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OK the tiny tachs are mounted together under the main frame attached to a peice of 6061, the wire is wrapped around the spark lead near the spark plug 4 times, the earth wire is between the pull start and the nancelle mounting plate and bolted to the holding the main plate on the nancelle under the washer at the bottom inner side near the pull start hanble, so about 5 inches from the spark lead.
At this point I should state that one works fine and the other is ok up to full rpm then goes off up to 9900.

And a side veiw of the wire between the nancelle and the pull starter. you can see the red wire leading to the spark wire and the black earth going to a nancelle bolt.

And a quick peek at where I am upto with series 111.

Wayne
You may have a bad ground, loose connection or a lead broken inside of the insulation. I would start by switching the tini tach pic up leads across engines to see if the condition follows the tach or engine. I experienced this when an ignition lead had broken inside insulation causing spurious signals that the tach interpreted as more sparks. It is normal for the C2 model of tach to stop reading at 10,000 rpm as this is the top of its range. Knowing if the problem is with the tach or engine will at least give a starting point for finding the problem.
Glen