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Old 12-19-2011, 02:57 PM
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Very interesting. The part that always cornfused me is the thought that vacuum advance actually advanced with acceleration. I'm sure it was a long-ago forgotten thing I learned in auto shop. I always did well in those classes, but I can't remember much from that time to know why. But that's another story for another time.

I have always thought that as engine speed increases, so does the advance, which of course is true with mechanical advance. But since manifold vacuum actual goes down with throttle plate opening, I always assumed (until today) that the reason you hooked the distributor to the carb was that they had a means of increasing vacuum with throttle opening. This is actually true, to a degree, with the old Load-o-matic distributor and carbs. This article, and a related link on vacuum advance, helped cure me of that assumption. Though this is true with this style carb, all distributors with mechanical advance, which the Load-o-matic distributors lacked, required manifold vaccum.

Bottom line is that vacuum advance is NIL off idle and during acceleration. It provides advance in lean-burn conditions, most common at idle and cruise. Lean mixtures need more burn-time (more advance) and rich mixtures require less advance. Mechanical advance is all RPM based and is the advance you have access to when you are accelerating, AKA "getting it on."

So, that being the case, the specs they provided you, Kenny, of:
Advance begins at 800 rpm (I assume all mechanical movement starts here)
22 deg of mechanical @ 2300
14 deg of vacuum advance (I assume they can check this below the 800 RPM)
36 deg of total all in at 2560 rpm
... would seem to imply that you add mechanical advance to the vacuum advance to get this total. Since they can apply vacuum at any point while on the test bench, this doesn't mean you will have 36 degrees of advance at 2560 RPM at wide open throttle (WOT) under load. So under WOT, I think your max advance may be closer to 22 deg. I can't see you getting another 14 deg out of another 260 RPM. So... how did they come up with this total?

On a side note, I stumbled onto a thread yesterday on the TFFN as I was looking around the Internet about what spark plugs people used when doing a DSII swap since I needed a set (this was on my phone - so it wasn't a serious search) and a guy did a bunch of elaborate mods to his 1100 carb to create "ported vacuum," which these guys on CI seriously denounced the use of. The guy on TFFN really thought this was a requirement to do a DSII swap.
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Last edited by Luva65wagon; 12-19-2011 at 03:02 PM.
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