Modern Vespa

I'm trying to adjust the air/fuel mixture using a tach/dwell meter to monitor the idle speed as it's not that easy to hear the changes.

Just wondering if anybody knows where to hook the connections up to?
On my old VW, I hook the red lead up to the coil and the black lead up to an earth.... not sure where I would connect it to on the vespa... any ideas?

i've searched the tachometer threads... but they all seem to be for more permanant solutions - rather than an quick idle speed check.

Something similar to this comes in handy for such things.

http://www.amazon.com/Docooler-Tachometer-Resettable-Inductive-Motorcycle/dp/B00F40Q5UA/ref=sr_1_5?ie=UTF8&qid=1446736628&sr=8-5&keywords=tiny+tach

fried okra

try this on for size.
Adding a Rev Counter / Tachometer to your scooter?