GRiSO Starting

Here's an odd one. My '15 GRiSO always started normally: A momentarily push the start button and the engine turns over 3 times and then starts. Regardless of whether you push momentally or push and hold, the engine would start with the same 3 revolutions.

But now, out of the blue, the button is behaving differently. If I momentally press and then release the starting operation stops, the speedo goes blank, and the bike does not start. However, If I push and hold the bike starts normally (3 revolutions and then start).

Yes the battery is fully charged. And once running the bike behaves totally normally.

Any ideas?

Your battery may be fully charged, but I think it might have seen better days.

Clean up the terminals if you haven't already, and if it still misbehaves swap out the battery for a new one.

Yup battery, gone or going.

I had the identical symptoms, new battery fixed it.

Just for fun, attach a volt meter to the battery terminals, and see what it registers while cranking.

Thanks for the tips. Will try swapping at the battery and report back...

Your late model GRiSO has some weird wiring
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It has 2 Start relays, the normal Start Relay (2) that the ECU holds On and the Startup Maintenance Relay (3), that you hold On with your finger on the button.

The speedo is going blank indicating that the Voltage is collapsing either the battery is dropping right off or you have Startus Interruptus.
A new battery will probably take care of it, if not a minor re-wire will.

If you were to watch the number plate light on the bike, it's wired to the same source of 12 Volts, you will probably see it dip down in brightness just before the relay drops out.

As Kiwi Dave says clean up the battery and smear some Vaseline on the connections just in case its not just dirty terminals.
Update
I suspect when you just jab the starter the ECU is detecting a low battery and dropping out the start relay

When you hold your finger on the button the ECU is still dropping out the start relay but the Start Maintenance Relay (held by the start button) is keeping the starter going.