"Mercurys are not my line of e
"Mercurys are not my line of expertise but your problem is a common one that could be attributed to any engine.
Since the engine does turn over normally at times, you can pretty well assume that the starter is okay (hopefully).
Usually that type problem is contributed to a slightly loose battery cable, or tight but dirty cable connection, or a cable that is corroded within the insulation somewhere.
After the engine has been turning over slowly for a few seconds, check by hand for a overly warm or hot cable. A hot cable will lead you to a faulty connection quite quickly.
This type problem could be at the battery, starter solenoid, the gound cable conection, or even at the starter.
It's unlikely that the timing is any part of the problem. If it was, the engine should kick back, firing against itself. Let us know what you find.
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