You know more than me, obviously. :P

Girlfriend has my car today, but so far in drives just fine.

 

Drove today. Vacuum certainly hasn't gotten any worse during cruise. I'll have to drive some more to confirm.

 

And what did you change, again? :-)

 

Went from 74 to 73 jets, and a longer but softer power piston spring. The green one from Cliff.

 

Interesting! In the mean time I have learned that some of these late 70's Olds units require bigger jets than others. For some reason.
Still no control over the APT ?

 

Which older Olds units?

Still no atp control, no.

 

I am not sure if the exact numbers, but the olds units from the late 70's. Your 57253 and a 59253 I have in my "shop" waiting for recalibration.
It's funny how your apt seems to refuse to co-operate. If/when you consult Cliff with this problem, please let me know the answer:-) just curious.

 

I'm banned from Cliffs website again (he keeps doing this every six months, I assume to avoid spam), so I'm trying here.

I found this quote on Wikipedia: "A pressure gauge measuring the manifold pressure can be fitted to give the driver an indication of how hard the engine is working and can be used to achieve maximum momentary fuel efficiency by adjusting driving habits: minimizing manifold vacuum increases momentary efficiency."

Does that mean that less vacuum means more efficiency, or does that simply mean that a vacuum gauge allows me to find out whether or not WOT really is zero manifold pressure? I'm confused.

If I enlarge my MABs to 0.070, do I need to make sure the ones in the air horn aren't any larger than 0.070?

Idle bypass: I'm getting choppiness at no/very light throttle cruise at very low speeds/RPM. I've read that this means the mixture's lean at idle - do you think I make too much vacuum with this cam to be using idle bypass air?

 

• A vacuum gauge can confirm whether you have any intake restrictions, such as a too-small carburetor, by indicating greater-than-zero vacuum at WOT at a given engine speed.

• Since vacuum level is a good indicator of throttle position and engine load, a vacuum gauge can help you improve your fuel economy by showing you when you are using more fuel, possibly without even knowing that you are.

High vacuum indicates light throttle / low load.
Low vacuum indicates heavy throttle / high load.

Vacuum does not directly indicate efficiency (which is the amount of power you get from a given amount of fuel), but many carburetors dump a fair amount of fuel when opening the secondaries, which is inefficient, so a vacuum gauge may indirectly indicate efficiency.

- Eric

 

Thank you!

As I understand it, the power piston on the primary side of the Qjet meters the amount of fuel given to the engine with engine vacuum, more vacuum means less fuel.

As you say, low vacuum means high load - but when is high load at low RPM more efficient than low load at high RPM? From what I can see, there's nowhere from 50-80 MPH where my 5th gear (1:0.68) gives me more vacuum than my 4th gear (1:1).

The lowest vacuum in 5th gear at those speeds is 11-12 inches, highest is 13. The lowest vacuum in 4th gear is 12 inches and the highest is 18-19 inches (this being at 50 MPH).

But we agree that the air pump in my engine compartment is only sucking .68 as much air in 5th as in 4th at the SAME vacuum setting, right? Going 60 in 4th is 1940 RPM, while it's only 1319 in 5th. If cruise vacuum is 18 in 4th, is 18*.68=12.24 inches in 5th going to move the same amount of air? And does that mean more or less fuel? :P

I have a 2.56 rear. Is my fifth gear "wasted"? :P

 

Right.

The speed of the air flowing through the venturis determines the amount of vacuum pulling the fuel from the float bowl, through the jets, and finally through the nozzles.

As the amount of air pulled in increases, its speed increases, and the vacuum pulling the fuel increases.
The sizes of the jets limits the amount of fuel that can pass through them in response to the pull from the venturis.
(The vacuum force created by the venturis is completely different from the manifold vacuum. The force pulling fuel through the nozzles increases constantly as the air flow increases, while the manifold vacuum decreases as the throttle plates are opened).

The variable primary jets in the QuadraJet allow more fuel to flow as the needles are lifted out of them, so manifold vacuum pulls the needles down into the jets against spring pressure, then as manifold vacuum decreases, the springs raise the needles, increasing the sizes of the jets, which allows more fuel to flow through them in response to a given amount of pull from the venturis.

Note that there are two factors influencing the amount of fuel dispensed:
• The amount of airflow, which will pull more fuel as airflow increases, and
• The amount of manifold vacuum, which will allow more fuel out as the throttle is opened wider, especially when the engine speed is still catching up to the throttle position (acceleration).
Therefore, at a given engine RPM (air flow rate), the mixture in enriched when vacuum is low, compared when the vacuum is high.



Good question.

I remember wondering that myself when I was a kid driving around in my Chevelle.

I never did figure out the answer, but I suspect that careful fuel economy records and trial and error driving on the same route will tell you.

In general, it is my impression that if the engine starts to lug, and the vacuum drops, economy will drop, as compared to a high-vacuum setting at a higher RPM, but if the engine can be driven at that road speed at a lower RPM without lugging it, mileage will be better.


I am not even going to try to go there.


Not if you drive as fast as I do.

- Eric

 

I like insight. Thank you.

Well, it doesn't lug (only in 1st gear at no/low throttle). I could do a road test to see if 4th is more effective, then.

Highway speed is 80 MPH/130 KPH, outside cities is 50 MPH, and cities are 30 MPH.

In any case, I need to get my ATP back in line. We'll see where that leaves everything else.

EDIT: Oh, and that my speedometer shows 14% too slow.

 

Aha! Testing the ATP, it shows that the power piston doesn't follow the ATP all the way down at all, leaving 1½ turns from the PP bottoms out until the ATP screw bottoms out.

EDIT: Also put in larger MABs (0.070") and put a screw in the idle bypass air to see if it improves choppiness at low/no throttle.

 

Did you ever get your pp properly dialed in?

 

I did not - after putting in the 0.070" MABs I had erratic idle and vacuum surges, so when I got home I pulled the carb apart and found the arms of the PP to be bent, one higher than the other. I switched to a different PP and haven't driven the car much since, because I had to fix a gas tank leak as well.

I'm driving far Tuesday, I'll let you know at that point.

 

Uneven hanger arms will cause all kinds of problems, for sure:-)
Btw you didn't need to replace the power piston. You could have just bent the arm to adjust them.

 

I know, but it was easier to throw in the spare than to try to make the old ones straight.