Remove the starter solenoid and reverse the "Shorting bar" on the end of the shaft. This provides better continuity where the bar touches the contacts and more current is available to turn the starter motor. I did this on a 1971 1750 spider, and the increase in cranking speed was significant.
Edit: I just realized you said there was no voltage drop over the solenoid, so my advice may have been in vain...
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1976 Spider (Dedicated Autocrosser, "SPICA, No Carbs")
1991 Spider Veloce (Retirement cruiser)
Scuderia Non Originale
Last edited by conedriver; 04-19-2017 at 07:19 PM.