It's the heat riser valve that you have stuck; read about it & its function
here. See if you can free it up- here's a recent thread
about one being stuck. I got lucky & a few light taps with a ball peen hammer dislodged mine. Being stuck 'open' or 'closed' is not ideal and will cause poor engine performance. I don't know if it could cause the smoking, however.
Keep mentioning symptoms & what you see (like this heat riser valve being stuck) and folks will jump in & help out. I wouldn't bust out the head-pulling gear just yet.
Brad
