I have read many treads about ignition timing, but could you please comments this?:

I have just myself checked ignition timing with timing light, at ca 750 rpm.

I saw the scale (12*/8*/4*/0*)) at upside of pulley (it went from zero degree (Top dead point) to 12 degree only) So I had to calculate the distance to the timingmark I saw with the ignitionlight. It was almost the same distance (12*?) as from the scale from 0* to 12*. Also, 12+12= 24*.

I have heard about timing tape, that could give a more exact readings. I know this is maybe a highly forwarded timing. Anyone else who have such reading?

I rev it a bit (to about 3000 rpm), and I saw the mark moved (calculated with my eyes as there was no scale there) and it moved smoothly about 20* points. And that harmonize with the standard spec. for centrifugal advance....

But I tried also this: When the engine idled, I took the hose to vacuum canister (that was disconnected from carb) and blow pressure (first carefully) in it with my mouth. But the timing mark didnt move a bit. I tried to blow as hard as i could too, but nothing happend. This cant be correct!? Is vacuum advance function maybe defekt? I once "sucked" too, but the mark didnt move either now.

I then began to adjust the ignition-timing by adjusting/turning the distributor. In fact I first turned it MORE advance timing. I could move it so much that the timing mark moved another 10/15* (a total of 24*+10/15*= 34/39* at idle) before I could feel that the engine began to shake a little bit (trying to kick back piston??). I retarded it quickly (but I couldnt hear "pinging" when I advanced it like this...).

I then retarded it lower, against 6-8*. The idle speed then drop, and I could hear it idled pretty roughly/bad. So in fact it looks like the engine likes itself around 24*...(?)

But if vacuum advance doesnt work, I realize that it in some conditions (low/high vacuum) compensate for the (perhaps) advanced timing I have. But of course, non-function vacuum advance is not optimal.

I also mention that I will estimate that the timing mark at the pulley was hitting about 11 o`clock when the ignition was 24*. When I rew it (+ ca 20*) I estimate it was about 10 o`clock. Does that make sense? I have read that some of you prefer 18*. But if I do the same, I`m afraid I will lose a part of the power I gained when I earlier adjusted it from only about 8*.....? (a workshop earlier put it there at 8*, and it runned like crap)

 

I have my base timing, or the timing at idle, without any advance, set at 6° before top dead center, that's 6° advanced. But timing is not set in stone. It's usually suggested to give an engine what it likes.

Now, when you say you had to visually "expand" the timing tape, that would probably indicate one of 2 things. Either the engine is so far advanced by that much (assuming this reading was still taken at the idle speed of 750 RPM). Or, another possiblity is that the balancer is actually slipping from the inner hub. On a factory balancer, there is the inner hub. This is the part that connects to the crankshaft. The crankshaft pulley is also bolted to this part. Just outside the hub is a little rubber-like ring called the elastomer ring. This is pressed between the hub and the outer damper. It is not unheard of for this elastomer ring to "slip", as in the factory setup, it is, in no way, attached to either the damper or the inner hub, rather, just pressed. So, if this slips, or doesn't even have to slip, but the outter damper can actually shift its position, in effect, changing what the timing will be. So either the timing is that far advanced or the outer damper has slipped.

Try this: with the engine at idle, rotate the distributor until the timing tab on the cover lines up with about 6-8° on the "scale' you refer to. See how the engine runs. Also, if it is the stock balancer, there will still be the factory etching in it. This is a line the factory uses so that some relation may be established between the balancer and the timing tab, hence, allowing the timing to be set. Try crawling under the car and bumping the starter over (may want to disconnect the distributor, if it makes you feel safer). Try to see this factory etching. See if it at all lines up with this scale you refer to.

Regarding the sucking on the vacuum line: Considering your cam and the rest of the build, i would guess that your engine produces anywhere from 14-18" of vacuum, more or less. Either way, this is a bit more vacuum than you can produce with your mouth, so no, I doubt you would be able to replicate the vacuum advance with just your mouth. I am assuming you have it plugged into the ported vacuum source on your carburetor. If this is the case, the vacuum advance canister does not see any "pull" at idle. Once the throttles are opened (you step on the gas a little bit), then vacuum is directed and the vacuum advance is activated. Try getting the engine up to maybe 1000 rpm's or so and unplugging the vacuum line and listen for any change. Don't go to wide open throttle or anything above about 1/2 as the vacuum advance really doesn't do anything "up there" in the rpm range.

By advancing the timing so much like that, you are obviously changing when the spark plug lights the cylinder. The proper time to light is is generally just before the piston reaches top dead center, so that by time the spark is lit and the cylinder reacts, the piston has reached top dead center and can begin the power stroke. By beginning the spark that much sooner, you are lighting the cylinder while the piston is still moving up. This reduces the amount of power that cylinder, and the other 7, in turn, can make. It also causes that little stumble. Lighting too soon can cause for engine knock or that little ping.

On a 305, the timing tab on the balancer is located at 12 o'clock, so that is ROUGHLY where the indicator should be pointing. 6-8° of advance would put the etching or whatever form of indicator you have 3/4" or so to the left (if looking from the front of the car) of the timing tab. If you are at 11 o'clock, you are really advanced, as you say, as much as 20° or more. ANd yes, as you rev it, the centrifigal advance and vacuum advance do perform their job and further advance the timing. So that sounds like it's operating as it should. From the sound of things, it basically sounds like the base timing is off by about 15° or so. Try backing it off (retard the timing) and see how it works. Try to get your timing indicator to the point where it is just a hair before what would be 12 o'clock. Try for, as I described, 3/4" or so.

 

You're talking about a traditional carb motor with vacuum advance distributor, i.e. no computer controls....

Most SBC motors will like to idle somewhere between 12° and 16° advanced, then centrifugal advance starting at about 1200 RPM and smoothly increasing the "total" advance to about 36° at 2500 RPM. Then about 12-15° of vacuum advance on top of that, with ported vacuum, such that there is no vac adv at idle (throttles closed), but the idle circuits of the carb adjusted such that it becomes active under any real-world throttle opening that might be in use at cruise RPMs, and at vacuum greater than about 10-12".

So, you might actually see as much as 50° of advance with a timing light, depending on conditions.

Set the timing to where the motor runs the best. I'll bet that if you set the static timing (the simplest dumbest most trivial part, where you twiddle the distributor) to about 15°, and then tune the weights & springs in the dist for 22° of added centrifugal advance with a RPM curve as described, and use an adjustable vac adv canister set for 12° of advance at 10" of vacuum, you'll have the optimum all-around performance street timing curve.

Meanwhile, give the motor what it wants (that is, make it run the best), and don't let the "numbers" surprise you. The motor doesn't read a timing light and decide "oh that looks unusual, I'm going to run bad".

 

Thank you guys. I will carefully read thru`your postings and come back after working with this.

I also found out that it`s not correct to blow at the vacuum hose. Of course this one should be "sucked"

 

Right.... suck, don't blow!!