kalli

i know what you mean. when you have the accellerator pedal at a certain position and it does drive but you have the feeling that you push the pedal a tiny bit further than you actually should have to.

drive in 1st gear at idle. (basically as slow as you can without using brake or clutch) straight and slightly uphill. if the car jitters at that condition the ignition is too far advanced (my guess is it would be).

where do you have the vacuum advance hooked up to? manifold vacuum or ported vacuum. if manifold vacuum then that 16 +vac advance is way too much and would explain problem. Maybe the curb idle is set a bit wonky causing some vacuum at idle. So check if there is a change in advance with vacuum connected and disconnected.
As said 16 is already a _lot_ and when you add a bit of vac advance this will cause problems

scootchu

My curb idle is 800rpm. It's a manual 302.
I pull about 14" vacuum at idle.
The advance is connected to the timed port on the Holley.
When the vacuum advance is connected and I use a timing light the advance goes off the scale of the balancer.
I have adjusted the vacuum canister before. I will see if I can get it pull a bit less as this it where the pinging happens.
Without the vacuum advance connected it runs like a raped ape.

kalli

---> When the vacuum advance is connected and I use a timing light the advance goes off the scale of the balancer.

that really sounds like manifold vacuum port, not timed vacuum.

The holley have the timed port at the front metering block passenger side. All other ports on carb below the float are man ifold vacuum

if you have a high vacuum on the ported (timed) pickup at idle you might want to investigate on that. maybe check curb idle screw wrong timing and you have to turn the screw too far in which can cause something like that.

urban_cowboy

You can run just mechanical advance (I do), but you should probably do so with a dizzy designed for just mechanical advance.

For mild street motors, vacuum advance can help fuel efficiency. I have attached an explanation from wikipedia.


Vacuum timing advance
The second method used to advance the ignition timing is called vacuum timing advance. This method is almost always used in addition to mechanical timing advance. It generally increases fuel economy and driveability, particularly at lean mixtures. Vacuum advance works by using a manifold vacuum source to advance the timing at low to mid engine load conditions by rotating the position sensor (contact points, hall effect or optical sensor, reluctor stator, etc) mounting plate in the distributor with respect to the distributor shaft. Vacuum advance is diminished at wide open throttle (WOT), causing the timing advance to return to the base advance in addition to the mechanical advance.

One source for vacuum advance is a small opening located in the wall of the throttle body or carburetor adjacent to but slightly upstream of the edge of the throttle plate. This is called a ported vacuum. The effect of having the opening here is that there is little or no vacuum at idle. Other vehicles use vacuum directly from the intake manifold. This provides full engine vacuum (and hence, full vacuum advance) at idle.

What do you mean that with vacuum advance, the timing was way too much? That tells me that your timing curve is too quick, you have too much initial timing, or you have too much total timing.

scootchu

Okay back to basics.
Inital timing 6 degrees
Back to 15L
Still have the light springs ( Total timing at 2300-2400 rpms)
Vacuum advance hooked to manifold port
Test drive indicates no pinging, great acceleration.

Hmmmmmmmmmmmmmm.

kalli

this way the engine should have a LOT of advance when idling and cruising and the bigger the load less advance from vacuum and the higher the rpm more advance.

I do understand correctly that the timing was set while the vac advance was disconnected and plugged. Only after timing you plugged it in. This should have caused the ingintion to be at ~20 degrees and the idle a lot higher.

If it's driving this way it sounds good to me. might even want to try 8 degrees initial

scootchu

Yep initial was set with the hose disconnected and plugged. The timing would settle after a few revs to about 18-20 degrees. Idle wasn't terribly high as I had it set at 650 and after the vacuum switch it went to about 850.
I have read where full manifold vacuum is how the early 302's ran and that the timed or ported vacuum was added, but not preferred by some.
I will get out to do some more testing, but on the first drive there was no pinging whatsoever and the performance was comparable to what I described earlier.

urban_cowboy

I think the main difference in manifold vacuum vs ported vacuum is:

Ported vacuum is 0 at idle b/c the vacuum pickup is higher than the throttle plates so you get no vacuum advance (or very little) at idle when connect to this source. This was done to increase exhaust gas temperature to aid in the emissions air injection stuff. (Needs more centrifugal or mechanical advance to get total timing correct)

Manifold vacuum will generate vacuum advance at idle due the the presence of vacuum at idle. (Needs less centrifugal or mechanical advance to get total timing correct)

Honestly, you can set your timing up for either source, but it may require reworking your timing curve, so pick one and stay with it.

Here is a decent forum thread about the subject.
http://www.corvetteactioncenter.com/...m-advance.html

kalli

read that article about a year ago, re-read it now again and picked up very inportant piece of info. the amount of centrifugal advance must be way lower when the manifold vacuum is used (instead of timed) ... that might help ;-)

scootchu

I read that too.
Here's the site it came from:
http://www.corvette-restoration.com/...o_articles.htm

Very interesting reading and well written.