Vacuum advance

1964 225 Valiant, completely rebuilt, 3 speed manual. Points ignition, all new quality parts chosen by what was recommended here. The original vacuum advance can wouldn't hold vacuum so it was replaced with a new one. I also installed a new drive gear on the shaft and set the end play per the FSM. I dug out my old test equipment and the dwell is 43*, right in the middle of the 40-45 spec. Currently set at 8* BTC static. I'll add more but that's at the limit of the adjustment slot on the distributor. (I know about the second slot underneath) It starts so quickly you can hardly hear the mini starter spin.

Future plans include converting to electronic distributor and HEI but I just wanted get it on the road and have some fun before winter starts.

It's running great and I've managed to put 180 miles on it in the last few weeks with the top down 90% of the time.

But.....when I connect the vacuum hose from the carb to the advance can, the engine bucks, misses and becomes undriveable. It backfired so badly it blew the bottom of the old muffler open so it sounds like an old hot rod. So I've been driving without the vacuum hooked up.

I hooked up a timing light and at about 1500-2000 rpm's the timing light stops flashing on #1 cylinder. I have not checked the others cylinders yet.

Anything?