I'm trying to tune my 350 crate motor, Edelbrock intake, 1406 carb, shorties headers with comp high performance cam and am curious about the vacuum advance can. I've been working with the timing, and when I advance it enough for the car to perform at its best, I have erratic idle and surging. As I've been digging through this car, I notice that the vacuum advance looks like the original one that came with the 305 in 1984. Should I change out the vacuum advance to something like an Accel Adjustable VA?

 

I'm assuming you're not running an ECM anymore, yes?

When you're adjusting the timing are you disconnecting the vacuum line to the vacuum advance?

 

No ECM, nothing but motor under the hood.

As for line disconnected/attached, have tried it both ways.... still not happy with it, going to change plugs, go back through the carb, but have yet to get the car to idle correctly while maintaining power. It idles high out of gear, low in in gear, but surges to the point of making sitting still at a stop sign hard. Surging is not a problem while accelerating which made me thing the current vacuum advance may not be suited to this set up. I went through and found and remarked TDC yesterday, reset the timing, but when I retard it to the point where it idles better in gear, I lose all kinds of power accelerating, over 3 secs 0-60

 

Block off the vac advance so you're just on mechanical. Set it up for best acceleration. Add an adjustable vac can connected to full manifold vac. Add as much as you can before it starts pinging when lightly loaded.

The extra vac advance at idle should help your idle problem.

 

No don"t. A vac can is great for operating vac assisted acsessories when you have low vacuum. Never use it on a vacuum advance.

Set the timeing with the Vac line unplugged. If you are running advanced it may run rough at idle, goes with the territory and a cam. You can change the weights in the dizzy to change when and how much advance you get as well.

I would recomend going to a distributor with an adjustable advance curve like a Mallory mechanical advance or similar distributor. That way you can set the timing to 15 before and dial in the total advance to where you need it.

 

?? Where else would I use it ?? I don't have any accessories in the way of heat, air, this car is stripped down to nothing but motor under the hood, anything extra has already been removed. The vacuum advance runs from the carb to the can already, are you saying I should remove the can all together and hook the vacuum advance line direct instead of installing an adjustable can?? I'm using the right side vacuum on the front of the Edelbrock 1406 currently.

 

What is initial timing at idle? the car may want more initial timing.

Vacuum advance will help idle only if you have enough engine vacuum for the vacuum advance to operate correctly. Most big cams will not produce sufficient vacuum to operate the vacuum advance,
which is why most racers omit vacuum advance and do one of two things.

1) Re-curve the distributor to get enough initial timing at idle for smooth idle and total timing is all in by 2500-3000 rpm.

2) Lock out the distributor so total timing is "all in" at all times.. this can make the car hard to start but several ways to get around this.

I chose option "1" because my cam produces only 2" of vacuum at idle.
initial timing is 19 degrees.. total is 34 all in by 2500 rpm.
idles all day at 900-1000 rpm.


you may also have to play with idle speed, a big cam will not idle like OEM, so forget about trying to idle at 700-800 rpm.

get a vacuum gage: set idle speed and mixture screws for highest possible vacuum reading, you can do the same with initial timing.