76 KZ900 Charging

Hey all. Due to my girlfriends dad buying a Harley, I have a near mint 76 KZ900 to ride all summer(he's owned the bike for 20+ years). The problem I'm having now is after installing a new battery(trickle charged for 16 hrs), and riding for 40 miles the headlight gets dim and the electric start wont work. Let the green machine set for 15 to 20 min and the battery regains its charge and you can be on your way, home. The coils have been upgraded, jetted and piped(4 into one LEFT hand exit).
Should I be looking at wiring, voltage regulators? Any advice would be great. Pics to be posted soon!

First thing you need to do is put a multimeter on it. See what you have for voltage after it has sat charging all night. Start the bike see what the voltage is at idle, check it again at around 4,500 rpm I think it is. There are some other threads on here where WiredGeorge gave some detailed instructions on how to check out the stator and voltage regulator/rectifier. I think you should see somewhere around 14 volts at 4500 RPM. If you are not seeing a significant increase over battery voltage with the bike running you know you have a problem. Battery voltage should be around 12.5 or so volts, once you start the bike I'd expect to see at least 13 volts, likely more like 13.5-13.8. I'm assuming you have no extra driving lights or aftermarket lighting? The charging systems on these bikes are pretty borderline to start with so extra accessories can be a problem.

I've gotten bad new batteries before so make sure that isn't the problem before looking elsewhere.