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bw46 · 10-07-2015, 08:47 PM

Maybe a vacuum leak around the intake manifold where it meets a head. Could explain the unbalanced idle a/f screw reaction.
I agree this sounds like an intermittent lean condition.
You should not need 16 degrees at idle. Remember that the mechanical advance from the MSD factory will add a bunch more, often well over 20, which would bring total to too much. When an engine needs a lot of initial it's because something else is off, or it has a big cam, which doesn't seem to be the case.
A big imbalanced vacuum leak could lead you to adjust idle up on the power circuit as well, to compensate. the guys in the thread that mention this are spot on, but I suspect if you close your throttle plate your engine won't run due to something else.
To look for a vacuum leak, you can spray small amounts of starter fluid around the suspected location. If the RPM jump up, you've found it.

All just a guess, but maybe...

10-07-2015, 08:47 PM	#16
bw46 Registered User Join Date: Sep 2015 Location: portland, OR Posts: 48	Re: strange engine issue from 900-1400 rpm Maybe a vacuum leak around the intake manifold where it meets a head. Could explain the unbalanced idle a/f screw reaction. I agree this sounds like an intermittent lean condition. You should not need 16 degrees at idle. Remember that the mechanical advance from the MSD factory will add a bunch more, often well over 20, which would bring total to too much. When an engine needs a lot of initial it's because something else is off, or it has a big cam, which doesn't seem to be the case. A big imbalanced vacuum leak could lead you to adjust idle up on the power circuit as well, to compensate. the guys in the thread that mention this are spot on, but I suspect if you close your throttle plate your engine won't run due to something else. To look for a vacuum leak, you can spray small amounts of starter fluid around the suspected location. If the RPM jump up, you've found it. All just a guess, but maybe...