I would not hook a car battery up to jump or start your bike.
One of the biggest shadows you're going to be chasing is "CCA" ratings by powersport battery manufacturers. None of them actually use the "CCA" standard and have their own formulas for coming up with what ever numbers they print on their website and they often don't line up from one company to another. I look for what companies document for "burts" rating etc. Amperes over a 10 second crank etc. I also don't mess with batteries that don't have a built in battery management System/circuit "BMS" but that's a different subject.
Have you removed the starter mounting bracket and the ground points and made sure there is no powder coat or corrosion causing resistance? My cranking improved a lot when I pulled the starter mounting bracket and sanded off all the black powder coat and then the red powder coat on the frame lug under the coil.
What's the actual battery voltage measure before you crank? What about during and after?
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