Author Topic: electrical backfire when shut off key. (Read 7921 times)

nomad20012 · « **on:** July 26, 2012, 10:41:29 AM »

Like the subject, I'm not sure what's happening when I shut off the key, I occasionally get what seems like and electrical discharge/backfire when I de-energize the bike.

cb750 Running a Pamco with barebones electrical, solid state reg/rec, new coils, battery, plugs, etc.

I know it's difficult to completely diagnose without a proper diagram but just looking for quick tips on what to suspect if any one has experienced this before.

Many thanks.

jamesb · « **Reply #1 on:** July 26, 2012, 10:59:22 AM »

mine just started to do the same thing it just makes a faint backfire sound is this what yiours is doing i'm going to check my grounds and power i'm running a barebones setup myself pamco stock reg, rec and harley coils.

nomad20012 · « **Reply #2 on:** July 26, 2012, 11:16:36 AM »

Well it's not so much a backfire sound as it is like a backfire of compression or electrical dump from somewhere in the bike. One time there was also faint smoke present.

I better check all my grounds, I just finished this wiring I may have overlooked something.

Duke McDukiedook · « **Reply #3 on:** July 26, 2012, 11:26:36 AM »

Sounds like capacitor discharge when the circuit is opened but that should not happen if the ignition key is turned off. Sounds like something is electrically awry and is completing the circuit to ground when you turn the ignition key to off.

lucky · « **Reply #4 on:** July 27, 2012, 02:54:33 AM »

Yes.... my Dyna ignition does the same thing occasionally.

Frostyboy · « **Reply #5 on:** July 28, 2012, 04:28:35 AM »

So we have 3 machines displaying the same symptoms & 2 different electronic ign systems.
Can I ask when this occurs exactly?
1. Is it when you turn the engine off from when it's running?
2. Or is it if you turn the ign on & off, but the engine not running?
If it's #2, has the engine been running just prior to switching key back on & off?

All I'm getting at here is that if one of the coils is still latched to ground via the ign control as the key is turned on & off and there is a fresh fuel charge left in the cylinders for that coil, the interuption of current flow through the coil via the ignition switch will cause the coil to discharge & fire the plugs attached to that coil.

Opinions welcome.

bollingball · « **Reply #6 on:** July 28, 2012, 04:33:23 AM »

Quote from: nomad20012 on July 26, 2012, 11:16:36 AM

Well it's not so much a backfire sound as it is like a backfire of compression or electrical dump from somewhere in the bike. One time there was also faint smoke present.

I better check all my grounds, I just finished this wiring I may have overlooked something.

What is a electrical dump??

trueblue · « **Reply #7 on:** July 28, 2012, 04:36:33 AM »

Quote from: Frostyboy on July 28, 2012, 04:28:35 AM

So we have 3 machines displaying the same symptoms & 2 different electronic ign systems.
Can I ask when this occurs exactly?
1. Is it when you turn the engine off from when it's running?
2. Or is it if you turn the ign on & off, but the engine not running?
If it's #2, has the engine been running just prior to switching key back on & off?

All I'm getting at here is that if one of the coils is still latched to ground via the ign control as the key is turned on & off and there is a fresh fuel charge left in the cylinders for that coil, the interuption of current flow through the coil via the ignition switch will cause the coil to discharge & fire the plugs attached to that coil.

Opinions welcome.

Frosty, I would have to hypothesise that is what is happening, I have read a few of these threads and it seems to be a common issue with aftermarket ignitions, even though it is feasible that it could even happen with points, it doesn't seem to be an issue.

trueblue · « **Reply #8 on:** July 28, 2012, 04:38:12 AM »

Quote from: bollingball on July 28, 2012, 04:33:23 AM

What is a electrical dump??

Shocking to wipe

.

jamesb · « **Reply #9 on:** July 28, 2012, 10:20:12 AM »

my cb550 has pamco ignition installed i have no problems with it it is all stock wiring. the cb750 has the same pamco ignition. it is a barebones simple wiring with aftermarket switch and it just started to do this now i noticed that my switch is a little touchy so i'm gouing to try another one and see what happens.

scottly · « **Reply #10 on:** July 28, 2012, 10:31:18 AM »

My Gerex electronic ignition sometimes produces a spark when the key is switched on. A couple of times, it has actually fired a cylinder and tried to start!
Mopar electronic ignitions always produce a spark when the key is turned on, at least in my experience....

TwoTired · « **Reply #11 on:** July 28, 2012, 10:42:27 AM »

Our ignition systems store energy in the coil before they are commanded to spark (dump their stored energy).
To command the coil to spark, the power driving it and holding the energy at it's peak, is shut off.
Turning off the key switch will look the same to the coil as a trigger command, from the aspect of commanding the coils to lose their stored energy (sparking the coils).

However, turning off the key is an unsynchronized event with respect to crankshaft position. Which results in a mistimed spark or random event. Backfires in exhaust or intake are possible.

Yet another opportunity is created by contact bounce within the key switch. During the contact break event, the passing 12v can pulse before disconnecting cleanly. Older key switches with dirty contacts would do this moreso than new pristine contacts. Anyway the twelve volt pulses resulting from this can also cause the coils to fire multiple times, out of sync with proper crankshaft angles.

I would expect "high energy" coils to exhibit the problem more than stock coils. And, the problem might be exacerbated with the plug cap and/or spark plug resistors eliminated, as those devices extend short spark events in longer ones.

Cheers,

dave500 · « **Reply #12 on:** July 28, 2012, 02:09:29 PM »

boyers make a spark when turned off.

Rookster · « **Reply #13 on:** July 28, 2012, 02:20:59 PM »

Lucas Rita EI does the same thing. I don't think is a malfunction but different characteristic of EI over points.

Scott

Frostyboy · « **Reply #14 on:** July 28, 2012, 04:32:50 PM »

Quote from: trueblue on July 28, 2012, 04:38:12 AM

Quote from: bollingball on July 28, 2012, 04:33:23 AM
What is a electrical dump??
Shocking to wipe .

phil71 · « **Reply #15 on:** July 28, 2012, 04:40:54 PM »

are you saying you get one last ignition event in a cylinder? If so, you're talking about a capacitor discharge ignition unit, so it will 'discharge' and light one more time. If the cylinder that sparks happens to come to a stop on a compression stroke, with a hot air/fuel charge in the chamber, it will light off. That's gotta be a 1 in a 100 situation tho, no?

trueblue · « **Reply #16 on:** July 28, 2012, 05:06:44 PM »

Engines almost always stop with a cylinder on compression stroke, it is the hardest one to overcome and stops the engine. A 4 cylinder 4 stroke engine basically has a cylinder on a compression stroke at all times. Also most engines almost always stop on the same cylinder, the one with the highest compression, because it is the hardest one to overcome.

w1sa · « **Reply #17 on:** July 28, 2012, 05:50:08 PM »

The spark energy remaining in the ignition when you turn off a running engine is depleted by the following strokes as the engine comes to a stop.

You would expect some unignited charge mixture to remain in at least one cylinder and possible two. Since the coil is energised during the intake and compression strokes, if you then cycle the ignition switch without starting, you can energise the coil associated with the cylinders most likely to have unburnt mixture in them....... electronic ignitions also exhibit better spark/energy characteristics than points only ignitions (stronger spark can ignite leaner mixture more successfully etc).......maybe that also has something to do with some EIs exhibiting this characteristic...

Hasenkopf · « **Reply #18 on:** July 28, 2012, 08:23:15 PM »

yeah, mine does this same thing, and I have a martek ign. kinda weird, but not a problem.

nomad20012 · « **Reply #19 on:** July 29, 2012, 07:09:32 AM »

Quote from: Frostyboy on July 28, 2012, 04:28:35 AM

So we have 3 machines displaying the same symptoms & 2 different electronic ign systems.
Can I ask when this occurs exactly?
1. Is it when you turn the engine off from when it's running?
2. Or is it if you turn the ign on & off, but the engine not running?
If it's #2, has the engine been running just prior to switching key back on & off?

All I'm getting at here is that if one of the coils is still latched to ground via the ign control as the key is turned on & off and there is a fresh fuel charge left in the cylinders for that coil, the interuption of current flow through the coil via the ignition switch will cause the coil to discharge & fire the plugs attached to that coil.

Opinions welcome.

#2 for sure.

This sounds like the most accurate and/or logical diagnosis in the thread so far for my particular situation.

It seems like it is backfiring thru the carbs, with a faint brief plume of smoke. Bike works great in every way, just need to know what to do to correct this or If it's going to cause any long term harm.

nomad20012 · « **Reply #20 on:** July 29, 2012, 03:36:05 PM »

Quote from: TwoTired on July 28, 2012, 10:42:27 AM

Our ignition systems store energy in the coil before they are commanded to spark (dump their stored energy).
To command the coil to spark, the power driving it and holding the energy at it's peak, is shut off.
Turning off the key switch will look the same to the coil as a trigger command, from the aspect of commanding the coils to lose their stored energy (sparking the coils).

However, turning off the key is an unsynchronized event with respect to crankshaft position. Which results in a mistimed spark or random event. Backfires in exhaust or intake are possible.

Yet another opportunity is created by contact bounce within the key switch. During the contact break event, the passing 12v can pulse before disconnecting cleanly. Older key switches with dirty contacts would do this moreso than new pristine contacts. Anyway the twelve volt pulses resulting from this can also cause the coils to fire multiple times, out of sync with proper crankshaft angles.

I would expect "high energy" coils to exhibit the problem more than stock coils. And, the problem might be exacerbated with the plug cap and/or spark plug resistors eliminated, as those devices extend short spark events in longer ones.

Cheers,

Any long term harm caused to anything as a result of this behavior?

TwoTired · « **Reply #21 on:** July 29, 2012, 04:09:54 PM »

Quote from: nomad20012 on July 29, 2012, 03:36:05 PM

Any long term harm caused to anything as a result of this behavior?

Yes. But, severity and length of the "term" are in question, and difficult to quantify.

The intake tract wasn't intended to encounter routine combustion. Soot deposits, intake valve seat contamination, rubber coupler deterioration, side pressure pulses against the slides, combustion deposits blown into the fuel jets and fuel channels, can't exactly be described as a good thing.

If the backfire was combined with an overflowing carb and pooling of gas in the air filter box and intake runner...Could have a hot time in the nether region.

Frostyboy · « **Reply #22 on:** July 29, 2012, 05:02:55 PM »

Quote from: nomad20012 on July 29, 2012, 07:09:32 AM

#2 for sure.
It seems like it is backfiring thru the carbs, with a faint brief plume of smoke. Bike works great in every way, just need to know what to do to correct this or If it's going to cause any long term harm.

How about you try using the kill switch to stop the engine rather than the key. Then leave the kill switch in the 'Off' position when you turn the key back on. Flick the switch back to 'Run' when you actually want to start it back up.
Then the coils won't be powered if for whatever reason you're turning the ignition back on/off.

nomad20012 · « **Reply #23 on:** July 29, 2012, 05:29:35 PM »

Quote from: Frostyboy on July 29, 2012, 05:02:55 PM

Quote from: nomad20012 on July 29, 2012, 07:09:32 AM
#2 for sure.
It seems like it is backfiring thru the carbs, with a faint brief plume of smoke. Bike works great in every way, just need to know what to do to correct this or If it's going to cause any long term harm.

How about you try using the kill switch to stop the engine rather than the key. Then leave the kill switch in the 'Off' position when you turn the key back on. Flick the switch back to 'Run' when you actually want to start it back up.
Then the coils won't be powered if for whatever reason you're turning the ignition back on/off.

Yes I should give that a try, thanks for the tip.

nomad20012 · « **Reply #24 on:** July 29, 2012, 05:31:33 PM »

Quote from: TwoTired on July 29, 2012, 04:09:54 PM

Quote from: nomad20012 on July 29, 2012, 03:36:05 PM
Any long term harm caused to anything as a result of this behavior?

Yes. But, severity and length of the "term" are in question, and difficult to quantify.

The intake tract wasn't intended to encounter routine combustion. Soot deposits, intake valve seat contamination, rubber coupler deterioration, side pressure pulses against the slides, combustion deposits blown into the fuel jets and fuel channels, can't exactly be described as a good thing.

If the backfire was combined with an overflowing carb and pooling of gas in the air filter box and intake runner...Could have a hot time in the nether region.

Yeah I hear you, thanks for the good info. Ill get this fixxed up.

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Author Topic: electrical backfire when shut off key. (Read 7921 times)

bollingball