[Mart]

It turns over slowly with 6v. It turns over slowly with 12V. It might be worth checking all your battery, solenoid, and earth connections. Don't forget the starter must be well earthed to the engine block. If the starter and block have been painted they will not be earthing properly.
Is the battery earthed direct to the block or the frame? If the frame, does the engine have a ground strap to the frame too?
Try feeding the starter independantly of the ignition, so the drag from the starter does not pull the ignition voltage down.
Try cranking with one plug wire connected to a spare plug laid on the head - does it spark?

Are the plugs wet?

Keep going, I'm sure it will run.

Mart.