Vacuum Advance Question

[Whitey]

OK...
I got a Mancini Distributor (non adjustable vacuum advance). I wanted to be able to FINE tune my timing so I replaced the non adjustable with an adjustable. When I started my Dart, I got a big 'pop', not like a back fire.
I disconnected the vacuum line to the distrib and restarted the car and it ran fine, just needs timing adjusted.
My question is, what should I do to set this up initially? I guess an example of what I'm talking about is the mixture screws on a carb; when you first set it up and you bottom them out then back them out so many turns.?
This is basically a stock distributor that I want to put a stock 'adjustable' vacuum advance on.

Thanks,
Eric

 

[Meep-Meep]

First you need to hook the vac advance to a ported port. On a Holley it's usually the little bent tube up high on the right side of the metering block. In this configuration you should not get any vac signal at idle and if you do you need to check the relationship of your throttle blades in the bores vs. the initial timing. About 1.5 turns of the idle stop screw from lightly seated should position the throttle plate correctly in the bore. Correct position is not to uncover the ported port and about half way up the transfer slot. Check the timing curve at various RPM's as described in the factory manual. You still may have to tweak it a bit but if you have a stock type setup it should be close enough to work well. You could easily see 50 deg of advance with the vac advance hooked up during cruise RPM.

 

[beeguy]

The way it was originally designed and FBO set ups require is to run manifold vacuum. ported vac advance started in the smogger era and isn't the best for most performance engines.

Set your initial wherever you like it and then hook your vac advance up to manifold vac, your engine will speed up and smooth out whick is what it's supposed to do so you'll then need to back your idle speed down, then check your total all in at 2000-2500. this way works best because as you open the plates in WOT your vac will drop to nothing and prevent pinging, as you close them your vac builds and increases your timing for a smooth efficient cruise range. you can play with your initial and your adjustable vac advance to get what works best, you may have to change your mechanical as well but do that as a last resort.

Your idel mixture screws are normally abouut 2 1/2 turns out for a starting point. it's best to use a vac gauge and work side to side a little at a time to adust your mixture.

 

[Meep-Meep]

Hmmmm, right or wrong I've never run manifold vacuum to the dist. I always had excellent results with the ported port and light springs, and like you said, all in by about 2500. I just set my buddies 390 GT Mustang up with the ported advance and it's responsive and no pinging on pump 91. He also cranks about 160 PSI compression pressure.

The other thing I've run into with manifold vacuum to the dist is a stumble due to the timing retarding as the throttle is opened. I'm sure the correct advance springs would compensate for the vac diaphragm snapping back, or perhaps it was a sticky mech advance assy, but the ported route seemed to work best for me. I'm pretty sure the factory AVS for 68 and 69 had a ported port for the vac advance. Now I want to go look at my carbs and consult the factory manual :edgy:

 

[beeguy]

I'm not a computer wiz by any means and I don't know how to paste stuff but if google " timing and vacuum advance 101 " you'll find a great read on the subject. somewhere I have the original full article and the authors name but you can get he gist of it on bronco.com and other sites.

Don at FBO sets up some of the best Mopar ignitions out there and he even runs manifold vacuum on moderaterace cars . manifold vacuum is the way to go on older cars , but you have to tune for it.

Most of us grew up with ported vac advance and don't question it, but that's because we are familiar with the smogger cars .