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Borrow a DC amp meter and check the cranking amps of the starter.
Check the positive cable for rot at the ends.
A poor ground can cause what youre describing too
I completely agree with checking the ground cable. A while back, my bike (2006 FLSTC) wouldnt crank with a full charged new battery and I was at my wits end. I thought what the hell, so I went to a local automotive electrical shop and they made a new ground cable. The starter hasnt spun that fast in the past 10 w.
Those are the ones I use on my 113 evo.. How old are they? If over 10 years, I'd cut the heat shink off near the lug and if not soldered, pull them all and solder them.
I had starter issues that I thought was battery or wiring issue. 1st issue turned out to be my ignition switch which is a known problem. They get carbon and crapped up. Cleaning the contacts and the connectors and applying dielectric grease fixed that.
Next up in bad cranking...again thought battery, replaced with a good AGM and seemed a little better except when hot. Got bad again a month later and it turned out to be the starter clutch. It was a giant PIA to change out, but bike has never cranked better and I've gone through one(!!) battery in 4 years. May not be your problem, just my experience with crankiness
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