My bike seems to have a misfire after being warmed up. The manual is a little confusing to me, because my electrical knowledge is not great. The resistance of plug caps, ignition wires, coils all turns out fine. What is the best way to test the spark units?
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Ignition system
- Thread starter brettp
- Start date
How do I determine which cylinder?
When misfiring condition exists pull spark plug boot off of each cylinder separately ( one at the time). If you have good spark at cylinder you pull the boot of your engine will change sound and run rougher, if you do that at the cylinder that has misfiring already nothing will change…
But
Not a bad idea. I will give it a shot, thanks!
Two words...insulated pliers. The cylinders that are firing will bite you in a hot second.
I wondered about that!
As luck would have it, no shocks when I pulled each plug. The bike died every time. I think my coils and spark units are good, I think my battery resting under 12 volts is my problem.
As luck would have it, no shocks when I pulled each plug. The bike died every time. I think my coils and spark units are good, I think my battery resting under 12 volts is my problem.
Your electrical current needs while bike is running are provided by charging system and not battery. Battery is storage for starting and is getting replenished by charging system. Now if your battery is in bad shape it will put extra demand on charging system and over time cause premature failure.
I don't think this is entirely true, and I used to think the same thing. Here is why.
The charging system does not feed (connect to) anything other than the battery. The battery feeds the bike. You can think of this as a battery connected to an electrical circuit, then place a generator "behind" the battery to replenish its charge. Now, it is true that if you have a bad battery, any charge from the generator going to the battery is "consumed" by the bike as soon as it gets to the battery.
Did that make sense?
The charging system does not feed (connect to) anything other than the battery. The battery feeds the bike. You can think of this as a battery connected to an electrical circuit, then place a generator "behind" the battery to replenish its charge. Now, it is true that if you have a bad battery, any charge from the generator going to the battery is "consumed" by the bike as soon as it gets to the battery.
Did that make sense?
That's right, I agree with that. To reiterate one of my statements, the power being demanded by the vehicle is passed from the generator through the battery to the vehicle.
Our debate is really just pedantic. At the end of the day, we both know what happens when either the battery or the generator die.
And on that note, I put a new battery in it and the voltage never dropped below 12.25V at idle. When the rotor comes in next week, I'll throw that in and see what happens to the charging system numbers.
Thanks, all.
Don't forget that the battery in most motorcycles also acts as a source of power and a "voltage stop" at low RPM because the output of the charging system drops at idle. If the stator isn't generating a full 12 volts then the voltage of the entire electrical system is the higher of what either the stator or the battery is making at that point.
So low battery voltage at idle could cause spark issues if the battery's resting voltage is low enough and the stator is not generating enough voltage to properly power the coils. Though in that case the battery would test low on the bench too with a multimeter, probably under 10 volts, and at idle the bike would test with abnormally low system voltage.
You are right in this context for DOHC CB750's but not all motorcycles are wired quite the same. For example my recently-acquired 1986 Yamaha XT350, which has a separate coil wound in the stator just for providing AC voltage for the headlight and taillight while running, can run perfectly well with a big fat capacitor in place of the battery because it has no electric starter and will run on a completely dead battery.
Double-check the wiring diagram for your bike. I just looked at my 1980 and the output from the regulator/rectifier doesn't flow power only through the battery. There are two separate positive voltage outputs and one ground. The red/white positive voltage output goes to the starter relay to charge the battery, but the second is black and goes directly to the ignition switch, kill switch, starter switch, and fusebox as their primary source of voltage. All the lights, while your CB750 is running, are powered directly by the stator and not the battery!
Your "never dropped below 12.25v at idle" is a sign of that brand-new battery bumping up the whole electrical system with plenty of juice, but that doesn't tell you what your stator is putting out.
A misfire when fully warmed up usually makes me think ignition coils are on their way out. Physically inspect the casings of the coils. The bike dying every time you pull a plug cap regardless of which one is not normal -- usually a CB750 will run on 3 cylinders.
Always good to hear from you, Sense!
Thanks for the info on the wiring, I heard conflicting info on the black and white wires being positives or negatives. I'm still unsure what those do. Yellows, I understand. Not the others.
I looked over the coils and the cases are good. When cold they measured 25k ohms (I think) about 3 weeks ago. I read for hours about coils and spark units. I decided to order new coils after reading they can perform poorly when hot before they die completely. The spark units' potting on the back was in great shape so I didn't buy those. The new battery I put in didn't change the misfire so I'm looking forward to putting in the coils when they arrive (hopefully tomorrow). At that point it will have a new battery and coils. The new rotor will be in sometime next week to hopefully cure the charging problem.
Thanks for the info on the wiring, I heard conflicting info on the black and white wires being positives or negatives. I'm still unsure what those do. Yellows, I understand. Not the others.
I looked over the coils and the cases are good. When cold they measured 25k ohms (I think) about 3 weeks ago. I read for hours about coils and spark units. I decided to order new coils after reading they can perform poorly when hot before they die completely. The spark units' potting on the back was in great shape so I didn't buy those. The new battery I put in didn't change the misfire so I'm looking forward to putting in the coils when they arrive (hopefully tomorrow). At that point it will have a new battery and coils. The new rotor will be in sometime next week to hopefully cure the charging problem.
I was talking about the wires that carry DC voltage coming out of the regulator/rectifier. On my 1980 K they're a red wire with a white stripe (positive), a black wire (positive), and a green (ground). The wires going into the regulator/rectifier, from the stator, are the three yellows and one black and one white. Freaky angry zappy pixies that run in both directions many times a second go through those, and any kind of bidirectional angry zappy pixies scare me so that's not something I know firmly about. I prefer my electrons flowing in exactly 1 direction.
Have you also checked for any direct lightning leaks? Like starting the bike at night or in a dark garage and staring at the spark plug wires as it idles? If your spark plug wires have never been replaced, they could be the culprit if the insulation has broken down enough to allow zaps to jump through occasionally. With a bad jacket they can still show up as "good" in a static resistance test with a multimeter, but the multimeter can't tell you how the wire behaves when thousands of volts are shot through it. I usually check how flexible the wire is and whether I can see any cracks both at rest and when bending. Cracks are an obvious sign of failure.
With spark plug wires: when in doubt, replace them. They're cheap. Your local auto parts store will sell you excellent copper-core silicone-jacketed wire for less than $2 a foot in any color you want as long as it's black. You can buy fancier colors elsewhere for more money, of course. On our bikes the wires are replaceable; there's a threaded brass fitting in the cap that you screw the wire into and a spike in the coil that you compress the wire into using a plastic nut.
Absolutely agree, when I got my bike spark plugs, spark plug boots, wires were on the list of parts to be replaced. Compromised plug wire will be very hard to detect, besides way of detection Sense said with dark garage, you can also listen for ticking sound which is created when spark ground itself through the wire against head. Also you can run your fingers around plug boots and wires while bike is running and if you get zapped, you found compromised plug boot or wire!
Thank you, gentlemen.
I replaced the coils, wires, and caps with a kit from 4into1 for $70 last night. After complete assembly, resistance on new coils was 3k higher than the old. The boots were worn and cracked and the wires were green inside, as were the coils connection to the plug wires.
I will take it for a ride tonight to see how she behaves. The only things I haven't changed in the ignition system are the spade terminals on the coils and the spark units.
I replaced the coils, wires, and caps with a kit from 4into1 for $70 last night. After complete assembly, resistance on new coils was 3k higher than the old. The boots were worn and cracked and the wires were green inside, as were the coils connection to the plug wires.
I will take it for a ride tonight to see how she behaves. The only things I haven't changed in the ignition system are the spade terminals on the coils and the spark units.
Use the rear axle bolt!
Ugh. I have to take my exhaust off (not just the mufflers) to get to the rear axle.
