When you say broken negative terminal inside battery, do you mean the negative battery terminal connector broke? You need 4 things for an engine to run. Air, fuel, compression and spark. If you have good compression and you have spark at the spark plugs and you have fuel, unit should run. The fact that you have spark tells me your electrical/ignition system is operating. If you spray ether in the carb and you have spark, it should at least try to run momentarily. Do you have a vacuum operated lock off or an electric solenoid type lock off for fuel? How many hours on unit? How do you know plugs are firing? Need more information to properly help you.