Spark Plug Firing With Key In On Position And Engine Not Running

[Quasikaze]

I'm still new to mechanical work so please correct me if I'm wrong, but a spark plug isn't supposed to be firing with the key in the on position. It should only fire when the engine is running. The plug on the right hand side (sitting down) is firing but the left one is not. So if they are supposed to be firing, then something's wrong with the left side. If they're not, then something's wrong with the right.

Thoughts?

 

[5twins]

As soon as you turn the key on and the ignition is powered, the plugs will spark as you turn the motor over. How do you think this or any motor starts?

 

[Quasikaze]

"as you turn the motor over"

That's the thing. I'm not turning it over. The key is simply in the "ON" position.

 

[5twins]

Well then, my best guess is that the motor just happened to stop at the point where that right cylinder was ready to fire it's plug. Dual points ignition I assume? Then this could happen. The later electronic ignition fires both plugs at once so only one firing might indicate a problem.

 

[Quasikaze]

Okay, that makes some sense.

Isn't the plug is supposed to fire when the cylinder is at TDC? I have it set to TDC, but on the left cylinder, not the right. Does that mean the timing is off and I need to set the timing or is that even something you do on a 1980 XS650 SG?

To be honest, I'm somewhat confused on the ignition system right now. The research I've done hasn't clarified it, yet. I think I have TCI because I have a button I can press to start it. I think that also means I don't have points. Please correct me if I'm wrong, though, because I'm certainly confused.

 

[5twins]

The plug actually fires a little before TDC. The pistons on these motors rise and fall together, in unison, so when one is at TDC, they both are. If you have a 1980 U.S. model then yes, it would have the electronic ignition (TCI). The start button isn't related at all. They all had that except for the first two model years ('70 & '71). If you do have the TCI ignition then there is no timing to set. It is pre-set by the factory and can't be adjusted, but it should never need to be.

 

[gggGary]

why a new thread?
AFAIK TCI will spark once when you turn on the key and, one more time when you shut it off. Not spark continuously. Both plugs fire together. This is known as a lost spark ignition. The side that doesn't need a spark gets it anyways. You have ONE coil with TWO high tension leads.

 

[Max Midnight]

Isn't the plug is supposed to fire when the cylinder is at TDC?

To be honest, I'm somewhat confused on the ignition system right now. The research I've done hasn't clarified it, yet. I think I have TCI because I have a button I can press to start it. I think that also means I don't have points. Please correct me if I'm wrong, though, because I'm certainly confused.

No, ignition will occur some degrees before T.D.C. depending on engine speed This is necessary to give the mixture time to be fully ignited at T.D.C. thus getting the maximum work from it.
As your bike is a 1980 XS 650 SG then I would assume you have TCI which would be set at 15° BTDC at 1200 r.p.m. and once correctly set would not normally need re-adjusting
The best 'research' you could do would be to confirm the absence of points (or not) by removing the top LH cover on the cylinder head.

 

[Quasikaze]

why a new thread?

Because it's a new problem? IIRC, I haven't posted about this one yet. Also, it makes it easier to search for others having a similar problem.


"removing the top LH cover on the cylinder head."

What does "LH" stand for?

 

[Max Midnight]

Left hand

 

[fredintoon]

Hi Quasikaze,
the first thing is to identify what bike you have and what a PO may have done to it.
Your posts say it's an XS650SG. Check for TCI (Transistor Controlled Ignition)
The round chrome covers on the LH (Left Hand) and RH of the cylinder head should have nothing in them. They used to house the points on one side and the centrifugal advance mechanism on the other on bikes up to 1979. It cost less to use up all those leftover parts than to redesign and retool.
There's a possibility that the PO has been "improving" things.
Perhaps a Pamco or other aftermarket ignition system. Look for stuff that ain't points inside the chrome covers.
Or even a rephase.
Poke a pencil down the sparkplug holes to see if the pistons move up and down together.
Note that there are some "improvements" that really are improvements. Check if your bike has them.
My shortlist:-
Replace the stock rototiller handlebars.
Install tapered roller steering head races.
Replace the stock M14-ended swingarm throughbolt with an M16-ended one.
Fit a front brake master cylinder with one that has a smaller piston diameter.

 

[phil1rowe4]

Hi,

If one side is firing with just the ignition key on, but the other is not, then (to me) that suggests a dual coil setup with a dodgy connection somewhere on the negative side of one coil or a faulty earth somewhere. I'd be checking coil and other ignition connections. Give the connectors a wiggle and see if it stops.

 

[Max Midnight]

For a coil to keep firing then the supply voltage would need to be constantly cut to produce the high voltage in the secondary winding.
I think that scenario is unlikely due to a bad connection.

 

[XSLeo]

Being that we don't know what the bike is, we can only guess what the problem is. How about giving us the year, model and any mods done to the bike? A lot of us put this in our signature, you see it right below every post we make.
If it has the TCI sometimes a bad TCI box can cause continuous sparking. Just why I can't say, not that electronically proficient.
I would start by checking every connection in the ignition circuits, just to be sure it's not a bad ground or poor power connection.
On these old bikes most of the electrical issues are bad grounds or bad fuses. Some bad connections between components or dirty switches.
Leo

 

[Klt200]

I'm still new to mechanical work so please correct me if I'm wrong, but a spark plug isn't supposed to be firing with the key in the on position. It should only fire when the engine is running. The plug on the right hand side (sitting down) is firing but the left one is not. So if they are supposed to be firing, then something's wrong with the left side. If they're not, then something's wrong with the right.

Thoughts?

Did you ever find your problem?

 

[gggGary]

Did you ever find your problem?

Quasikaze Last seen Sep 4, 2016
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