[many birds]

Under moderate-to-heavy throttle (still in the primaries), going up hill, my car has very mild pinging right after the 2 to 3 shift (2800 rpm). It has excellent throttle response and idles pretty good at 650 in gear. I have my initial timing set to 10 degrees btdc. The mechanical advance adds 23 degrees (all in by about 2500 ish using the heaviest springs) and the vacuum advance is set to add about 8 degrees. It is attached to the ported vacuum on the carb.

The combo is a 69 firebird with a 1968 400, with #15 heads (74 cc measured chambers), voodoo 703 cam. My cranking pressure is between 175 and 180 PSI in all cylinders. TH400 and 3.55 rear end with 26.5 inch tires. The carb is a 1973 q-jet 3266 with 74 jets and 44 rods--modified with Cliffs parts and recommendations. I have the accelerator pump connected to the outer hole. It used to be hooked to the inner hole, but made no difference changing the holes.

I'm wondering if my pinging issue is distributor related or am I running lean on the upper end of the primaries? I'm assuming the vacuum advance isn't really adding much vacuum at that level of throttle, although I haven't tried disconnecting it yet. I use 93 octane gas. And my gas mileage is pretty bad around town--about 10 mpg.

[David Jones]

How old are those springs?

[many birds]

About 6 mos. old. Got them in the summit kit. The center cam and weights were junk! Totally not accurate to the chart they provide.

[Kenth]

The factory "all in" mech advance occurred at 4600 rpm´s and this when fuel was "good".
With today fuels "all in" at 2500 rpm´s you just have to expect some ping.

[geeteeohguy]

The factory also used manifold, not ported vacuum to the distributor. You might make the switch, re-adjust your base idle and mixture, and give it a shot.

[many birds]

Originally Posted by geeteeohguy

The factory also used manifold, not ported vacuum to the distributor. You might make the switch, re-adjust your base idle and mixture, and give it a shot.

I’ll try that since it would be easy, but my engine doesn’t like a lot timing at idle—it starts to surge/miss with too much advance at idle.

Also, I’m not sure how this would affect the timing at the time when the pinging is occurring since I’m pretty deep into the throttle at that time. And the manifold and ported vacuum should be the same under heavy throttle. But I’ll give it try this afternoon.

[JLMounce]

If you're seeing idle surge by going to manifold vacuum, it's possible given your advance curve that even at your idle speed of 650 rpm, you may be seeing your mechanical advance starting to come in. On it's own a half a degree, degree or so may not be noticeable, but add the vacuum can to the mix and that change in timing may cause a vacuum change enough to add a bit more timing, which can cause hunting and surging as you describe.

I think the first thing you really need to do is get your mechanical curve sorted. I don't think anybody is in disagreement here that your curve is likely too aggressive. If you have access to a wideband oxygen sensor it may also be helpful to observe what the fuel curve is doing when this is occurring. It's certainly possible the issue is either fuel or timing related, but could as easily be both.

[PDC]

I’ve had good luck with 20* all in by 3k with base timing set at 14*
That way total is 34* but not all there til 3k. I am running D-Port E-Heads with the heart-shaped chamber.

[many birds]

So, I switched to manifold vac. It instantly picked up some rpm. I brought it back with the idle screw and readjusted the mixture screws. Oddly, it wanted them just about 1/2 turn richer. I thought manifold vac was for leaner idle mixtures.

Vacuum at idle has gone up by one inch, 13 inches of vacuum now. I set the idle to 650 in drive and went for a spin. And the idle is 800 in drive. 5 minutes later on the same drive, it dropped 650 again. The engine was fully heat soaked before making these adjustments. I haven’t taken it to the hill yet to check for the pinging, yet.

[many birds]

Originally Posted by JLMounce

I think the first thing you really need to do is get your mechanical curve sorted. I don't think anybody is in disagreement here that your curve is likely too aggressive. If you have access to a wideband oxygen sensor it may also be helpful to observe what the fuel curve is doing when this is occurring. It's certainly possible the issue is either fuel or timing related, but could as easily be both.

Any suggestions on where to get stiffer springs? I was looking at SLC wide band controllers from 14pioint7. Seem reasonably priced. Does anyone have any experience with them?

[lust4speed]

If it pings on the hill, stop and disconnect the vacuum advance and go back up the hill. Could still be enough vacuum coming in to screw things up - especially with the quick advance.

[Kenth]

Originally Posted by geeteeohguy

The factory also used manifold, not ported vacuum to the distributor. You might make the switch, re-adjust your base idle and mixture, and give it a shot.

For the engine in question Pontiac used ported/timed vacuum for the ignition vacuum advance, not constant manifold vacuum, unless with cold or too hot engine.

[Cliff R]

Pinging in the mid-range at moderate to heavy load is NOT going to be how you route vacuum to the vacuum advance.

The engine not wanting a lot of timing at idle is common when the camshaft chosen is making adequate cylinder pressure for efficient combustion as idle speed. My last three engine don't like, want, or respond well to adding any timing at idle speed. They can be set-up to use manifold vacuum to the advance, but when I hook it up the engine speeds up very little and develops a "skip" or miss to the idle note. I back off the speed screw about 1/4 turn or so and it's still a little unhappy. It also drops off a tad more when the trans is placed in gear as the throttle plates aren't open quite as far.

What needs to happen is to retard the timing for no ping and see where it ends up. It's OK if you are down at 28-30 degrees total timing by then. Some higher compression engines with cams having good cylinder filling abilities will not take much more timing than that. If you need 10 degrees initial for best idle and it quits pinging with a little less total then take some timing out of the mechanical advance or down to 10 degrees (20 at the crank) instead. I'm also betting you can run a little more vacuum advance once this is done which will help fuel economy as it is only applied at light engine load.......Cliff

[Kenth]

Cliff, excellent posting, as always.

[Half-Inch Stud]

I need something to do...better have me come over to drive it to tune it all up. PM me.

[455rebel]

Did u degree the cam, a retarded cam will cause this. My 2802 retarded 2 deg pinged with timing set at stock and below. Advanced ca m 2 deg, where it should be, and all is well.

[shaker455]

[QUOTE=
I'm wondering if my pinging issue is distributor related or am I running lean on the upper end of the primaries? QUOTE]

P-rod size may also be a factor,
Todays gas has not got better in past ten years so I would advise dropping P-rod size to about .041-.042
I personally measure rod installed height in relation to jet
Also, disconnect vac adv while testing & use one heavier spring to slow advance rate.

[many birds]

Cliff...agree, with the manifold vac, I’m getting that little bit of a skip at idle. I might try going back to ported and setting initial timing to 8 and see what happens. That’ll give me a little more room for the vacuum advance at light throttle.

455rebel...no I didn’t “degree” the cam. Stupid mistake. I even had the degree wheel sitting there next to the engine for six mos while the car was getting body work done.

Shaker455...how do you measure the installed height of the rod? What’s it in reference to? And what is it supposed to be?

[darbikrash]

Originally Posted by many birds

Any suggestions on where to get stiffer springs? I was looking at SLC wide band controllers from 14pioint7. Seem reasonably priced. Does anyone have any experience with them?

I have used the Zeitronix ZT-2 wideband controller on a couple of cars, and I like it. It is also a datalogger, so you can get O2 along with vacuum (MAP), RPM, throttle position etc as you drive.

http://www.zeitronix.com/Products/zt2/zt2.shtml

About $419 with Bosch O2 sensor, AFR display, and datalogging box that connects to your laptop. You have to buy extra sensors if you want more than just AFR and RPM.

[shaker455]

[QUOTE Shaker455...how do you measure the installed height of the rod? What’s it in reference to? And what is it supposed to be?[/QUOTE]

Measuring rod height is a process I developed to 1st make sure rods are even and same height, 2nd to track how much is engaged into the jet to determine it's square area.