My '73 440 in a street vehicle generally runs great and starts fine after sitting for 1 or 2 days, but if it sits for a week it is very hard to start, backfires and runs really bad for 5 or 10 minutes before "cleaning up". Once clean it runs great again. It has been doing this for close to a year, 5000 miles, but I put it down to poor tuning as I was doing other things.

Now the Thermoquad is great, after a lot of help from this site, and the problem still exists.

Some information, bottom end is old original (200,000 miles +)but still sound, heads were rebuilt with all new parts a year ago, moderately flowed and ports matched. Standard cam. Vacuum is fine with about 17 - 18 ins at 2500rpm cruise (65mph). The low garage temperature is about 50F and when good the car starts without needing choke.

I have one thought but would like to hear what people think could be the problem.

Thanks,