Welcome to the Corvette Forums at the Corvette Action Center!

Ignition Timing... What I want or what the engine wants.

  • Thread starter Thread starter sscam69
  • Start date Start date
S

sscam69

Guest
That is the question and I need some help from you experienced people out there.

Now that I know the timing marks are right I did some experimenting.

I set the timing at 12 deg, set the idle screw, A/F idle screws etc.

But the engine did not sound to good, "healthy" so to speak.

So I loosend the hold down screw and turned the distributor in the counter clockwise direction. When I did this the engine sounded a lot better, "healthy", and the idle rpm went up. So I readjusted the idle to 800rpm along with the A/F ratio screws and the car sounded a whole lot better.

On another note at the 12 deg timing the vaccum was reading approx 12" and when I readjusted I was getting close to 15".

Next test was seat of the pants. So me and my brother went for a few spirited sprints. I told him to remember how the "seat of the pants" feeling felt. We came home and then I readjusted the timing and went for the second round. Sure enough I could feel a difference and then I asked my bro and he said "oh yeah, definitely more".

I checked the timing and the yellow mark was way off the tab. My guess is 16 deg.

Oh and another thing, I installed a advance curve kit because of problems I was having with pinging at higher RPM. No more pinging.

What exactly is going on here? I think I know but I want to hear from you guys what could be happening.
 
In the "OLD" days, we would advance the timing until it pinged and back it off 2 degrees. Then make sure it turns over easy and starts without laboring.......with that done you got what the engine likes.
 
which dist springs?

sscam69 said:
On another note at the 12 deg timing the vaccum was reading approx 12" and when I readjusted I was getting close to 15".Oh and another thing, I installed a advance curve kit because of problems I was having with pinging at higher RPM. No more pinging.

Re advance kit: Which springs? ... lite, mid, hvy?
JACK:gap
 
sscam69,
did you disconnect your vaccum line from the distributor then time it?
I experianced the same problem yesterday. The previous owner set the timing to 14-16 btdc. I reset it to 6 btdc and it does not respond the same after i re-hooked the vaccum advance. I did notice that when i reconnected the vaccum advance it moved significantly. I wonder if the advance has lost some of it's tension?
I guess the tough thing is timing to spec or timing to performance (which you hope the two were the same).
I too would like to hear what the other more experianced folks have to say.
Shawn:upthumbs
 
cruzer82 said:
In the "OLD" days, we would advance the timing until it pinged and back it off 2 degrees. Then make sure it turns over easy and starts without laboring.......with that done you got what the engine likes.

I agree with this meathod. Use all the advance it will take without pinging. Even at 10 to 12 you will notice a difference in throttle response and acceleration. If you can't avoid pinging at anything over the factory specs (which is a big compromise so your engine will run on most any fuel you may buy) install an adjustable vacuum advance unit and adjust it stiffer until you tune out the pinging.

Tom
 
It's difficult to judge accurately, but I think you might still be one tooth retarded on the distributor. Try moving it ahead one more tooth, then try resetting the timing again. My guess is that you have brought the initial timing into the range where it will run OK, but to do so the ignition timing must be advanced off the scale.
Once reset, the timing when checked with your strobe should be well within the timing range on the scale.
Good Luck! Let us know how you made out.
vettepilot
 
Being one tooth off on the distributor (if that's the case) makes no difference whatsoever in how it runs, unless you can't turn the distributor enough to set the timing you want without the advance can hitting either the plug wire support or the intake.

Assuming your centrifugal advance is working properly and providing 20-24 degrees (and not beginning below 700-800 rpm), initial timing doesn't mean much. Disconnect the vacuum advance, gradually rev it up to 3500-4000 (or when the light shows it's not advancing any more), and set the distributor so you show 34-36 degrees advance at that rpm (a dial-back light helps here, as the mark will be well off the tab), then see what you have at idle - should be 10-14 degrees. Then connect the vacuum advance and note how much it added - should be 14-16 degrees more advance now at idle than the base timing reading. With the right combination of springs, ideally you want your centrifugal to start about 100 rpm above normal idle and be "all in" by 2800-3000. Whatever you do, don't use the weights that come in the "advance kit" - they're junk - use the originals.

You also want your vacuum advance fully-deployed at idle, so the can should be calibrated to be fully pulled in at a vacuum reading at least 2" Hg. less than your engine shows at idle - this will give a stable idle and provide better idle cooling.
 
Heres some answers to some of the Q's

Re advance kit: Which springs? ... lite, mid, hvy?
I went with the light springs because as I was installing the kit I noticed that the "hard" springs were the same ones that were already on there. Oh by the way its a replacement HEI distributor that I bought at Autozone. I was going to work my way in but since I was going to install this new cam I didn't think anymore tuning was necessary because its going to change.



did you disconnect your vaccum line from the distributor then time it?
I even checked it twice because I was retarding the timing so much.

unless you can't turn the distributor enough to set the timing you want without the advance can hitting either the plug wire support or the intake
Thats what happened. I could not turn the distributor counter clockwise anymore because I was hitting the manifold with the canister. There could be more potential there, but because I will be installing a cam its going to come off again and I am going to make sure I address this issue.

Whatever you do, don't use the weights that come in the "advance kit" - they're junk - use the originals.
Why are they junk. I figured the springs were not standardized and worked best with the weights already in the kit. Thats why I used the complete kit. My brother did notice that the onces on the replacment distributor are heavier than the onces in the kit. I was trying to take the dyanmics involved when swapping out the kit and the effects of not having the right springs with the right weights.


JohnZ I am going to try what you suggested but that brings me back to the same questions is "it what I want or is it what the engine wants".

What I am getting at is should we pay attention to how the springs/weights/Vaccuum advance control the combustions or should we pay attention to how the engine reacts to the ignition?

Frank
 
sscam69 said:


JohnZ I am going to try what you suggested but that brings me back to the same questions is "it what I want or is it what the engine wants".

What I am getting at is should we pay attention to how the springs/weights/Vaccuum advance control the combustions or should we pay attention to how the engine reacts to the ignition?

Frank

Normally, you'll find that the engine is happy with a proven recipe, and is driveable, has crisp throttle response, a stable idle, good WOT performance, and decent fuel economy. You have to consider the full range of your daily driving conditions, not just how it feels at WOT. Small-blocks generally like 34-36 degrees total advance (20-24 of it in the distributor, starting 100-200 rpm above idle and "all in" by 2800-3000), and 15-16 degrees of vacuum advance at idle. Unless you have a dial-back timing light to set up and "map" your advance curve and have the parts you may need to limit the amount of centrifugal advance if it's excessive, your best bet is to pull the distributor and take it to a shop that has a Sun distributor machine and have them set up the curve and check the vacuum advance on the machine. You'll probably find (as I have in years past) that the weights in the "advance kit" are so heavy that the curve comes in too soon and you can't get it to idle without already being "on the curve", which leads to idle instability and having the curve "all in" much too soon. Once it's set up "to the recipe", it'll stay that way, and both you and the engine will be happy.

If you start throwing "race parts" at it, all bets are off, and you have to experiment to find your own "recipe" that works best with the modifications you've incorporated, and that's when you have to start making compromises you probably won't be happy with. "Race motors" are easy to tune, if all you do is run at WOT; but if you want a "race motor" to run well under everyday street driving conditions, that's another matter entirely. Some people just don't understand this - usually their sig has ten lines of Summit and Jeg's goodies, and their post is titled "How Come My Car Runs Like Crap?" :)
 

Corvette Forums

Not a member of the Corvette Action Center?  Join now!  It's free!

Help support the Corvette Action Center!

Supporting Vendors

Dealers:

MacMulkin Chevrolet - The Second Largest Corvette Dealer in the Country!

Advertise with the Corvette Action Center!

Double Your Chances!

Our Partners

Back
Top Bottom