Well, it's been a while because of a few hurricanes and Halloween, but got back to work on the 68.
Checked and re-checked everything possible and found nothing to mention.
We re-checked the firing order several times, I verified each plug wire was firing strong, re-did the compression test, adjusted the carburetor floats, checked the timing and tried to fire it up, same results. Backfire and flame out the carb.
This time we went farther. Pulled the valve covers and re-checked all the valve adjustments. All fine, no collapsed lifters. Tested the advance on the distributor, all fine. Pulled the carb and took it back to Gearhead so they could test it. Floats working fine, power valve working fine, but they found the choke was not functioning, and once the engine started, it stayed in full choke, which allowed the engine to run at high rpm, but once it idled down, the engine started to to flood.
That explains why the plugs were fouled so bad so quickly, but did not explain the backfire.
Their BBC guru asked about the timing. I said it was static timed for 7 degrees BTDC with the vacuum advance plugged. He said, in his experience, BBC with any amount of cam needs about 15 degrees of static advance. That surprised me, seemed like a lot, since I did not have that much cam, but I went home, re-installed the carb and left the timing at 7 BTDC. Started it and instant backfire.
Increased the timing to 15 BTDC and tried again, this time boy did it roar to life.
Set the total timing to 36 degrees at 3000 rpm and it idled down fine, but rough. Did not like to idle below 900, would not idle hardly below 700-800 rpm. Looked up my cam specs with the cam grind number on Comp Cams site, and discovered the mild cam I bought was not so mild after all !!!
So now the engine runs, the cam is broken in and I can move on to the next phase, installing the tilt nose hinges.