When you set the total timing without the vacuum hose connected to lets say 36* @ 3000 RPM; Is locking an adjustable vacuum advance at 36* doing the same thing? When the vacuum is set with the hose disconnected at 36* and then you connect the hose to the vacuum source, the timing will advance a lot higher than 36*, right? My main question is, could my engine be spark knocking because my vacuum advance is not adjustable, therefore I cannot keep it from advancing past 36*? I've run the timing at 36,35,34,33,32 and it still wants to knock some. The lower I go, the less "PEP" it has, naturally. I SHOULD be able to easily run this engine with 36* total timing, but it's proving me wrong. I haven't tried to run it without the vacuum connected to see if the spark know goes away. This is a stock GM HEI with a standard vacuum advance. I've installed the heaviest advance springs, and run 93 octane fuel, but it's still knocking. The plugs look to be burning normal. The compression is about 9.8:1 with iron heads. Input welcome!!

What is the engine combo?

Leave the vacuum advance unhooked. Set the total timing by revving it while watching the timing with a light. When it has reached total advance (hopefully by 3000 rpm or less), then set it at what you think will be about right.
SBC with conventional chamber heads (Sportsmans, double humps, old smog castings, etc) will probably be in the 36* area. Vortec or similar chambers will likely be in the 30-32* range.
BBC's range usually higher 34-40+.

OK,
Had to run for a sec.

Set the total timing to what you think is good. Go make a few WOT runs with it all warmed up and listen for detonation. If you have it back off at least 48 from ANY audible det.
Once you figure good total, (your base will be too slow probably) plug the vacuum in to full manifold. This will bring the idle timing up from base to a reasonable level.
It will also bring timing up from total to 50+ degrees, this is normal and is only there during cruise/light throttle, it goes away under WOT.

If you have a radical cam and low manifold vacuum, you'll have to go to locked out timing or a very narrow mechanical advance to make it work, and then use no or ported vacuum.

Most of the factory HEI cans have about 20° of pull to them, so it doesn't surprise me that you get some ping. But, when specifically is it knocking?

When are you getting the spark knock? Is it at cruise or WOT.

The vac advance will not add any timing at WOT, only adds timing to part throttle cond.

If your pinging at WOT then it's from total timing (not counting vac advance), plugs too hot, carb too lean, etc.

If it's pinging at part throttle cond. then I would say your stock can is adding too much advance and you need an adjustable one to linit the amount.

On my set up (10.18:1, 272/284* cam, iron heads) I had to set total to 34*, drop down to R43 heat range plugs, limited vac advance to 10* (stock cans adds anywhere from 15 to 24* depending on what it came from)

It's knocking at 1/3-2/3 throttle. It doesn't do it everytime I drive it either, but it does it enough to get my attention. This is in my '80 Z. It's a .060 over 350 with 180cc intake Pro Action heads. 64cc chambers (9.85:1 to be exact). CC XE274 cam. I know these cams are notorious for valve noise, but I am certain this is spark knock/ping. I ALWAYS run 93 octane gas and sometimes have added octane booster with no improvement. I run a Holley 650 (#4777).

When I set it, I ran the engine to get it to operating temp. Disconnected the vacuum advance on the carb end and plugged the carb inlet. I revved it to 3000 rpm and turned the dizzy till the timing light blinked right on 36*. Reconnected everything and took it for a spin.....PING PING PING! Followed the same proceedure and turned it down to 34*.....PING PING PING. Again, dropped it to 32* and it didn't PING near as bad, but the lower rpm torque went away too. A local guy known for hotrodding told me to set it back to 36* and leave the vacuum canister unplugged and see if that works. I haven't had time to try this, and I was wondering if an adjustable vacuum advance accomplishes the same results. I understand if the advance has no vacuum, then the timing won't be able to advance past where it's set. This is a 80% street motor so that's why I don't want to give up any low rpm torque.

Yeah sounds like your vac advance is adding too much timing. Get an adjustable one and once you tune it, you'll be good to go.

Being a street engine I would not run w/o the vac advance. The benifits of the extra timing is too great. Ckeaner plugs, more power, better throttle responce, etc.

You'll probbly end up only needing 8 to 12* of vac advance to get detonation free part throttle driving.

Question for you all that can help him too...

Scenario: Lets say he buys an adjustable can and he adjusted it to the lowest setting, or the setting that requires the least amount of vacuum to pull the arm..but he finds that it still takes more vacuum to be "all in" than full manifold vacuum has. (Random example: 12" for all in but full manifold is 10"). He cannot use full manifold vacuum... so he'd hook it up to the timed port. But, when using the timed port, what should his vacuum advance be adjusted to ..to get "all in"?

Can anyone help me with my last question? Its something that confuses me...

Personally I've never messed with an adjustable vacuum advance. Luckily I haven't needed to.

But, based on what I read, you can adjust them down as far as nothing. Although it would pointless to even have one at that point.

This one says "infinite adjustment to both amount and rate of ignition advance". (http://www.jegs.com/webapp/wcs/stores/servlet/product_10001_10002_30890_-1)

Maybe what's confusing you is that the vacuum advance will be highest when the vacuum is highest. The more the throttle is closed, the higher the vacuum reading will be. So by setting a vacuum advance can to the lowest setting would be the last thing you'd want to do if you're having the same trouble as GoldenOne7710.

By setting it "lower" would require less vacuum to advance the timing.

As you open the throttle to go faster, vacuum would drop and so would the amount of timing from the vacuum advance.

Does that help, shawntmartin?

I don't think the adjustment has anything to do with the amount of vacuum available, just the amount of advance the can will pull?

If you have to plug it into a timed vacuum port it definitely becomes a "try it and see" thing. Having a vacuum gague duck taped to the windshield while you drive around normally for a while can also give you an idea where you might want to be. If you never see more than 8-10" driving around like you would normally there's not much point having it all in at 12", for example. Get it all in at 10" and then play with the AMOUNT of max. vacuum advance from there to see what it likes.

Vacuum readings on the overrun/decel are irrelevant, by the way. You need the timing to be right under load- even if it's modest load like idling against the converter in gear, a steady highway cruise or modest part throttle acceleration, for example. Obviously, timing is critical under WOT conditions (where vacuum advance doesn't come into play), but under part throttle/partial load is where having the vacuum advance dialed in properly matters.

With a well matched combo it's not usually too tough to get it dialed in. If you've got a mismatched combo (cam too big for the stall and gears, for example) that's when it gets a lot trickier to optimize it for all conditions.

AJ_72,
No I'm not having the same trouble as GoldenOne7710, I was just wondering where I should have the vac. adv. adjusted if using the timed port. In an another post Damon explained that if I want to use the full port on the carb then the manifold vacuum at idle (in gear)had to be more than the vacuum required to pull the vac adv can arm all of the way or I would suffer from an unstable idle... which I was. If my manifold vacuum was not strong enough then I have to use the timed port. The confusing thing *WAS* if I was to get the vacuum adv. to be all in at a vac. guage reading that is lower than the idle manifold vacuum readingthen I would have to lower/loosen the screw. but you say looseing the screw advances the timing more which can cause "popping". So there are two situations here:

1.) Loosening the screw so that the can will be "all in" at a level lower than the manifold vacuum (like Damon explained needs to happen to use the full port) but this advances timing and can cause popping like GoldenOne7710 is experiencing.

2.) Tightening up the screw to lower the amount of added timing to get rid of the popping, but now the amount of vacuum required to pull the arm "all in" is more than what the manifold has... so now you have an unstable idle. And will now have to use the timed port.

So my options are to find another vac. can that can be set all in at a sub 10" reading (my manifold vac reading at idle, in gear) or I can use the timed port and advance the vacuum advance as far as I can without pinging. Correct?