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Author Topic: Distributor vacuum advance: how much pressure?  (Read 2485 times)

attkrlufy

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Distributor vacuum advance: how much pressure?
« on: July 13, 2011, 09:36:00 AM »

I posted this on Allpar but I didn't really get much help and the one answer I got makes no sense to me - even though I really don't know what I'm doing - so I thought I'd try you guys:

I recently scrapped the lean-burn in my car for an orange box
electric ignition.  A mechanic friend came over to help me set the timing
 ('79 360 & 4v).  We pulled the vac line from the carb to the
distributor and with the engine running tested how much vacuum the
distributor needed before advancing the timing.  Turns out it won't
advance the timing until there's about 15-17 in/hg pressure.  My friend then
 said it should start to advance at 5 in/hg and so I need to replace the
vacuum advance diaphragm on my distributor because it's waiting too long to advance the timing.  The only reason I want a 2nd opinion is because the
distributor is brand new from the elec ignition conversion kit, so I'm a bit wary of replacing brand new parts.


So here's the response I got:
Quote
The purpose of the vacuum advance is to ONLY advance the distributor at
light throttle. At light throttle, the engine can use the extra advance
for better fuel efficiency. At full throttle, the vacuum drops and the
mechanical advance built into the distributor is the only one used or
you will experience "pinging" or in other words "detonation" which leads
 to mechanical failure. It sounds like yours is working correctly.
There are 2 things I don't understand about that reply.  1) I thought when you have an elec ignition, you don't have weights in the distributor.  I thought weights were only with points.  2) if he says vac advance is for "light throttle" - and my vac advance "is working perfectly" when it kicks in at 15 in/hg - how can that be considered "light throttle"?  To get that much vacuum from my thermoquad, I'm practically at WOT.  If my mechanic friend is saying the vac advance needs to kick in at 5 in/hg, then that sounds more like "light throttle" than 15 in/hg does.

Anyone know who's right?


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1979 New Yorker - 360 4v, 2.71:1 rear, factory moonroof, factory road wheels


Tom Dawson

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Distributor vacuum advance: how much pressure?
« Reply #1 on: July 13, 2011, 11:48:26 AM »

You still have weights in an electronic dizzy, the lean burn dizzy does not.
Sounds like the dash pot  for the vac advance is going bad, you should see advance at the slightest pull of vac. If you have the HP dizzy, there is an adjustment in the nozzle of the dash pot, need an allen head to adjust.


Tom



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Steve

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Distributor vacuum advance: how much pressure?
« Reply #2 on: July 13, 2011, 12:42:53 PM »



I need to read this
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Distributor vacuum advance: how much pressure?
« Reply #3 on: July 13, 2011, 01:59:07 PM »

You should have mechanical counter-weights as noted, vacuum is set up that the higher vacuum is at idle and low throttle to advance and cruise, low vacuum bleeds off the vacuum to reduce the pinging. If the vacuum is not increasing above idle, as said, the dashpot, if it has a hex end to it, then an allen wrench can be put in the end and the amount of advance speed to let you control the point and speed of advance.
 
If you go back over to allpar and look under B bodies there is a long dissertation I wrote up about rebuilding and checking a distributor for operation that explains what you were asking over there. It basically was the same thing, how to take the distributor apart, check things, adjust the vacuum advance....long post and all, should help, but send me a pm if you need more, or I will answer questions here.
Ed
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Jacques

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Distributor vacuum advance: how much pressure?
« Reply #4 on: July 13, 2011, 10:25:39 PM »

Ed, do you have a link of your thing on Allpar? I couldnt find it, but its an interesting subject I like to read about.
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Distributor vacuum advance: how much pressure?
« Reply #5 on: July 14, 2011, 06:54:06 PM »

No problem, we went through the whole thing in checking and setting it up for this guy's 383 '68 Charger (I feel obligated to help any first generation Charger person), so the distributor stuff is gone over pretty complete, including the vacuum advance means/method/tuning and adjusting.
 
Hope it helps!
 
http://www.allpar.com/forums/topic/134504-yet-another-timing-question/
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attkrlufy

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Distributor vacuum advance: how much pressure?
« Reply #6 on: July 16, 2011, 03:20:02 AM »


Quote from: dana44
No problem, we went through the whole thing in checking and setting it up for this guy's 383 '68 Charger (I feel obligated to help any first generation Charger person), so the distributor stuff is gone over pretty complete, including the vacuum advance means/method/tuning and adjusting.
 
Hope it helps!
 
Awesome.  Thanks for that link.  I'm traveling now so I can't get to my car.  But when I get back, I'll be sure to use the info in those posts to sort this all out.


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1979 New Yorker - 360 4v, 2.71:1 rear, factory moonroof, factory road wheels


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