I'm having a really hard time getting my Y Block running smoothly.  Bear with me - I'm still learning.  I have a 292 in front of a Ford-O-Matic in my '59 Country Sedan.  About 7 or 8 years ago, I had the engine rebuilt to stock specs (but bored .030 over) but it never ran quite right afterwards.  I was always very inconsistent - I'd get it tuned and going well, but once it ran for a while it would sputter.  Then the next time I turned the key it'd run poorly again.  So it mostly sat and now only has maybe 8-10hrs of run time and fewer than 400 miles on it.  Before today, it hadn't been started for any real length of time in over a year.

I recently got a renewed interest in the project though so after a fluids change I decided to try to get her running this afternoon.  Following the shop manual, I started by warming her up and tuning the idle speed on the Autolite 2100 (rebuilt with the engine), then the fast idle.  The idle was slightly lumpy and I had to raise the RPMs to 550 since 450 was too rough.  Then I moved on to the mixture.  The manual says to lean it out until it runs rough, then enrichen the mixture until it "rolls", then turn the screws in until it runs smoothly.  The first step was easy, but backing the screws out would smooth it out immediately and then produce no change no matter how much I turned them.  I finally settled on an arbitrary one full turn from lean but the slight lumpiness remained.

This is where the trouble began.  I gave it a quick rev and got a backfire out of the carb.  The idle then got really rough and it produced a good amount of white exhaust.  I tried playing with the mixture again but it didn't help much.  I haven't double checked the timing yet, but it should be at spec since it was set during the last tune.  Any ideas what the problem could be or what else I can check?  I'm hoping I didn't permanently damage something...