Any ideas please????

My 1980 runs well, plenty of power, regular highway use, pulls cleanly to as high rpm as you want.... but have just run into a problem, on deceleration to idle the engine dies - if I catch it and give it a bit of gas it will idle but if i dont it just goes lumpy and dies - starts again no problem.

Have checked plugs and points - all is well

Reading posts the coasting valve seems a common problem - before I go to the expense of replacing has anyone other suggestions what this problem might be caused by or is it most likely the valve?

Thx

Chris