Is it because fitting the spark plugs is making the engine turn slower , battery voltage drops and then no spark ?
Think it needs more than 9 volts at least to get a spark under load .

Good point mactavish.
Alands,I take it you are also starting with a strong, charged, correct battery? Preferably tested with a discharge tester for remaining CCA power.

Hi
If I put tester on the battery when turning it over will this tell me the voltage? Or is there something else I need to do?

Battery is good
Drops to 11.5v when cranking with the plugs in.

Air is an electrical insulator.
Due to compression you have 10x as much insulator between electrodes when the plug is fitted.

A healthy spark has to jump 10-15mm outside the engine, to safely work on a 0.8 mm gap inside the engine.

There are more than one possible reasons for a weak spark.

Partly shorted out ignition circuit (moisture on high voltage cables, inside killswitch or ignition switch)
Side stand switch playing up
Old ignition coil(s) with insulation damage
Bad grounding on ignition coil(s)
Engine in newly painted frame or rusted engine mounting bolts
Bad spark plug caps (corroded resistor connections)
etc etc

All of that high voltage stuff can not really be diagnosed to be in working order by measurement with a digital multimeter. It is only good to detect faulty components, but component inside specs on a low voltage measurement may fail instatntly when confronted with much higer voltage/ampereage in real life.

Check the spark plug electrode gaps and set to recommended/minimum gap.
Try with new spark plug caps
Put an additional grounding wire between ignition coil, engine block and battery negative terminal
Unplug kill switch (or bridge it, depending on the type of ignition).
To rule out the wiring loom, try wiring up a basic ignition loom outside the main loom.
Keep in mind that a CDI/TCI may have some kind of security built in to prevent starting with gear engaged or riding off with side stand out, so depending on the bike you may need to take care of that and bridge/separate some of the CDI/TCI connectors.

This can be sorted out with the correct wiring diagram, and maybe an OEM workshop manual.

Andy
who could help with some more info on the bike/ignition in question.

Hi Again
Up date
ive tested the kill switch
side stand switch
clutch switch
and all work as they should.
ive added a extra earth lead to the coil and tried another coil and get the same results.
A good spark from the leads to the engine casings at a gap of about 10mm with the caps off(bare leads).
Then as soon as i screw the plugs in i loose all sparks even though i am testing the same way as above.
ie the leads held 10mm from the engine casings with the caps off(bare leads)
anymore ideas ?????

any chance of a video of what you are demonstrating - with and without plugas = perhaps one plug in and bare cap for the the other?

I’ve done a couple of videos but don’t know how to put them on here?????
Up date
I have done some more testing:-
Voltage at coil when switched on only 13.1v
Voltage at coil when turning over with no plugs in 10.5v
Voltage at coil when turning over with plugs in 11.3v

Voltage at CDI when switched on only(Red/Whited wire.Input) 13.1v
Voltage at CDI when turning over with no plugs in 11.2v
Voltage at CDI when turning over with plugs in 10.9v

Voltage at CDI when switched on only(Brown wire.Output) 0v
Voltage at CDI when turning over with no plugs in 9v
Voltage at CDI when turning over with plugs in 11.2v

I would run a separate battery feed to the coil and test, Seems the starter is sapping the supply to CDI & coil... I'm having a problem with a aftermarket CDI suppler for a separate problem just now and his mantra is the CDI should never fall below 12volts.

I’ve already done this and still lost the spark