battery too weak?

I hooked up my 1978 kz650b electric starter and the starter barely turns. I did a battery test the other day and it said the battery is good. So why is the starter having a hard time turning? Its a wet cell battery.

What is the battery voltage before and while cranking?

If your battery is or is going bad, it can destroy your electrical system.
Steve

If the battery is bad replace it; if it needs to be charged disconnect it from the bike and charge it at LOW amperage (2 amps or LESS). Do NOT try to start your bike with a battery charger hooked up to it or by using jumper cables that are attached to a running vehicle as that can cause problems with your bike's electrical system. Ed

Before 11.3 while cranking dipping to 5

11.3V? Battery is cactus. You can try charging it (low amperage as stated above - if you use a high amperage car charger it can boil it).

11.3V indicates that the battery is at about 10% charge. Might be past the point of no return, depending on how long it's been that way for.

You can jump a motorbike from a car, but best not start the car like you would if you are jumping another car. You don't need the extra starting voltage - just jump from the battery alone.

Get a battery. I get my batteries from High Tech Battery Solutions on line. Under $50 delivered with a one year warranty. AZ and others have a 90 day only.
Steve

Make sure the battery is charged. You can get a cheap charger from Harbor Freight.

Yeah I'm going to look around for a new battery

Not the numbers 12N etc.
Steve

And make sure it comes with a bottle of acid.... some DO NOT!!

What do the battery numbers mean? I'm having a hard time finding one that fits the battery holder. Went to AutoZone and they didn't have a battery that fit.