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I had to jump mine nnce (with battery in the front and positive terminal under the fender). Hooked the negative wire to battery and positive cable to starting solenoid on firewall. Nothing to it.
Steel may not be as good a conductor, but with the frame, you have massive amounts there. I suspect the extra connections of using the frame to be the sore spot. At any rate, your extra copper cable seemed to make a significant difference, and really, in the end, as long as you get the results you want, whatever you do is OK.....er.....legal things I mean.....LOL
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