[vrooom3440]

I am preparing to do some in depth carburetor tuning and I want to make sure I have the ignition right first. I have been doing some reasearch to better understand the ignition advance system. Based on what I have learned I think I have some changes to make.

The current setup: BB 402, Crane HR296 234/[email protected], probably about 10:1, aluminum Edelbrock heads, and an unbranded "performance" HEI distributor from Tognottis (probably an import MSD clone). I set my ignition timing to hit 36 full without vacuum, and I think I was in the area of 14-16 initial. I think that should be pretty close but I plan to verify it again to make sure (and take notes so I know exactly where I am...). I just ran the vacuum advance as it came out of the box connected to ported vacuum. Turns out that "out of the box" in this case is vacuum advance starts at 5", full at 15", and estimated 20 degrees advance based on measurement of mechanism movement.

I have noted that sometimes there is a slight surge at highway cruise and I sometimes hear pinging with slight acceleration. I have also found that I can occaisionally have dieseling problems at shutdown, like if I do not run 91 pump gas.

My research, backed up by the symptoms noted above, suggests I need to limit the vacuum advance to about 12 degrees. I have read that there is a template floating around but have not been able to find a copy. Can someone point me to one?

Next I am under the impression I should use manifold vacuum for advance and set the adjustable can to come all in just below idle vacuum, or about 8" in my case. That seems pretty low so I would like to validate that. I cruise at 15" of vacuum so there would be a long way for vacuum to drop from cruise before the advance started backing out.

TIA

 

[vrooom3440]

No responses... so here is an update.

The vac can on the dist was an adjustable can. However the range of adjustment was about 2" total. Not good enough to come close to what I needed to do.

I checked my old dist and the can on it was labeled "UNI 9-11" but I never knew just what that meant. After checking it out with the hand squeeze vacuum pump (a very handy tool BTW) I found it started advance at 3", full at 9", and 20* total advance. So I guess that label indicated max advance at 9" and 11* degrees distributor (22* crank). MUCH closer to what I needed, so I decided to give it a try.

I made up my own vacuum advance travel limiter out of some scrap .030 stainless steel I had and cut it for 6* distributor or 12* at the crank. This reduced and raised the vacuum range to 6"-9" as well.

Put it all back together and reset the total timing, learning that the mechanical is all in by 2000. Might have to tweak that a bit too...

So I have around 36* total and 16* initial. With vacuum advance connected to manifold vacuum I idle at 28* which sounds like a lot. But I do have a fairly serious cam for a street motor. Cruise is at 48*.

Bottom line is that it seems much happier and I have not noted any pinging yet. It shuts down with out dieseling. My vacuum at idle has gone from 8" up to 10" and idle bypass requirements have been almost entirely eliminated (I was not able to get idle speed high enough previously without opening up the ported vacuum so I had put in a controlled adjustable vacuum leak to compensate).

The one thing that I had meant to do and did not was tweak the idle advance around and find what value the motor liked best. I was running out of time and in a bit of a hurry. But I still think that would be a useful piece of info to put into the puzzle.

 

[mr 4 speed]

Good info.
Usually limiting the vacuum advance 10* when you are running 36-38* total is about right.