04 DT125re Plug only sparks on release of the start button

[Stoners]

Firstly thanks in advance for your help, This has been doing my head in.
There's no kickstart on the bike just electric.
When the starter motor is turning over there's no spark at the plug but it will spark briefly when the start button is released.
I've changed the CDI
Fitted a new battery
Checked all the connections
By passed the kill & side stand switches
Checked all the earth connections

Now heres the weird bit, when I remove the starter motor lead from the relay & directly connect it to a seperate battery & hit the start button (just to power up the live to the CDI etc) it sparks perfectly!
Any ideas?

[Hugh Farking Cant]

I reckon your bike has a TCI rather than a CDI ignition unit.
Most Yamahas do .
A Yamaha TCI unit requires a voltage of above 11.5 IIRC in order to
provide a satisfactory spark.
Your starter motor is drawing excess voltage from the system and the TCI will only provide spark when the load has been reduced.

This is proved by your test.
I reckon if you had jumper leads from a heavier battery to your bike battery you will find it starts.

Either the starter motor requires overhaul/replacement or the starter lead and how the starter is earthed ( engine ) requires inspection. You say you checked the main earth to frame/ engine.

Check the battery voltage with a m/meter whilst cranking and post results.
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Remember; Someone on here always has a blacker cat.

[TUG]

Given the bike is over 10 years old, like said in previous reply the starter motor might not be working perfectly and drawing too much power, if not that then is it possible you replaced the battery with the same one that was fitted upon purchase but haven't checked with a yamaha dealer what battery SHOULD be on it?

Failing the 2 options above it could have something to do with your wiring possibly shorting somewhere in the starter button circuit, but that is a guess at it's finest and I don't have a wiring diagram to look at and trace for you.
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Haz ER-5, innit!

[Hugh Farking Cant]

To diagnose problems with the starter button circuit without the aid of a m/m.

Unplug wire from starter button at solenoid.
Ignition on.
Connect light duty test cable from battery positive to solenoid connection previously unplugged.
____________________
Remember; Someone on here always has a blacker cat.

[Stoners]

Thanks everyone with your very plausable answers

Multimeter results

Battery off load 12.97V
Ignition on 12.58V
Starter button presed to engage relay 12.33V (motor disconnected)
Starter button pressed with starter motor reconnected 11.47V

The results didnt change even with a 105Ah battery connected with jump leads

[Hugh Farking Cant]

Are you sure the second set of readings with the jumper battery are correct ?

A 105 Ah automotive battery in good condition would not drop voltage at all with those loads placed on it.( It's not a leisure battery is it ? )

Have you checked for sparks by using a jumper wire replacing the starter button circuit as in my other post. ?
____________________
Remember; Someone on here always has a blacker cat.

[Stoners]

When using the 105Ah battery, I basicaly had a very cheap set of jump leads from my sprinter van on to the bike battery.

When I used a wire to by pass the push to start switch,
the start button was pushed with the motor disconnected the relay engaged and the voltage accross the battery terminals dropped from 12.58v to 12.33v, I didnt think that warranted further investigation, had their been an issue it would have dropped signifigantly more.

It may have nothing to do with this but the previous owner of this bike told me when it rained the bike drove like a dog, loss of power, missfiring then dead altogether for a couple of days until it dried. I never experienced this but I never drove it in the rain. I cleaned all the ignition related connections to avoid this happening again & it drove fine.

[Stoners]

The kill switch was faulty & the bypass wire I was using had a break in it.
Most kill switches stop the starter motor but not the DT125 it just kills the plug.
Thanks everyone for your help