1968 Idle Going Lean

[robert campbell]

Rob

Thanks. You're correct. At idle with vac advance hooked to manifold I get 13* + 15* = 28* which is, BTW, exactly what I would get once I cracked the throttle with the vac advance hooked to ported vacuum. So the only difference is that I already have the advance engaged at idle. And you are spot on in that the engine is much more sensitive to vac fluctuations with manifold when the vac drops below ~14" (above that the vac advance is full on so any fluctuations in the range 14"+ vac has no effect... I checked this using my Mityvac hooked to the vac advance). It is also true that with the large advance at idle the idle screws are only out about 1 turn and the throttle plates are more tightly closed... all making the engine more sensitive to changes to fuel and air vs using ported vac (the upside being better fuel economy and a cooler running engine). To your suggestion about trying ported vac hooked to the advance... to be clear, *I have already tried it* (previous to this current issue) with an increased initial timing of ~15* and, frankly, my engine runs better with manifold vac... that is the main reason I use it. Having said that, I still think that there is some fundamental issue that the engine has always had with a vac leak (not external). With no vac advance at idle and ~12* initial I get 15" of vac with +/-1" flutter... the upside is that that it sounds like I have a more radical cam :wink:. Assuming that the cam is stock (?), I should be getting ~18" vac steady even with 10* idle advance. As the idle advance increases (keeping the RPM the same), the vac goes up and the flutter stops... all consistent with a vac leak. And that is why (I think) my engine seems to "like" manifold vac vac advance... it's covering up/correcting for this internal vac leak.

As usual, thanks for your insight/input. Always appreciated.

James

James,
Cam is a wild card here. Your engine should idle just fine with only 12 to 15 degrees of initial. 28 degrees is way to much at idle.

I have pointed this out before, but as you step on the throttle with "manifold" vacuum to the vac advance, you actually lose advance due to the loss in manifold vac. With ported vacuum when you step on the throttle yes, the vacuum starts to bring in the vac advance, but in a gradual method. Just as the engine demands it and just how the engineers designed the distributor curve to work for basically stock engines.

When you mash the throttle with your setup you actually have an immediate loss in advance of 15 degrees. Amazing that you do not have some sort of a flat spot. With ported vacuum the centrifugal starts to pull in and if you dial in 15 to 16 degrees of initial and use ported vacuum, the engine starts from a better spot.

Hate to say it again, but you need to try 16 degrees of initial, ported vacuum, and readjust your carb to this setting. I am truly convinced it would run a lot better!

What have you got to lose? At 28 degrees of advance at idle I would love to see a dyno pull curve from idle to full throttle. Your engine would fall flat on its face right off the bat and then finally catch up at around 2,500 rpm.

Rob

 

[p51]

James,
Cam is a wild card here. Your engine should idle just fine with only 12 to 15 degrees of initial. 28 degrees is way to much at idle.

I have pointed this out before, but as you step on the throttle with "manifold" vacuum to the vac advance, you actually lose advance due to the loss in manifold vac. With ported vacuum when you step on the throttle yes, the vacuum starts to bring in the vac advance, but in a gradual method. Just as the engine demands it and just how the engineers designed the distributor curve to work for basically stock engines.

When you mash the throttle with your setup you actually have an immediate loss in advance of 15 degrees. Amazing that you do not have some sort of a flat spot. With ported vacuum the centrifugal starts to pull in and if you dial in 15 to 16 degrees of initial and use ported vacuum, the engine starts from a better spot.

Hate to say it again, but you need to try 16 degrees of initial, ported vacuum, and readjust your carb to this setting. I am truly convinced it would run a lot better!

What have you got to lose? At 28 degrees of advance at idle I would love to see a dyno pull curve from idle to full throttle. Your engine would fall flat on its face right off the bat and then finally catch up at around 2,500 rpm.

Rob

Rob

I appreciate your passion about this vac advance issue but I am not at all sure you even have looked at the magazine articles/papers I posted about this... one by a GM engineer who was the Viper plant manager when he retired. His explanation about how vac advance works is clear and concise. It is also clear on at least one of the reasons why ported vacuum was introduced. And there is another guy who graphed the vac advance of ported vs manifold vs throttle opening... the advance curves pretty much followed each other with the only exception being at idle. So, again, once you crack the throttle and the ported vac port is exposed to the manifold vacuum, the canister advances the timing exactly the same as it would with manifold vacuum. Here is the graph...
http://www.gofastforless.com/ignition/vacuum_chart.jpg

Here is the post that refers to the graph
http://www.gofastforless.com/ignition/advance.htm

But regardless of the articles/papers/internet posts, I think you missed the following in one of my replies above...

"To your suggestion about trying ported vac hooked to the advance... to be clear, *I have already tried it* (previous to this current issue) with an increased initial timing of ~15* and, frankly, my engine runs better with manifold vac... that is the main reason I use it."

Until I figure out why I have that low vac w/flutter (this is without the vac advance hooked to any port, just an initial timing of 12') there is no reason for me to retry ported vacuum because I would get the same result I got when I first tried it. If there is anything you and I can agree on its got to be that trying the same thing multiple times and expecting different results is a sign of insanity :wink:

James

 

[franklinair]

All very interesting - But - I'm a simple man, preferring to keep things simple. My vision of performance upgrades for a 289 consists of a 4 BBL intake & carb, no more than 500CFM. (A basic 289 can't suck more air than that, so any more is a waste of $$.)
A 302 already has a 4BBL set up, so the only improvement there would be a Pertronix II ignitor & coil.
Accurate tuning will give the small block all it needs for proper operation.
Our GT/CS's are a unique Mustang, and I choose "not to confuse their identity" as a friend of mine used to say.
Although I DO make some modifications (Mini Tach, AM/FM radio, gold CS striping), only an MCA judge would know it. Even a Mustang enthusiast, let alone a casual observer doesn't know enough about the GT/CS to recognize minor alterations.
Well, that's the old man's opinion. Anyone wishing to build a 400 HP 289, be my guest. Just not my cup of tea.

Neil:smile:

 

[robert campbell]

Rob

I appreciate your passion about this vac advance issue but I am not at all sure you even have looked at the magazine articles/papers I posted about this... one by a GM engineer who was the Viper plant manager when he retired. His explanation about how vac advance works is clear and concise. It is also clear on at least one of the reasons why ported vacuum was introduced. And there is another guy who graphed the vac advance of ported vs manifold vs throttle opening... the advance curves pretty much followed each other with the only exception being at idle. So, again, once you crack the throttle and the ported vac port is exposed to the manifold vacuum, the canister advances the timing exactly the same as it would with manifold vacuum. Here is the graph...
http://www.gofastforless.com/ignition/vacuum_chart.jpg

Here is the post that refers to the graph
http://www.gofastforless.com/ignition/advance.htm

But regardless of the articles/papers/internet posts, I think you missed the following in one of my replies above...

"To your suggestion about trying ported vac hooked to the advance... to be clear, *I have already tried it* (previous to this current issue) with an increased initial timing of ~15* and, frankly, my engine runs better with manifold vac... that is the main reason I use it."

Until I figure out why I have that low vac w/flutter (this is without the vac advance hooked to any port, just an initial timing of 12') there is no reason for me to retry ported vacuum because I would get the same result I got when I first tried it. If there is anything you and I can agree on its got to be that trying the same thing multiple times and expecting different results is a sign of insanity :wink:

James

James,
Read them all. Vacuum flutter at 15 inches is common with more overlap due to a "hotter" cam. Good look on your quest. My thoughts is that if your car was in my garage I could tune it to run far better than it does currently with far less than 28 inches of advance at idle. Even a full on race motor does not have that much advance at idle. Personally I feel that your throttle plates are far to closed to support the extreme amount of advance at idle. With far less advance at idle would allow proper alignment of the throttle plates to the transfer slots and a much better idle signal. This would improve your AFR ratio at hot idle.

Sorry to say this but it is my feeling that you have not went the full mile to adjust your combination to work with ported vacuum.

Good luck, but you seem to think that the engineers that designed your engine, and all engines in the 60's, did not know what they were doing. A couple articles by others may or may not be true. In a high performance world, they may apply. And trust me the high performance world is my wheelhouse.

I have a suggestion and that would be a dizzy with no vacuum advance!

Rob

 

[p51]

...I have a suggestion and that would be a dizzy with no vacuum advance!

Rob

Hmmm... no vacuum advance at all?... Interesting. Now that is what I call a compromise :wink:

And just to be clear, I think my cam is a stock cam. The rest of the engine is stock (eg heads, intake, etc) and when dynoed it pretty much matches the power put out by a stock 302... ~200HP at the wheels.

Anyway, it looks like I figured out my "leaning out" issue and am back to how the car was running ~4 weeks back. Again, thanks for the advice/help with the intake manifold. fuel filter (I had always wondered if air in the fuel filter was a problem), etc

James

 

[robert campbell]

James,
Glad you found a solution!

Rob