The Vacuum Deal

[wxman1]

Greetings All:

I'm running a 69 C10 Pickup with a 350CID
engine that uses a 600cfm Holley 4 barrel.
I'm posting a link to a pic of the engine below:

http://mywebpages.comcast.net/wxgeek/holley_600cfm.jpg

Ok, here's the question. When I disconnect the
vacuum advance line from the distributor
I see NO change in the timing mark for number one
cylinder. The line is connected per Holleys
instructions to the Vacuum Advance Port. The port
below is the full manifold vacuum port that the
modulator for the transmission (TH350) is
connected to.

So whats the deal...Is the vacuum advance can
bad or what?

Engine timing at idle with the vacuum advance line
disconnected is around 11 deg BTDC.

Jim M.
Mt Juliet TN

[RON WOODGEARD]

You are either hooked up to the wrong vucuum port or, the diaphragm in the advance canister is bad.

[bigjimzlll]

Hi..your hooked up on ported vacuum right? This port has no vacuum at idle, it builds vacuum as the throttle is opened..I use the ported vacuum for my th350 and the full vacuum (the lower port) on the vacuum advance

[Solver]

I am not a motor head but I don't understand why you would want full vacuum at idle. Is'nt the whole idea of vacuum advance to increase timing at higher speeds (ie higher vacuum)?
If you hook it up to full vacuum port isn't that the same as just advancing the timing to that point initially and skipping vacuum advance unit?Like I said I am no motor head but was just wondering about this

[bigjimzlll]

not quite..if you have your initial at 10 deg. then add full manifold vacuum limited to 10-12 degrees this gives you 20-22 degrees at idle...where your motor likes it...then when you accelerate the vacuum drops off and the mechanical picks up..giving you 20-22 degreees of mechanical advance plus the initial of 10..giving you a total of 32 at 3000 rpm..now if you use ported vacuum..you will have 10 deg initial plus 20 degrees of vacuum plus 20 degrees of mechanical thats 50 degrees at 3000 rpm..can you say detonation??? GM used ported vacuum with real stiff springs on the mechanical for emissions...they would have 2-4 deg. of initial with the vacuum doing all the work until 5000 rpm...you will find more performance with non proted vacuum....
Jim

[Huck]

Assuming you set your initial vacuum at let's say 10 degrees advanced with the vac advance disconnected. Now if you have the original stock ignition, you would hook up to the full manifold vacuum and you should see the timing change for more advance. That would be correct ~~~~~~~. IF you have HEI ignition, you should hook up to the ported vacuum which would not produce any advance at idle as discussed above.
I would try the ---hook up to manifold vacuum with a timiing light--- to check the advance can on the distributor. If you do not get timing change = bad advance mechanism. Now a lot of what has been said depends on things being original like springs on the timing weights in the distributor. So, sometimes our recommendations will need tweeking to allow for 30+ years of some butthead modifications. Huck

[crazy longhorn]

If you are running a stock or very mild cam , the vac can will probably do better hooked to ported vacumn. for the guys running hotter cams, fat carbs ,ect......i feel you will do better with the vac advancehooked to full manifold source at the carb. To set up this way, you will need to do some recurving & go to an adjustable can(I run a Crane can). I run 14 initial, 34 total(with the can unhooked & plugged). The can is set to give 8 degrees, & hooked to manifold source. This brings timing @ idle to the 22 degrees (with can hooked), that Big jim was talking about, & totals @ 42degrees with can hooked & no load. I am running very close to 10 to 1 on compression, so it was nessissary to trim the vac timing back a little from that 10-12 degree figure. To answer the original Q, if you hook the vac can to manifold vac......& the timing doesnt change, the can is bad. good luck.......crazy AL

[wxman1]

Update on timing/vacuum deal. Got a good vacuum
meter. Hooked to manifold port on engine
reads 15 in at idle and goes up to 22 inches at 3000 rpm. Hooked to port vacuum above the
butterflys and reads about 1 in at idle and goes to around 22 inches at 3000 rpm. Disconnected
vacuum advance line from carb and connected it
to my hand operated vacuum pump. Applied about
10 in of vacuum and while watching timing light
on timing tab the spark advanced to about 18 deg
BTDC. So I would have to assume that the vacuum
can is working.

Thanks to Huck and others for cluing me in on
the HEI systems not have any advance at idle.
All the engines I had fooled with always had a
few degrees of advance at idle because they
always tell you to disconnect the vacuum advance
line before checking the ignition timing. this
is the first engine that I've had that had an
HEI distributor.

Bye the bye...I'm posting a link to a pic of my
distributor. It a little out of focus, but its a
Delco-Remy. It has some markings on the cap but
none of them are model numbers/serial numbers.
Is anybody familiar with this distributor?
Know what model number it is?

Link is:


Pic of my distributor

[bigjimzlll]

Im glad it worked out for you...but...the statement that HEIs do not have vacuum at idle is erronous...It depends on the setup..I have a HEI hooked to manifold vacuum, and it works great..it depends if you want performance or not, as crazylonghorn stated, if you have realitivly stock set up..the ported vacuum will work fine..but as soon as you step up to a performance build, you'll want to try the manifold vacuum

Jim

[Blackhawkdc]

Your distributor looks like just a stock setup. Might wanna look into an adjustable vacuum advance can so you can tweak the timeing a little.

[Bill72BB]

[QUOTE][i]...I'm posting a link to a pic of my
distributor. It a little out of focus, but its a
Delco-Remy. It has some markings on the cap but
none of them are model numbers/serial numbers.
Is anybody familiar with this distributor?
Know what model number it is?



The #'s should be stamped on the side of the Distributor if you can post them someone will tell you what model it came from .

Good luck