Need some help, Troubleshooting my car Options

[914curt]

I am having trouble with my 914. I have a 1974 1.8L that is bored to a 2.0L with dual 40mm carbs on it, that is about all i know about the motor. I purchased the car with this engine in it and was told it had a 1.8 turned in to a 2.0. The problem i am having is that it has completlely run out of power it was running good last summer but at the end before i put it away for the winter it got really bad. it will not accelerate at all. But sometimes it does. it is intermitent. Today i tool it out and it ran fine at first and them went to crap. it had no power and was back firing like crazy. I have put a new ignition system in the car, and changed the plugs today. when i checked the plugs all of them looked good except cylinder three. It was completly black, like covered in oil. the rest of them looked great like they were supposed to be. after changing the plugs there was no difference in the performance in the car. i was thinking that the problem could be the rings in cylinder 3 but i am a very novice mechanic. this is my first time trouble shooting a car by myself. Could anybody give me some advice or ideas on what it could be or if i am right about the rings.

Curt