Vacuum timing

[James903]

I’ve got a quick question about vacuum advance. I’m setting up a tunnel ram and my base timing is 20 deg and I have the 18 deg timing bushing for the mechanical. So basically I can get 38 total and it runs good I just have to set the carb set up a lil better. It’s running a lil lean. I’ve got a vacuum can on my msd I still haven’t bothered to run the lines to yet. My question is obviously the vacuum advances runs off manifold vacuum and it’s a 10 deg canister. So now will the vacuum canister give a additional 10 deg of timing on top of my total timing? Or does it just give 10 deg when there is higher vacuum pressure at lower rpms? I feel like I know the answer from stuff that I’ve read but not sure. You can only get the advance you have set mechanical right? The vacuum canister just helps advance when there is vacuum, and pulls the weights open in the distributor right? Thanks

[Captainfab]

If you connect your distributor's vacuum advance to manifold vacuum, it will advance the timing at idle and drop off after the rpm's increase. If you connect it to ported vacuum, it will not affect the timing at idle, but it will advance the timing as the rpm's increase and the vacuum increases. The vacuum advance does nothing with the weights under the rotor. The vacuum advance moves the base plate in the bottom of the distributor

[James903]

Cool, thanks for the response. Tuning this tunnel ram has been somewhat fun in a weird way. I like what you said about ported and vacuum off of the manifold, that never quite crossed my mind. I did try hooking up the advance (ported) a couple times after I set the base timing.it seemed all good but then when I gave it throttle it wouldn’t come back down to idle that I had it set at. So I’ve just left it off while I was getting the jetting closer. Not sure if the vacuum canister was just sticking after it started to advance. Now that I’ve got the jetting closer I’m thinking about trying to set it up with the advance. I need to put the bigger accelerator pumps in this weekend and plan on hopefully getting it all dialed in. I might just end up not running the vacuum advance. It seems to run good the way I have it set up now just a bit lean.

[Dead Parrot]

The behavior with ported could be due to your setup being on the very edge of starting to pull ported vacuum at idle. When you pop the throttle, the vacuum advance comes in, speeds up the idle enough to put enough vacuum on the ported port to hold the advance in place. Try it on manifold and readjust your idle.

Vacuum advance is a part throttle efficiency device. If it is working properly, it is out of the picture at or near WOT.
So at WOT: timing = initial + mechanical, in your case a max of 38*.
At part throttle: timing = initial + mechanical(varies with RPM) + vacuum. Under light high RPM throttle, you can have close to 50* advance.

[James903]

I did try the manifold vacuum today and it was the same result. I haven’t given up on it yet. I changed a bunch of things on the carbs and ignition. Different springs and advance stop in the msd. The carbs I did bigger accelerator pumps, different power valves, squirters, and main jets it made a huge difference. No more lean popping and got it to idle at lower rpm. So I’m getting closer and it’s definitely fun to drive. I did drop the timing down to about 16deg. If I had more daylight to mess with it I was thinking about dropping the timing a bit and trying the advance canister again. Now that it’s idling better my vacuum at idle is about 9-10hg. I noticed that the vacuum canister I have is a b26 which I believe is rated around 10 for vacuum. I want to try a different canister or find a adjustable canister