geehumshriber

I switched to the non-CC carb a while back, and now I realize that there's two different types of vacuum advancement ports on the carb; timing and full. How do I know which one I use? It's an Edelbrock 1406, and before I go knocking around under the hood, which type of advancement is in a distributor and how do I find out? To my knowledge it uses centrifugal AND vacuum, if that offers any help to you.

Then, how does this change how I have to tune up my carb? I'm worried it's too rich.

How does this all tie together?

SPIKE

iTrader: (1)

if your using a standard dizzy you will see little weights and springs under your rotor thats your centrifugal. and if your dizzy has a little canister with a vac line on it thats tour vac advance. these are for adjusting how fast and how much timing advance is put to your motor. as far as using ported or non ported vac to your dizzy is see what one gives the best results when you punch it. some setups run better on one or the other. try them both. there are some real good posts on here that get into this in great detail if you search.

toms84ss

iTrader: (1)

The vacuum advance is usually run to a "ported" vacuum source. This means it will add timing under partial throttle or high vacuum. If you run it to manifold vacuum. It will be there at part throttle and gone under full throttle. You would be better off with more initial timing then trying to use the vacuum advance for anything other then part throttle timing. Its supposed to help with fuel economy.

atc3434

iTrader: (4)

Lets here a little bit more about the combo your running... what manifold, heads, cam, exhaust, cubic inch, and tranny/rear-end are you running? If the 1406 is in stock configuration, I would be amazed if it was somehow running rich, I jumped to a .42/.70 rod and a .101 main jet for the secondaries to get things right with my mildly cammed 305, stock the carb was super flat on the top. Accelerator shot is still the middle hole, thats working pretty good for off idle response, running the factory springs and the transitions on and off power staging are clean and smooth. This is all with 14* of initial adv, without vacuum. My advance at idle is actually quite a bit higher, I'm of course getting adt'l timing with the vacuum advance connected. I don't have a total advance number as yet, I'm running 1 light and 1 medium spring, so I should be seeing about 30*-34* total timing in by like 2800rpm.

You want to get your initial timing dialed in before you start worrying about any carb settings, you're initial timing will affect how you setup everything else. You want to have an idea on how much total timing you want - you'll need to know how much your distributor will mechanically advance, you'll take that number, and then add enough initial timing to be sure you're acheiving as much total advance as your combo wants. SBC's like 32*-38* of total timing, in by 3000rpm or less... the total advance and curve is gonna depend some on your comp ratio, cam, jetting, etc, but 34* in by 3000rpm serves most applications adequetly.

Once you've determined your initial advance and got that were you want it (probably between 8* and 16*, again depending on your total mechanical adv) you'll want to hook up the vacuum to the distributor and get started adjusting the idle circuit. The 1406 will idle pretty darn well with 1.5 turns out on each idle air bleed screw, you'll want a vacuum gauge to tweak from there, adjusting for maximum vacuum at idle.

Once you've got that squared away, you'll wanna start tuning the various circuits on the car, the stock calibration should run on most cars, but may be far from ideal. Just going from stock rods at .47/.73 to a set of .42/.70 made a tremendous impact on my lowend and part throttle response, but I was still flat on top. I jumped from the stock secondary jets at .95 to a .101 and the topend came alive pretty good. I'm sure I've got a little more left on the table still, but I've gotten things all running smooth, clean, with relatively good gas mileage, I'm on the safe side of rich right now, and will slowly lean my secondary jetting in conjunction with total timing to get things optimal.

geehumshriber

Okay, as for engine parts, it was a scrap piece that the last owner put together. As far as I know it could be top of the line or bottom of the line stuff. It's a 305 with nothing special. Posi and a TH350 (auto).

It's both a vacuum advance and centrifugal, on the carb there are two ports for the dizzy; full and timing. The question is right for a dizzy with both vacuum and centrifugal advancements.

For timing, it's an 8 degree. I just recently did the timing with the old carburetor, so to my knowledge, the timing is done right. Mechanic did it.

Any further advice?

atc3434

iTrader: (4)

Well, I'm guess its not clear as to what your trying to acheive. If you want maximum performance, follow the process I outlined above. I'd run with full vacuum on the advance, timed is usually for specific distributor applications, if your ignition doesn't specifically call for timed vacuum, I would run full vacuum. If you're just interested in getting the car running ok, and not full out performance, I'm sure the stock jetting with the 1406 will get you there just about right. If you're looking for more than that, again, see above.