[czar97]

Just looking for any insight while I'm at work before I start adjusting idle mixture screws tonight.

Problem: Stalls when I put it in gear.

Last night:
Set base timing at 8 degrees.
Watched centrifical advance for another 16.5 degrees.
And got another 19 degrees out of the vacuum advance.
=43.5 degrees of total timing
(note: one guy told me to shoot for 34 degrees of total timing)

1972 Pontiac 350 (8:1 compression)
1971 Quadrajet with Cliff's rebuild kit
SUM 2800 camshaft 208 214 .421 .444 112 degrees lobe separation
MSD Pro-billet PN 8528

Currently the idle mixture screws are out 8 quarter turns (2 full turns). I see the google machine tells me to start at 1.5 turns out. Maybe a half turn in on each is a good place to start tonight.

Thoughts?

[Cliff R]

Start out about 3 to 3 1/2 turns with the mixtures screws and work from there.

16.5 degrees mechanical advance is s little short for that engine, I'd rather see 20-22 degrees starting around 900 rpms and all in by 2800-3200. Another 15 degrees from the vacuum unit and good to go.....Cliff

[steve25]

How much vacuum do you have at idle?

Did you degree in the Cam?

[czar97]

Didn’t degree the cam.

17 psi vacuum at idle.

Put a vacuum gauge on the back of the carb and turned IN the passenger screw until it started to stumble and sound awful (and vacuum went down). Then backed screw out until vacuum went up to 17 and wouldn’t go any higher. Just kept turning it out multiple turns and never really changed at all. Went back IN to rough idle then back out to the first part of smooth idle and 17psi. Then did the same on the driver side.

Put it in gear and no more stalling.

Now time to start a new thread regarding voltage at he coil (9.43 and if that’s too high).

[Schurkey]

Originally Posted by czar97

Problem: Stalls when I put it in gear.

Last night:
Set base timing at 8 degrees.
Watched centrifical advance for another 16.5 degrees.
And got another 19 degrees out of the vacuum advance.
=43.5 degrees of total timing
(note: one guy told me to shoot for 34 degrees of total timing)

Vacuum advance is NOT included in "total timing".

You have 8 + 16.5 = 24.5 degrees of total timing. NOT ENOUGH. Are you sure the advance mechanism was DONE advancing? Some need 4000+ RPM before they're "all in".

19 degrees from the vacuum canister is "probably" too much and may need to be restricted to 10--15 degrees.

Originally Posted by czar97

Currently the idle mixture screws are out 8 quarter turns (2 full turns). I see the google machine tells me to start at 1.5 turns out. Maybe a half turn in on each is a good place to start tonight.

Turn the idle mixture screws to where the engine runs best at curb idle, in gear. This is easiest to see with a vacuum gauge--set the mixture screws for highest vacuum.

Then turn both the screws IN (lean) enough to reduce vacuum by about half-an-inch. (1/4 inch each) Turn the screws an equal amount in--maybe 1/4 to 1/2 turn. This reduces tailpipe emissions considerably but has little effect on idle quality.

[Half-Inch Stud]

measure Total ADV without VACuum Advance hose, and shoot for 34* at 2800-3000.

Initial ADV sounds low if you have a cam "068 or bigger".

Plenty of Vacuum signal!

[dataway]

While there are some smart people listening.

Below is the charts for the various spring/weight/dist-cam combinations I've tried. (currently #5 dist-shaft, 524 plate (bushed), 37 weights, one kit HD spring, one OEM spring)

Currently I'm using the lime green line. Initial set at 9, mech kicks in at about 1000 and maxes out about 3200 at 28-29 total at 3100-3200 rpm.

Stock engine with 068 cam and Q-jet.

What I am wondering about is why my idle mixture screws are only 1/2-3/4 turns out. I can close them both down and kill the engine, but anything past about 1.5 turn out and it starts to load up. Carb is in perfect condition, tuned by Cliff (basically all stock) and setup on his test engine. (704 carb).

Manifold vacuum at idle is 25 inches ... engine seems to produce a ton of vacuum.

What I surmise might be happening is that my initial timing is too low ... which reduces idle rpm, which means the idle speed screw may be holding the primaries open a tad to maintain idle rpm, so I am not running on just the idle circuit ... hence the idle mixture screws being in so far and not having a very pronounced effect.

Engine is not real happy without the vacuum advance plugged in, and idles roughly on just initial and mech. With vacuum advance plugged in it behaves beautifully as far as starting, idling etc. Of course with that kind of vacuum it's pulling max vacuum advance at idle. (have yet to tune the V-adv for a max of about 15).

Should I add more initial?

[tooski]

[QUOTE=dataway;6043565]While there are some smart people listening.



Should I add more initial?

I'm ot one of the smart ones, but I pay attention. I'm not sure what your compression ratio is. The info that I've garnered from this site say a Pontiac engine with stockish heads should like 32 - 36* total mech timing. I believe you still have the engine on the test stand? It would be fairly easy just to do some testing at different initial timing.

[Schurkey]

Originally Posted by dataway

Initial set at 9, mech kicks in at about 1000 and maxes out about 3200 at 28-29 total at 3100-3200 rpm.

If there's no detonation, add some more initial.

Originally Posted by dataway

What I am wondering about is why my idle mixture screws are only 1/2-3/4 turns out. I can close them both down and kill the engine, but anything past about 1.5 turn out and it starts to load up. Carb is in perfect condition, tuned by Cliff (basically all stock) and setup on his test engine. (704 carb).

No nozzle-drip? No signs of too-high float level? Doesn't matter how many turns out the screws are--as long as they're responsive, and you can turn them "too lean" as well as "too rich".

It's just not a concern if you can adjust them for best idle. The engine cares about fuel mixture, not screw position.

[dataway]

Schurkey ... no nozzle drip, looks dry as a bone in there during idle ... I can see just a tad of wetness on the throttle plate screws, like you wiped them with a damp cloth, but nothing approaching a drip from the nozzle or a puddle on the plates.

Float level appears spot on (was set by Cliff)

Yep, that's kind of what I hoped for ... they can be turned so lean it won't idle, and they can be turned too rich.

I've tried it at 12 degrees initial ... and it idles WAY better, just that three degree seems to make a big difference. It's at about 10:1, .040 over YS400, running on 91 non-ethanol. I was keeping the initial at 9 because the Vac advance is kicking in about 20 and I didn't want the total with vac/adv to be too much ..... I know ... I should limit the vac/adv can, or put the adjustable one on.

[lust4speed]

Also if it is dying when you drop the car in gear, you might have the "cliff syndrome" (no relationship to Cliff R.). Basically idle is happy in park with vacuum advance added and centrifugal advance probably has already started. When you put a load on the engine by dropping it in gear, the vacuum drops a little, RPM drops a little; and then things snowball with vacuum and mechanical advance both dropping lower until the now unhappy engine drops off the edge of the cliff and dies.

While you are getting things sorted out, disconnect the vacuum advance and check timing at normal idle speed. Then drop idle down as far as you can and check timing again. If you have lost several degrees the curve is starting too soon. You need to have the curve come in later and above idle speed so a load doesn't drop timing with the normal drop in RPM.

I have had trouble with my AC cars and erratic idle and took the easy way out by setting initial at 14, 36 total, and then used ported vacuum so it wouldn't influence the idle. I'm with the ported vacuum guys and I seem to remember the field was pretty split down the middle between manifold and ported vacuum users.

[many birds]

Curious to see how this is resolved. I’m struggling with the same issue, but I am running ported vac advance. Car intermittently just dies in drive— unless my idle in drive is set above 800.

[Formulajones]

Originally Posted by dataway

I've tried it at 12 degrees initial ... and it idles WAY better, just that three degree seems to make a big difference. It's at about 10:1, .040 over YS400, running on 91 non-ethanol. I was keeping the initial at 9 because the Vac advance is kicking in about 20 and I didn't want the total with vac/adv to be too much ..... I know ... I should limit the vac/adv can, or put the adjustable one on.

That's about spot on the direction you need to go. I run basically the same type engine in my formy with same cam, on the same 91 octane.

I pretty much don't like single digit initial timing on any engine here, stock or modified. Simply because I don't like to rely so much on the vacuum advance putting that much in there, which is how you're currently setup.

I run about 12-14 initial on mine, but you'll have to go in and limit the breaker plate for advance to stop about 20 degrees. So your total doesn't exceed the amount you're looking for. I limit mine to 34 degrees, that's where it runs it's best times. It's all in at 2600 rpm.

On the vacuum advance I have it limited to an additional 12 degrees, that's it. Which gives me about 46 degrees at very light cruise and works fine on this engine, and returns 17 mpg highway.
I modify the adjustable units, and have the spring tension set to it's lightest setting, which starts around 6-7 inches of vacuum and is all done at 10-12 inches of vacuum. Very similar to the performance cans offered back in the day. Then limit movement towards the can (pull side) to about .200" which will give you roughly 10-12 degrees at the crank. That's about all you need.

I prefer to have all this done before I even attempt to tinker with the carb. With timing setup like this, it should produce good vacuum and a strong carb signal with better idle quality, then you can back off the idle screw which should put the blades in a proper position to run on the idle circuit without triggering the main circuit during idle conditions. Then I proceed with carb adjustments.

[dataway]

I got rid of the erratic idle that I had initially by using stiffer springs on the mech adv. (as per advice on this forum) which got the mech advance starting point up around 1000-1100 rpm. Keeps the weights from flopping around too much at idle and the timing got much more stable.

I'm using manifold vacuum by the way.

As you can see from the chart (Lime Green Line) the springs and OEM weights I am using now are producing a fairly linear mech advance from 1000 to 3100 rpm. Chart is only initial plus mech advance, no vacuum.

Formula, yep, that's what I'm looking for, I think I've got the mech advance bushed to the right place now, but need to take about 5 degrees out of the vacuum and replace it with a few degrees of initial. Next week I'll move it to 12 degree initial and run some more tests. Your engine sounds just like the way this one was 35 years ago, ran and idled great, not pinging and I also got exactly 17 mph on the highway.

[Formulajones]

Yep that's similar to what I attempt to do here.

I'd leave the springs and weights alone because the curve is nice and doesn't start too early.

Only thing I'd change is initial and total numbers. If I'm reading your post right you have 9 initial and 29 total. Now if you're sure it's all done and all in at that point, then you don't even need to modify the breaker plate because it sounds like it's already limited to 20 degrees.

I'd just move the distributor to about 12 or 14 initial. That will give you 32-34 total which is where that engine will make best power on pump gas, then I'd leave it alone.

The only thing you need to do at that point is modify a vacuum advance unit like I described above and you're ready to rock. I also run mine on manifold vacuum as the original carb doesn't have a ported source. I get rock solid idle quality setup as I described, with only a 200 rpm change in and out of gear.

https://youtu.be/uIFkggwDY-8