1988 Plymouth Reliant Car wont start regardless of no known

Engine Performance problem
1988 Plymouth Reliant 4 cyl Front Wheel Drive Automatic? Miles

I bought the Reliant about a month ago. The guy I bought it from said it hadn't been driven in a while because of a family problem.( I personally think its a load of bs, but oh well) We ran out the stale gas in the tank and started running it on unleaded, it ran for about a month even though it would idle high and you would have to sometimes drive two footed or when you came to a red light put it into nuetral to keep it from dying out. Now it wont start at all! It has a brand new radiator, distributor, carb, fuel pump, and fuel filter. It tries really really hard to start but wont. I got it to start yesterday by unplugging the MAP sensor, but it died immediatly after starting. I checked the computer codes there were none. The only ones I got were 12 and 55, which mean that the battery had been disconnected in the past 50-100 ingitions and the 55 meaning the end of the test. What could be wrong with this car to make it not start?