For a couple years, I've been running a Pertronix 510 hp digital ignition box and MSD distributor with mechanical advance only (no vacuum advance). I seemed to always be chasing a misfire here and there. Sometimes just a slight miss at cruise or acceleration, other times bucking and backfiring. I've worked through several issues, including a defective air filter, bad plugs/wires, charging system failures, and most recently, complete loss of spark due to a bad Pertronix ignition box (although I found the MSD distributor also had issues with internals rusting, which I understand has been an issue in recent years).
Being fed up with quality of expensive aftermarket "performance" parts, I decided to swap back to a Duraspark 2 setup. I already had everything...all parts either new or slightly used, having installed it in the past, chasing down other ignition gremlins.
Upon installation, the car ran 100% better. Even a long-running low-rpm "hiccup" (slight misfire) was cured. I had previously chalked that one up to a carb issue, but I guess I was wrong.
As it is now, car runs great. It cruises up and down the road with zero issues and seems to have plenty of power. Initial timing was set to 12 degrees btdc and total mechanical timing is around 35. Decent throttle response. No bogging off the line.
The only issue is that my vacuum advance is unhooked. When I attach it, idle goes to crap. Any blip of the throttle causes backfiring through the exhaust. Timing is erratic. Basically, the engine just hates it. I tried Adjusting the vacuum advance, retarding/advancing the timing, recurving (springs only), adjusting the carb, and even swapping out the vacuum advance can. I've tried manifold and port vacuum.
What am I missing?
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