Classic/Antique Car Repair: 76 cutlass, gas filter, obdii

Question
sir i have a 76 cutlass with a 455 engine which has run beatifully until last week when it would not go over 35 mph now it will not go over 5 mph could you tell me what this might be or where to start looking thank you  

Answer
LEE
Well that is hard to tell you it would have to be looked at, can you have a OBDII put on it to see what is wrong, i wonder if your fuel filter is plugged up.or the carb is hanging up, get a can of spray carburetor cleaner, remove air filter and spray the carb all over in and out and the side where the linkage is and the choke in and out. this might help.and change the fuel filter also check the air cleaner filter. the points, rotor,cap,cond.wires,etc. could be bad but - I would go with the carb. and gas filter first. and add a can of gas treatment.