If you are putting a fresh battery in the 914 and it still won't start then the battery isn't the problem.
Your optima could still be discharged after letting it sit for 3 months. Optimas have special charging procedures when it is fully run down. They supposedly need a high amperage shock charge followed by a trickle charge to get them going again. I found this on the Optima web site:
34 & 34R - 34/78 - 6V - 25 & 35 - 75/25
These batteries are designed for engine starting applications. They are NOT recommended or warranted for use in deep cycle applications.
Recommended charging information:
Alternator:
13.3 to 15.0 volts, no amperage limit.
Battery charger:
13.8 to 15.0 volts, 10 amps maximum, 6-12 hours approximate.
Rapid Recharge:
Maximum voltage 15.6 volts (regulated).
Maximum current: No limit as long as temperature < 125°F (51.7°C).
Maximum recharge time: Charge until current drops below 1 amp.
Float charge:
13.2 to 13.8 volts, 1 amp maximum current, time indefinite (at lower voltage).
I also highly recommend a CTEK battery charger/maintainer for cars that aren't used every day:
http://www.ctek.com/US/chargers.aspI have the US800. Works awesome.