Weber Guru Question: Jetting and AFR

[jkh2cpu]

Hi. I put PMO webers on my trusty original-owner 1973 911T (MFI) back in 1987. The webers were an improvement over the MFI, but they always ran a bit rich. Here is the jetting from 1987:

Chokes: 30
Air Correction: 170
Mains: 130
Idles: 55
Emulsion Tube: F26.

Reading around, I see that the Air Correction jets are now recommended to be 180. I'm doubting that will affect the AFR range on the idle circuit.

Driving down the road at 70 MPH, I'm sure that I am still on the idle circuit, and I see that the LM-1 pegs the AFR at around 12. If stand on the throttle in 5th gear, the AFR goes up to around 15. I'd test the car at 100 MPH, but I think that the local Sheriff or Michigan State Police would nab me, and they would not be look kindly upon my experiments to improve my fuel economy.

Today, I got out the STE, balanced things up, opened the air bypass jets about two full turns, and moved the idle AFR from 9 to 11 or 11.5, which is an improvement, but the AFR is still around 11 at 40 in third and fourth gears (I'll try it later at 60 on the back roads).

Something ain't right. If I put 50 idle jets into the carbs, the car has trouble running, and the AFR moves up to 15 or 16 just putting around the neighborhood at 25 or 30, and it bucks going down the road, with really bad transition.

So...

1) Will changing the air correction jets from 170 to 180 make any difference?

2) I'm thinking that reaming my 50 idle jets to 52, test with LM-1, and then to 53, test with LM-1 may solve my problem.

Or...

3) Is there something that I have over looked? (It's not plugs, and I just changed those, and the fuel pump is the Bosch recirculator for the Zeniths, fuel pressure = 4.5)

TIA.

John.

[Halm]

Call Richard Parr at PMO. Tell him exactly what you have, that you think you are running rich and what would he recommend. Buy the parts from him and it will be a win-win phone call.

[911pcars]

You could try 125 mains (leaner). Before you do, consider that the idle mixture screw settings affect part-throttle cruise A/F via the idle jets (could also go leaner). Notice how far your foot presses the go pedal at cruising speed. Not much; the throttles are barely open. Setting the idle mixture with a wide band O2 sensor or even an inexpensive CO meter (Gunson) is recommended. One can make dramatic changes in idle mixture with only slight rotations of each mixture screw. Pete Zimmerman has described a pretty good basic setup routine. Check the archives.

Main and air correction jet sizes affect upper rpm mixtures more than idle and cruise operation.

Use an air bypass jet only to bring the throat producing the lowest air flow up to the threshold of the other throats (air bypass screws closed). Thereafter, leave them alone. Use the idle speed screw on each carb and the 6 mixture screws to set idle speed and idle mixture, remembering that idle mixture settings do affect the cruise mixture.

Sherwood

[2.7RACER]

Two things stick out from your description, well maybe more. The Webers like 3.5 psi, not 4.5.

Since you opened the idle air correction jets with only a small change in the AFR at idle tells me your engne is idling with the idle stop screws turned in a full turn or more against the stop.

This opens the butterflies enough to bring the lowest transition port into action at idle.

It also takes some control of idle away from the idle fuel mixture screw and the idle air correction screws.

Your engine can and should idle with the butterflies closed. Yes I said closed.

Even closed, the butterflies pass some air. But closed butterflies take the lowest transition ports out of the idle adjustment. This is good. You want the idle set with only the idle port in action.

If you haven't had the carbs off the engine recently, I would pull the carbs and check the butterflies.

With the idle speed screw fully backed off the stop, all three butterflies should be equally closed.

It's difficult to balance the carbs at idle if one or two buterflies remain open.

I would also check the float level with the sight glass tool and engine idling.

The air correction jets affect idle, transition and cruise over 3,000. 180's may help.

Check the O-rings at the idle fuel mixture screw and idle jet holding screw.

Any doubt I would get new O-rings from your local auto parts.

I'm sure there can be more but this can get you started.

[jkh2cpu]

Thanks, 2.7Racer. I think I'll start with getting the car to idle with the butterflies completely closed. I did exchange the 1.30s with 1.25s, and the AFR is at a safe measurement going down the road and at full throttle.

John.

[RoninLB]

I don't take 2nd or 3rd gear readings seriously. 4th is a maybe for some things. These carbs can't be perfect for all conditions.

5th gear can be tough for checking out mains and E-tubes as they may only overwhelm the idle jets at rpms over 3.8k.


I figure you can't go to far off by setting your idle screws all equally.

Start lean and work your way rich a little bit at a time. 1/8 turn should be a slight effect.

[911pcars]

"Your engine can and should idle with the butterflies closed. Yes I said closed. "

This is an interesting method of setting idle speed; that is, using the air bypass screws instead of the idle speed screws. I will admit this can provide the most mixture control for each cylinder. However, how would you adjust the idle speed thereafter? With your method, you would have to individually adjust each idle air bypass screw instead of using the single idle speed screw at each carb. That makes slight idle adjustments problematic.

Sherwood

[2.7RACER]

Hello Sherwood,
The idea here is to set the idle without opening the butterflies so much that the lowest transition port is passing fuel while adjusting the idle.
A very slight opening is OK but not necessary.
With the butterflies closed, the idle fuel mixture and idle air have more control.
I have noticed the need to examine the butterflies before the carbs are installed. All three butterflies need to be closed the same amount.
Not one closed and two slightly open or two closed and the remaining one slightly open.
Once the 3 throats are adjusted and balanced on both carbs, I balance left and right with the linkage at 3,000. I then rebalance the left to right idle with the idle air correction screws.
My 2.7 will idle at 700 quite smoothly. I then set the idle speed stop screw for the desired idle rpm's. In my case between 900/950 rpms.
All throats are balanced at idle as well as 3,000 rpm.
Balance at 3,000 the most important in my view.
If the butterflies are not carefully set with each other, it is necessary to open or close one or more of the idle air correction screws more than necessary.