
If your sure it's an ignition problem; it's carbs....

Offline DaveBarbier

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Re: Advice needed - random ignition problem
« Reply #8 on: September 16, 2019, 06:54:28 am »
I’m assuming this is so quick and intermittent that you can’t pull over when it happens and test with a multimeter. Or even reach down and pull the #1 plug when it’s happening to see if that further bogs the engine (meaning that cylinder is firing properly). Maybe you can rig up an LED as a timing strobe on your dash for 1/4 & 2/3 and see which light stops as it cuts out. Not sure how, but I bet it wouldn’t be that hard. OR live with it until it gets worse enough to be able to be easier to diagnose.

Offline Henning

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Re: Advice needed - random ignition problem
« Reply #9 on: September 16, 2019, 09:23:47 am »
I’m assuming this is so quick and intermittent that you can’t pull over when it happens and test with a multimeter. Or even reach down and pull the #1 plug when it’s happening to see if that further bogs the engine (meaning that cylinder is firing properly). Maybe you can rig up an LED as a timing strobe on your dash for 1/4 & 2/3 and see which light stops as it cuts out. Not sure how, but I bet it wouldn’t be that hard. OR live with it until it gets worse enough to be able to be easier to diagnose.

Dave has understood the problem  ;D Thank you all for your replies.

Spanner, the bike once cut out while I was waiting for the lights (very embarrassing). I was able to start it up again but it only ran on two cylinders for about five seconds until the others kicked in and away I went. So I don't believe it's an over-voltage issue.

Anyway, I went to buy some new plugs today but all they had were NGK resistor plugs (which they still insisted were correct). So now I have ordered new plugs and caps online which should be here for the weekend. Plug caps are NGK SD 05 F and VD 05 F if anybody's interested, same as what I have on the bike at the moment. Plug caps are five years old, so maybe time for a change. The plan is to change plugs and caps and see if that makes a difference. If not, I still have the old coils and ignition and will start by swapping in the coils. I really hope I don't need to swap in the old ignition.
71 or thereabouts 750 K1 - this one should have been put down

Offline DaveBarbier

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Re: Advice needed - random ignition problem
« Reply #10 on: September 16, 2019, 09:30:14 am »
Before swapping out the plug caps, test their resistance. Only swap out the ones that are out of spec (if they are).

Offline Spanner 1

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Re: Advice needed - random ignition problem
« Reply #11 on: September 17, 2019, 06:59:50 pm »
Still a good idea to test the alternators output at the battery when the bike has been run for at least 30 mins ( after electric start ). The max. voltage @ 5,000rpms should not exceed 14.5v. If it does then it's still a culprit with a cutting out Dyna ;) Your bike is going on 50 yrs old and the wiring harness and the bullet and socket connectors are likely all oxidized to where the Black 'sensor' wire to the regulator may be 1-2 volts below actual voltage at the battery, so the alternator thinks it needs to be at max output to get the battery up to full charge. The result is like, 15v applied to the battery which the Dyna does not like at all ! The only fix is a complete cleaning ( shining ) of all the connections in the harness and applying di-electric grease to each connector to avoid them oxidizing again. It's not a huge deal to clean them all and also ensure each plug-in is tight. A nice afternoons work and what most every one of our old bikes need ! Good luck !
« Last Edit: September 17, 2019, 07:15:03 pm by Spanner 1 »
If your sure it's a carb problem; it's ignition,
If your sure it's an ignition problem; it's carbs....

Offline scottly

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Re: Advice needed - random ignition problem
« Reply #12 on: September 17, 2019, 08:38:09 pm »
There was a plague of Dyna S failures with units of your vintage, that peaked about 5 or 6 years ago. For a while, every spring there were numerous reports of random ignition problems, caused by Dyna S failures. Earlier units had no problems, and at the peak of the plague Dyna changed the design, but the units made during the suspect period still crop up from time to time. The new units are visually different, as in not as squared off, and so far there have been no reports of problems that I've heard of.
You might try running the bike with the points cover removed to see if it helps.
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Offline Henning

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Re: Advice needed - random ignition problem
« Reply #13 on: September 18, 2019, 11:27:45 am »
Spanner, appreciate your time. I will check the battery voltage Friday when I get back from work, as well as take a look at the regulator and reg. connecting wires, the ground point and whatever else I can think of.

Scottly, I bought the Dyna S plus coils in Feb. 2008. I'm aware there was a bad period but I'm thinking (hoping) that it was later than mine. Whatever, nothing lasts forever and I will still be reinstalling the points plate as a final resort to confirm it is the Dyna S at fault.
71 or thereabouts 750 K1 - this one should have been put down

Offline Henning

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Re: Advice needed - random ignition problem
« Reply #14 on: September 20, 2019, 11:28:19 am »
Another week, another 125 miles on the clock. While commuting is not the sexiest of things, you do get the miles in. I live in the country (think England) and luckily have a really pleasant ride to work. Winter is coming and this morning it was cold and clear with the temperature dipping to 45 °F. I had the pleasure of seeing the big red orb of sun rising above the horizon a few minutes to seven on the way to work. The bike is still playing up but I have faith that it won’t die completely and leave me stranded.

Well, the plugs and caps didn’t arrive as expected, probably down to the underfunded and incompetent Danish postal service. That left plenty of time to take a look at the charging system. It was kickstart only Thursday and Friday to prep the battery for a charge test. I hooked up the voltmeter as soon as I got home and it said 12.8 V. Twenty minutes later and after a change of clothes, it was down to 12.6 V. I turned on the ignition and watched in surprise as the voltage fell to under 12 V. That’s when I kicked it into life and revved to 5k RPM. I watched as the Volts, measured directly on the battery terminals, rose to 14.8. That’s when I hit the kill switch. Clearly not all is well in Chargeville.

So I pulled the regulator; the terminals were tight and clean there where it mattered, no corrosion at all. And it’s the first time it’s ever been opened; there was a red dot of paint on one of the screws, and it’s never been adjusted, red dot of paint on the adjusting screw. Then I tried to remove the rectifier (which could also do with a bit of love) but gave up because the nut is rusted pretty solid and I don’t want to trash it now. So on to the common ground wire, attached to the seat lock.  The top bolt was tight, the bottom bolt  where the ground point is, was “cough” loose; a bad earth connection. It must be at least ten years since I last touched that seat lock.

It’s well known that a bad earth connection can cause all sorts of strange symptoms but I’m not calling this sorted yet until a week long road test. Thanks to all but especially to Spanner 1 for making me look at the charging system.

This is where I put the headlight relays; it does compromise access to the regulator.

71 or thereabouts 750 K1 - this one should have been put down

Offline Jerry Rxman Griffin aka MuthaF'er

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Re: Advice needed - random ignition problem
« Reply #15 on: September 20, 2019, 11:34:18 am »
Spanner, appreciate your time. I will check the battery voltage Friday when I get back from work, as well as take a look at the regulator and reg. connecting wires, the ground point and whatever else I can think of.

Scottly, I bought the Dyna S plus coils in Feb. 2008. I'm aware there was a bad period but I'm thinking (hoping) that it was later than mine. Whatever, nothing lasts forever and I will still be reinstalling the points plate as a final resort to confirm it is the Dyna S at fault.

Without reading all this and since you have excellent charging as to the 14.8V, have you fully charged it overnight and taken the battery (without riding it) in for a load test? You can have that 12.8 or 12.6 and still have a crap battery that acts as you state.
As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)

 

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