Quote:
Originally Posted by 61ragtop
Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.
Again Greg I am NOT doubting you just trying to understand it all and explain why I had it set up the way I did. When I used the vac adv hooked up to manifold vac my car responded real nice to it in performance and starting, idle, MPG.......
I DO value your help, just as you say it is hard to deal with these issues ''online'' via posts or email. It is also hard to convey how things are being said weather it is genuine or not, so dont take my questions and or ideas the wrong way (not that you are  ) as I dont mean anything of what I type to belittle any of your knowledge. 
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I never got any feeling of your not being fully engaged and willing to discuss these issues - nor did I ever get a sense that you did not believe me.
SO -- After reading your post about the vacuum advance --- I get all the way to the bottom... and BINGO -- he says EXACTLY what I said about the vacuum "dithering". He used different words -- but said the exact same thing. A MODIFIED engine works completely different than a stock engine.
Having said that -- this is why I said give the thing some more initial timing and go back to a ported vacuum. Basically this is the same "idea"/"theory" that a manifold vacuum advance is doing at idle (as he correctly explained) -- BUT without the dithering as he calls it (I called it vacuum bounce). But when you advance the initial timing - you MUST pay attention to the TOTAL timing that is set up in the distributor. Going to ported vacuum will give you all of the "advantages" that a vacuum advance has but with a cam - at idle - you're going to have timing all over the map. If you don't think so (or one way to actually see it for yourself) - stick a timing light on it and watch it. Also --- does your vacuum gauge bounce around? How much vacuum do you see. Have you run a long vac hose into the passenger compartment and gone out with a friend - and RECORD the vacuum at various driving conditions?
That information is very usefull.
Also - you can "map" your mechanical advance - while sitting in the driveway. Get your timing light -- DIAL BACK ONLY -- and with no vacuum advance hooked up (plug it) - record your initial timing - then bump the motor 500 rpms - record that timing - go another 500 rpms and record again - UNTIL you get no more change in timing. That should be around 3000 rpms or so. Maybe as late as 3500.... but you shouldn't see any change above that. That will give us a timing CURVE.... from initial to total. That would be good info to work with. Tells us a bunch about the distributor you have in there.
Remember that TOO LEAN can give you the same high CO readings that too Fat can give you.