Quote:
Originally Posted by cycleguy55
I've got the car running again, but get a surge / miss on light throttle cruise between 2,000 and 2,500 RPM. There's also more exhaust 'popping' or 'burble' on throttle-closed deceleration than before I replaced the ignition box and coil. There are no apparent issues at idle, when accelerating, or cruising < 2,000 RPM. I have not yet checked the timing but, other than replacing the magnetic pickup and distributor cap, the distributor was not touched.
Environment: - MSD 8477 distributor (magnetic pickup swapped from old MSD 8577 distributor in process of getting car running)
- Summit Multi-Spark digital ignition control box p/n , mounted behind dash ( https://www.summitracing.com/parts/sum-850610 )
- Summit Epoxy coil ( https://www.summitracing.com/parts/sum-850500 )
Questions: - Does this sound like an ignition or a fuel issue?
- Where do I start?
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I've got it running well again. The fix was to disable the vacuum advance ("VA"). Right now it's running with only centrifugal advance ("CA"). Connecting the VA to either manifold or ported vacuum gives me the same result - popping and strong misfires - which I don't have when running only on CA.
Here's my thinking: There's enough timing (16° BTDC base, 21° CA in by 2,500 RPM) already, and any additional VA means it's too far advanced. So, why did I not have this problem with the old, analog, ignition box? My belief is the old box didn't react or respond as quickly to the signal from the magnetic pickup, whereas the new digital box does, making it more sensitive to the advance. If there's less inherent 'lag' in the new box, I suspect that will be more pronounced as the RPMs increase, though I haven't had the opportunity yet to give it a good road test.
FWIW, I've run across a few Internet threads to indicate I'm not the only one who has experienced this issue. Some suggested using an "inline smog vacuum reducer that only gives it 3 lb of vacuum", but that seems a waste of time, energy and money. I mean, what's the point of having VA at all if it's dialed back so far? Why not just go to a distributor w/o VA and keep things simpler?
Thoughts? Is there a fix for this, or should I just yank the MSD 8477 and put my old MSD 8577 non-VA distributor back in?