 stupid question: timing w/vacuum advance theory, trying to understand.... |
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| Dr. Roger |
 Aug 20 2006, 07:20 PM
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#1
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A bat out of hell.  Group: Members Posts: 3,944 Joined: 31-January 05 From: Hercules, California Member No.: 3,533 Region Association: Northern California  |
i understand that initial mechanical timing is important because it needs to be retarded enough to allow the engine to turn over but advanced enough to start.
i also understand that mechanical advance increases as RPM's increase. i understand that the vacuum advance hose needs to be disconnected and plugged to time ignition at idle. (no vacuum advance). after timing, the vacuum hose is re-connected. now where i'm confused is the vacuum advance dials in advance under high vacuum states (idle and mid throttle). this means less advance at wide open throttle. makes sense so far. where i'm confused is if i start my car, and do not touch the gas pedal, won't the engine get high vacuum turning over the engine and kick back the starter? |
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| bd1308 |
Aug 20 2006, 07:26 PM
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#2
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Sir Post-a-lot Group: Members Posts: 8,020 Joined: 24-January 05 From: Louisville,KY Member No.: 3,501 |
ah but here is the other thing.
the centifugal portion of the dizzy doesnt kick in until ~1000 RPM or so ( a little less). At idle total advance = initial advance = ~7.5 BTDC (IMG:style_emoticons/default/smile.gif) |
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| Dr. Roger |
Aug 22 2006, 12:37 PM
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#3
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A bat out of hell. Group: Members Posts: 3,944 Joined: 31-January 05 From: Hercules, California Member No.: 3,533 Region Association: Northern California |
right, so when I turn over the car the mechanical advance is minimal but vacuum advance is getting full vacuum, right?
wouldn't that be too much to turn over? what am i missing? I need to understand this. =-) |
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| phantom914 |
Aug 22 2006, 12:41 PM
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#4
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non-914-owner non-club member Group: Benefactors Posts: 1,013 Joined: 24-February 04 From: Covina,CA(North ofWest Covina) Member No.: 1,708  |
QUOTE(Dr. Roger @ Aug 22 2006, 11:37 AM)  right, so when I turn over the car the mechanical advance is minimal but vacuum advance is getting full vacuum, right? wouldn't that be too much to turn over? what am i missing? I need to understand this. =-) The port for the vacuum advance is usually placed such that you do not see full vacuum until the throttle plate is open slightly. Plugging it when setting the timing is just a precaution since you will not normally see full vacuum when the throttle is fully closed. Andrew |
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| bd1308 |
Aug 22 2006, 12:45 PM
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#5
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Sir Post-a-lot Group: Members Posts: 8,020 Joined: 24-January 05 From: Louisville,KY Member No.: 3,501 |
OK
Here we go... 7.5 initial advance 15 vac advance (or so) 15+7.5=22.5 degrees AT idle. But what the key here is how much CAN the engine be advanced to run well at idle? Retard comes in with open throttle, as vac advance cuts down. I think you're on the right track to figuring out why it ended up being disconnected at the factory after 74. Personally, I never HAD the advance side connected, but the retard side rubber diaphragm was always toast. The advance side was very responsive and tight, which also led to my believe and supporting the advance side unhooked therory. After thinking of my total advance, I was basically running a 009 anyway, so I just bought a 009. 32 degrees is enough for me. b |
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| KenH |
Aug 22 2006, 02:01 PM
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#6
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Senior Member Group: Members Posts: 680 Joined: 16-January 03 From: Gilroy, CA Member No.: 156 |
The vacuum advance pick-up hole is above the throttle plate. There is NO vacuum advance till the throttle is opened.
Ken |
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| Dave_Darling |
Aug 22 2006, 10:14 PM
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#7
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914 Idiot Group: Members Posts: 15,063 Joined: 9-January 03 From: Silicon Valley / Kailua-Kona Member No.: 121 Region Association: Northern California 
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Ken is correct. The vacuum advance port will only be pulling a vacuum when the throttle valve is open a very small amount. So the vacc advance is not active at idle in any way shape or form--in fact, the vacuum retard is active at idle, because it gets its vacuum from below the throttle plate.
--DD |
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| Dr Evil |
Aug 22 2006, 11:02 PM
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#8
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Send me your transmission! Group: Members Posts: 23,034 Joined: 21-November 03 From: Loveland, OH 45140 Member No.: 1,372 Region Association: MidAtlantic Region
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For a more indepth explanation done by Lapuwali see my MJLJ thread and my advance quesitons.
http://www.914world.com/bbs2/index.php?show...mp;#entry750441 Missin ya, James |
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