My 305 doesnt idle correctly and it stumbles when i give it some gas.

If you are certain that the valve stem seals are "broken" then you have oil from the cylinder head getting into the combustion chamber which will create smoke and seriously foul the plugs.

is it posible that codes be stored without a check engine light. I dont have a check engine light on, and my car is carburated. The cat converter is clogged but i made a big hole in it.... i didnt run the car for a year, i let it idle today for a good 20 mins, it seemed to get better.

Thanks, il try a new O2 sensor and later get my carb overhauled. i heard to stay away from remans. my original has only 152,000km on it.