kz750 not firing

Hey guys,

My bike ran fine yesterday and today it wont start at all.
I have narrowed down the problem to this:

When I take out the spark plug and keep it against the the cylinder head (with the cap on) I get no spark when cranking the engine.

BUT if I keep the spark plug very close to the engine (so a small gap is formed between the spark plug and the engine) then I get a nice spark between the plug and the cylinder head.
Make the plug touch the engine and the spark disappears again.

What could be wrong ?

Bad plug, it's got a short circuit internally.

If it was a two stroke I'd say you just whiskered a plug

Thanx !

But are you sure...cause I dont have a spark on two plugs (1-4) ?
Could one bad plug effect both lines from that coil ?

/Arn

If you think it's the coil you could temporarily swap the coils and see if the problem moves to cylinders 2 & 3. Ed

If you're description is accurate, then my diagnoses is accurate.
You could have more than one problem.

If you pull the plug wire off one plug, you'll kill the spark on the other plug on the same coil.

Arnold wrote: Thanx !

But are you sure...cause I dont have a spark on two plugs (1-4) ?
Could one bad plug effect both lines from that coil ?

/Arn

YES. The plugs are in series, so one fouled or open plug will kill the spark from both.

bountyhunter wrote:

Arnold wrote: Thanx !

But are you sure...cause I dont have a spark on two plugs (1-4) ?
Could one bad plug effect both lines from that coil ?

/Arn

YES. The plugs are in series, so one fouled or open plug will kill the spark from both.

Not on the Dyna green coils. When I had a #4 plug foul out I pulled the wire off #4 while riding and there was no change, the engine missed on only 1 cylinder. When I pulled the wire on #1 the missing got worse, now running on only 2 cylinders. My conclusion, coil secondaries are in parallel. Don't try this in the rain. :ohmy: :woohoo:

High intensity voltage travels through the secondary loop.

When conductivity through the secondary loop is truly prevented, neither spark plug can fire because there's no voltage running through the plug.

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When removing a plug cap makes no difference, and the other plug in the loop is firing, voltage is somehow else being completed through the secondary loop, perhaps via shorting from the plug wire through the engine head.

When removing a plug cap makes a difference, both plugs are then not firing, whereas only one of them was firing before the cap was removed.

Good Fortune!

TomW wrote:

bountyhunter wrote:

Arnold wrote: Thanx !

But are you sure...cause I dont have a spark on two plugs (1-4) ?
Could one bad plug effect both lines from that coil ?

/Arn

YES. The plugs are in series, so one fouled or open plug will kill the spark from both.

Not on the Dyna green coils. When I had a #4 plug foul out I pulled the wire off #4 while riding and there was no change, the engine missed on only 1 cylinder. When I pulled the wire on #1 the missing got worse, now running on only 2 cylinders. My conclusion, coil secondaries are in parallel. Don't try this in the rain. :ohmy: :woohoo:

I don't think the coil outputs can be in parallel, they have to be in series like the drawing shows above. If they were in parallel, only one would fire which dumps all the energy in the coil at whichever plug arced first.

As shown above: if a plug wire shorts to ground (either externally or inside the coil) you can have one plug sparking.

Patton wrote: High intensity voltage travels through the secondary loop.

When conductivity through the secondary loop is truly prevented, neither spark plug can fire because there's no voltage running through the plug.
When removing a plug cap makes no difference, and the other plug in the loop is firing, voltage is somehow else being completed through the secondary loop, perhaps via shorting from the plug wire through the engine head.

When removing a plug cap makes a difference, both plugs are then not firing, whereas only one of them was firing before the cap was removed.

Good Fortune!

That's exactly what happened with my #4 miss. Pulling the #4 cap caused no change. Replacing the #4 and pulling the #1 caused the engine to run worse, hitting on 2 holes. I'll back up a bit here and call it a series/parallel circuit. The common ground of the plugs is the tip-off. In a true series circuit, one plugs shell would be connected to the others center electrode not the grounded cylinder head. With one plug cap off the other plug on that coil continues to fire. I know this empirically, I tried it with the Dyna coils in my bike. The current from that firing plug has to get back to the coil secondary to complete the circuit. The only suggestion I have is inductive coupling, the same thing that generates the high voltage from the collapsing magnetic field in the primary. The coil is in effect center tapped. :S :huh:

Although it might seem counter-intuitive, firing through the secondary loop travels across the plug gaps from center to ground in plug A and from ground to center in Plug B.




Try this test to visually observe which way the spark jumps.
Gonna have to look really fast! :woohoo: :lol:



Good Fortune!

The FSM is misleading when saying that one non-sparking plug results in both plugs not sparking.

One plug in the secondary loop may indeed fire alone, provided the secondary loop isn't broken.
For instance a non-firing plug may have a carbon-filled gap allowing voltage to pass through.
Or a plug wire may be shorting to the engine head in a manner whereby the secondary loop is completed.

Good Fortune!