Odd electrical phenomenon

[uudfourty]

1981 GS550L

After sitting for 8 hours or so, when I go to turn the bike on, I'll put they key in, turn to operate, and switch the engine switch to run-- and without me pressing the start button, I hear something like a "whump"... like a cylinder fireds without the starter being pressed or the engine turning over.

Doesn't always happen. Just sometimes. And it never happens unless it's been sitting for a while.

What?

[Zooks]

You been sharing grannies weed again???

[Steve]

It would not surprise me if you had said that it happened right after you shut it down, but not after 8 hours.
There is a slight chance that a cylinder drew in a charge of mixture and did not light it.
Turning on the ignition might just light that mix, but I certainly would not expect that mix to still be there that long.

.

[reddirtrider]

Does that bike have an electronic ignition? I can't see it happening with a points based ssytem as that would require the engine to rotate for the points to make and break.

An electonic system might be able to fire if defective. I say might since it would be possible to design a system that would do this if a failure was introduced.

As Steve stated though, I suspect that this would occur a short time after a ride, not the longer it sits. Gasoline evaporates fairly quickly.

[Dave8338]

Are your plugs black?

[posplayr]

My $0.02

I hear something like a "whump"... like a cylinder fireds without the starter being pressed or the engine turning over.

Hard to imagine even a defective igniter spontaneously firing, but if it were to happen,

a.) the igniter would be in a ground state on at least one coil
b.) there would be a pause for the coil to energize after the key/on switch were activated
c.) for some inexplicable reason the igniter would go open and fire a cylinder without having the signal generator pulse the igniter.

If it happened once and you could get it to happen again by repeating the process, I would start to worry about the igniter possibly being fried or otherwise on the way out and replace it.

Alternatively it is not something like loose seat bolts and you shifting your weight when starting the bike ?

Pos

[tkent02]

My Dyna-s makes one spark every time I turn the key on.
Been zapped a few times by this.

Now all you need is some fuel fumes, got a leaky petcock? Might be going down the vacuum line into the one cylinder.

[posplayr]

Humm

My Dyna-s makes one spark every time I turn the key on.
Been zapped a few times by this.

That would be an unfortunate "design feature". I will check mine to see if I get a spark on powerup .

Pos

[uudfourty]

My Dyna-s makes one spark every time I turn the key on.
Been zapped a few times by this.

Now all you need is some fuel fumes, got a leaky petcock? Might be going down the vacuum line into the one cylinder.

Whew.

Forgot to mention in original post that I, too, have dyna-s.
So.... whump, there it is.

[tomcat24551]

I have also noticed the "whump"--and I have Dyna S, too! Doesn't happen often, but just happened last Sunday on my '79 850.

Tomcat

[wazz]

I have also noticed the "whump"--and I have Dyna S, too! Doesn't happen often, but just happened last Sunday on my '79 850.

Tomcat

Same thing happens with the Martek ignition....."Whump" there it is.

Haven't had time to worry about it myself.

-greg

[reddirtrider]

There it is then. Either these are all defective or it's a design flaw.

[posplayr]

Makes more sense

Whew.

Forgot to mention in original post that I, too, have dyna-s.
So.... whump, there it is.

The OEM remote pulser which is passive would need to somehow generate a pulse.

With the dyna S since the pickup circuit and the igniter are tied together in the same unit, it is much more plausible that a startup transient fires the igniter.

Still is an unfortunate design .

Pos

[tkent02]

There it is then. Either these are all defective or it's a design flaw.

Don't see how it's a problem, once one learns not to hold a plug and turn the key on.

[reddirtrider]

Don't see how it's a problem, once one learns not to hold a plug and turn the key on.

It's all in how you define a problem. As an engineer I would not consider a design that allows the engine to fire inadvertantly something that I would sign off on. So as I said, I would consider it to be a design flaw.

Incidentally, it's something that would be easilly correctable while in the design stage. If they knew about it, which they may not.