Spark Plug Gap

Hi all. I’ve been rebuilding a KZ750 I purchased a couple of years ago, so far so good. Only thing I’ve yet to figure out is the following. If I gap the spark plugs to what the manual recommends .7mm, they don’t produce spark. I can tell thanks to the timing gun. If I set the gap to something like .05mm (excuse my metric) a gap ridiculously small as if it were a racing bike, the engine starts right up. What could be causing the spark to be so weak when gapped to the appropriate spacing?
For reference, I’m running new coils with the appropriate resistance on both the primary and secondary winding. Running a new fully charged battery and the dynamo/regulator are working flawlessly.
Thank you in advance.

Points or electronic ignition?

It's a KZ750B Twin with Points. One thing caught my attention is that when the ignition is set to ON the resistance from the battery to the engine 0Ω when the point system is open. When the points are touching it jumps to about 17Ω. Should this be happening?

You need very close to full battery voltage at the coils to get a good fat spark.

Luis,
Let's check some voltages.

Bike ignition on but engine not running.
Meter set to DC V

Black meter probe on negative battery post
Red meter probe on positive battery post.
Note voltage

Rotate the engine with a wrench till points are closed and touching.

Black probe on negative battery post.
Red probe on coil 12v+ wire
Note voltage

Move red probe to the negative coil wire.
Note voltage
Rotate engine till points are fully open
Probes in the same place
Note voltage
Turn off bike---coils may overheat if left on for too long.

I double posted...too much coffee

Let's check some voltages.
Meter set to DC V
Black meter probe on negative battery post
Red meter probe on positive battery post.
Note voltage - 12.6V

Rotate the engine with a wrench till points are closed and touching.
Bike ignition on but engine not running.
Black probe on negative battery post.
Red probe on coil 12v+ wire
Note voltage - 11.5V
(interesting thing happens here: the wire that goes from the ignition switch reads 12.5V, but when I connect it to the coil it jumps to 11.5V)

Move red probe to the negative coil wire.
Note voltage - 12.5V

Rotate engine till points are fully open
Probes in the same place
Note voltage
coil 12v+ wire: 12.6V
negative coil wire: 12.6V

Ok
meter on DC V
Bike ignition on but engine not running
Points closed

Red meter probe on negative battery post
Black meter probe on coil negative wire(blue wire).
Note voltage
move black meter probe to the points screw with pink arrow.
Note voltage

The reason your voltage drops when you connect the wire at the coil is the circuit is now under load.
Current is now flowing.
When you do voltage drop testing the circuit has to be fully connected and on.
It's pretty easy when you understand the technique.


photo by charka129

I'm getting a number very close to 0V. If I open the points it jumps to the battery voltage about 12.6V.
I’m doing all these tests with the engine STOP switch set to RUN.

what was the voltage at the negative terminal of the coil?
I need 2 voltages.
The one at the coil and the one at the points.
Label which one is which.

Battery voltage 12.9V

Points Touching
Coil Negative Terminal : 0V
Coil 12V+ Terminal: 11.82V
Points: 0V

Points Open
Coil Negative Terminal : 12.88V
Coil 12V+ Terminal: 12.88V
Points: 12.88V