87_FC

Well when i first got my 87 RX-7 N/A the guy that I bought it from told me that when the engine warms up it sometimes start running rough. But I still bought it ($800 yes it was a bet), what it did when it warmed up was like if it wanted to turn off and when put in gear and wanted to take off it would take off really slow because even with the gas pedal all the way down it would still not go like normal but really slow I would just let it sit for acouple seconds and the engine will go back to normal. I did the tune up spark plugs, cables, oil, oil filter, and air filter and I also bought a fuel injector cleaner and it did not do that anymore the car was now perfect. But just yesterday about a month of driving the car daily with no problems it started doing that again but only when it warms up and sometime if turns off or if I turn it off it has a hard time starting until it cools down for about five minutes and then it is good to go again. I checked the oil and it is full, I was wondering if it is because my straight pipe exhaust is ripped by the cat converter or if i need to change the fuel filter. Well I really have no idea why it does this someone please help me why does it do this? And today I have also noticed that when it starts to do all this it like shakes and then white smoke comes out of the middle of the car by were the cat converter is and at the end of the exhaust pipe the smoke comes out brownish. Today we changed the fuel filter and it stil had the same problem checked the fuel pump and it still did it im gettin a pretty good sparg and their seems to be no water or gas in the spark plug could the problem be the cat converter of too much unburnt gas collecting their because when the car starts to do this it smells like unburnt gas.. or could it be some kind of sensor.....Some one please Help Me Out...Sorry for being a long post just wanted to explain everything happening...