Weber/PMO idle jets, idle airs, idle mixture screws.

[jameshtaylor]

Can someone explain what the relative contribution of these three adjustable points is in the idle circuit? I know the idle airs make more subtle changes than the idle jets but that's about it. And does the mixture control offer a still more subtle adjustment? Do all do whatever they do over the entire range served by the idle circuit? Sounds like the mixture screw, being intended to deal with idle, probably has no function past some low RPM level, perhaps controlled by the transition ports? A lucid explanation of all this would be very helpful. I'm especially interested in what should happen when changes are made in each.

[chris_seven]

This seems to be a very confusing area of carb operation and this is partly due to the different names used to describe the various parts of the idle and progression circuits.

These issues are common to all Fixed Venturi carbs such as the Solex 40PI, Weber 40IDA3C and Zenith 40TIN.

If we start with the idle jet it is important to understand that this jet does not govern the idle mixture.

The fuel used at idle is drawn through this jet and metered by the idle mixture screw.

The idle jet controls the off-idle progression between closed-throttle and acceleration as well as part-throttle driving.

If this circuit is too weak then the engine will stutter when the throttle is open and if it is too rich the engine will tend to hunt and surge particularly when hot.

I the ensuring progression circuit is weak the car will tend to nose-dive with large throttle opening.

The progression circuit can be changed by adjusting the size of the idle air bleed as this changes the quantity of air that emulsifies with the fuel drawn though the idle jet.

If changes to the air bleed are not successful then the idle jet size may need changing.

Adjusting the idle mixture screw has no effect on the air/fuel ratio but only affects the volume of the mixture provided.

The idle mixture is controlled by a combination of the idle jet, the idle air bleed and any by-pass circuit that may be present in the carburettor.

The values of these components is normally determined for a specific set of engine characteristics. When cams, compression ratios are changed and engine behaviour becomes erratic or develops flat spots we often try to sole these problems by adjusting the idle mixture screw which is often unsuccessful and more detailed changes are required to the progression circuit.

The idle circuit itself has two outputs, the basic idle circuit and the progression drillings. At the lowest idle position the progression drillings are basically open to the high pressure side of the carb and do not se manifold vacuum and these drillings operate as an additional air bleed as all of the fuel mixture passes through the main idle port drilling.

As the throttle plates open the holes progressively move from the high pressure side of the carb to the vacuum side and they change from air bleeds to providers of fuel mixture hopefully matching the requirements of the air flowing around the throttle plate.

If the idle air bleeds are too small then the manifold vacuum can drop too low and cause a stutter cause a stutter but a smaller idle air bleed can also prolong the idle circuit

I would think that idle jets and associated air bleeds are likely to affect progression behaviour up to at least 3000 rpm and in some cases higher depending on the nature of the cams and the WOT throttle behaviour which will be strongly influenced by the Venturi.


If the idle transition and cruise are good but the idle is poor then adjustment of the idle mixture screws will most likely fix the problem.

If the idle is good but the transition and cruise are poor you would probably need to adjust the idle jet and the air bleed.

The Weber IDA3C family of carbs and the Zenith 40TIN don't have adjustable air bleeds, just a fixed diameter bush.

I believe that the PMO can be adjusted.

The Solex PI has the idle air bleed jet position either blanked or fitted with an undrilled jet. We are trying to find a way of making this part of the circuit work.

For Webers and Zeniths we modify the bodies with a small Timesert.



... and fit a small calibrated jet

[1QuickS]

If you want more reading to get a similar discussion in different words then try my web site discussion:http://www.performanceoriented.com/c...r-operation-1/

Also, if you are interested in another approach to Tuneable Idle Air Correction jets then I also have a remedy for Weber & Zenith (scroll down the page to get to the discussion): http://www.performanceoriented.com/p...modifications/ I have a DIY kit for the Weber Zeniths with eight sets of correction jets and the instructions with tools for modification for $135.

Solex carbs are easily modified in a similar way for tuning the idle air correction jets but to date I have not provided this service except for myself. I tap & insert threaded jets into the side of the jet carrier. Using the provisions for air corrections jets on the top of the Solex jet carrier exposes them to debris that is less susceptible to blocking them when the bleed is on the side of the jet carrier.

[chris_seven]

I have always wondered why the Solex 40Pl isn't fitted with an idle air bleed and it would be interesting to know where the air for the idle mixture enters the circuit.

Volume 1 of the Workshop Manual clearly shows the carb fitted with an idle air bleed but this part is always deleted.

Early carbs were fitted with a brass plug which screwed into the jet carrier while the later carbs had no screw thread and were plugged with an aluminium insert.

The only place for air to enter at idle is through the 3 ports above the throttle plate when the engine is idling but as they are uncovered this air is no longer available and I think that this must influence the early stages of progression.

I guess as many of the Solexes we rebuild are used on Race Engines fitted with very high lift cams this may be a moot point but it is interesting.

[jameshtaylor]

Thanks to both Paul and Chris for this very helpful discussion. Do I have this right: When the engine is idling with the throttle fully closed, air-emulsified fuel is only flowing thru a metering hole directly into the venturi? In other words all transition ports are inactive due to throttle position?
And Paul, this sentence confused me: "The idle and progression circuit is comprised of an idle jet and its holder, an idle air bleed jet... and the progression holes drilled into the throttle bore, all of which are supplied with fuel delivered through the main jet and via fuel delivery galleries." Does fuel pass thru the main jet before making its way to the idle jet and progression circuits?
Finally, I'm still not clear as to whether the mixture control screw affects mixture from idle all the way up to 3500-4000rpm when the main circuit takes over?

[1QuickS]

Chris: Idle air correction for the Solexes is a 1.10mm dia hole drilled through the side of the jet carrier assembly. This is where I locate the brass screw with metered hole for tuning transition. Handy to have a little transition tuning to adjust roll-on power at corner exit.

James: All fuel passes through main jet first. Fuel leaves main jet & enters the emulsion tube well at the bottom of the well. Near the bottom of the well is a passageway that directs fuel up a fuel gallery that eventually feeds the idle jet. This gallery is what I fondly refer to as the "Hidden Gallery" since it is routinely clogged in unused Webers and can become partially clogged from age...kinda like "hardening of the arteries" with plaque build-up on the walls. Actually, not ALL fuel passes through the main jet, fuel used for the accelerator circuit has its own, unique passageway.

Since the mixture screw is an active fuel delivery orifice like the progression hole drillings in the side of the throttle body it stands to reason that it supplies fuel for the duration of progression which lasts through 3000 RPM. As progression system loses effectiveness the main circuit gains effectiveness. This region of operation is referred to as "transition" since you transition from idle/progression circuit to main circuit operation. The summation of fuel delivered by the idle/progression plus that from the main circuit is ideally correct for engine demands. This is where the idle air correction jet is particularly useful. If you are trying to satisfy a lean or rich tuning issue at transition you can manipulate idle jet size, main jet size, main air correction jet size & emulsion tube selection. The ability to adjust idle air correction adds another variable to twiddle with. While there are many variables to adjust, some are more effective in the transition region and it is well to leave the main circuit operating to satisfy mid-range and WOT operation vs. transition tuning issues. The idle air correction jet allows you to adjust the fuel delivery curve where changing just the idle jet changes the richness across the entire idle/progression operational band.

[jameshtaylor]

This has been an interesting experience. Helped by the information above, I returned to my PMO tuning issue.
After installing an AFR gauge a while back I saw that my PMO’s ran quite rich on the idle circuit… a level light cruise at 3k rpm was almost always in the high 11’s. After fooling with settings and balance etc for a while I eventually changed the idle air jets from 130s to 140s This helped, getting me into the 12’s, but not into the slightly leaner 13’s where the experts say I should be.
My idle jets are 55s, about as small as Richard Parr at PMO ever recommends for my setup, so I decided not to fool with them.
I was about to exchange the 140 idle airs for leaner 150’s when I came upon Parr’s statement to the effect that loosening the idle air adjustment screw a "turn or two" would produce leaner operation without adversely affecting transition behavior. I was skeptical because I have always heard that the only thing these screws do is adjust the airflow to allow balancing the carbs among themselves. However I made the change and, lo and behold, 3k light cruise AFR’s moved into the low 13’s… exactly as he said and as I wanted them. So while the adjustment didn’t do anything to the mixture in the idle circuit, it did apparently affect the AFR in the combustion chamber.
Because I had tampered with the airflow into the carbs, I rebalanced them with a syncrometer. In the past I’ve always tried to get the syncrometer to read 4.5 at about 1200rpm. I realized doing this that Parr’s advice was in effect to seek a higher reading at the same rpm… so I set them to read 5, not 4.5 at 1200 rpm. Car runs beautifully. 3k cruise is in the low 13’s.
I have never heard anyone emphasize this in talking about adjusting carbs. Most of the advice I’ve seen suggests balancing the carbs to the highest reading among them… with little reference to what this level is. Nor do I remember anyone saying you must be at X rpm or you won’t get it right. But my own experiment suggests both the reading and the rpm level at which you set it are important. Comments?

[1QuickS]

Well, if you really want throttle plates to be adjusted to their optimum position then the process is to remove carbs & mechanically adjust throttle plate location to just block the first progression hole. The intent is to have the first hole exposed upon throttle activation from idle. This leaves the only way of adjusting air flow at idle as by the idle air screws. New carbs with closely fitted throttle plates can take advantage of this but worn carbs typically require throttle valves to be more closed to achieve a low RPM idle. I have mentioned this adjusting procedure in some of my previous tech writing but it is somewhat more technically correct than practical; sometimes it is much more pragmatic to Keep it Simple.

Since progression mixture is adjusted by the throttle valves sweeping past the progression holes it stands to reason that throttle plates exposing the first progression hole at idle will require less fuel from the idle mixture screw for idling mixture strength since some fuel is contributed from the exposed first hole. This leads to a lean progression since the fuel from the idle mixture screw is closed from what it would be if the first progression hole was not contributing.

Another situation would be if the throttle valves were somewhat below the first progression hole at idle. In this situation, the fuel delivery would be upset in that as the throttle valves are opened and there would be a lag before fuel is delivered from the first progression hole and a resulting stumble during off-idle throttle application.

Performing "Lean Best" idle mixture adjustment will minimize the potential for the first situation and STE readings in the 4.5 to 5.0 range also help confirm that "Lean Best" has been achieved. Higher readings indicate the throttle valves are too far open to supply air to burn an overly rich idle mixture adjustment. As Lean Best idle mixture is achieved, the engine will speed up due to a more efficient combustion mixture which will necessitate closing the throttle valves which will reduce idle speed and STE readings.

As for using the idle air correction screws to help add a little extra fuel at cruise: the air correction jets can provide this effect and allow the air screws to be used for balancing & setting throttle plate position correctly. By opening the screws you are decreasing the suction at the progression holes, this suction is a function of inrush air passing between the wall of the throttle bore and the edge of the throttle valve. If air is bypassed through the air correction gallery then this air is not contributing to drawing fuel from the progression holes plus this bypassed air dilutes mixture strength.

If this approach is adopted then idle mixture strength will need re-adjusting to achieve Lean Best with a resulting increase in engine idle speed & STE reading. You can mechanically adjust idle speed back down with the idle speed stop screws but this will adjust the alignment of the throttle valves with the first progression port.

In my conversations with Richard, I have found he tends to recommend the air screw adjustment procedure over jet replacement since it is easily performed and does not require a more tedious idle air jet trial & error testing program with multiple jet installations along with the associated cost of jets.

Idle jet size is easily known to be correct if in basic slow speed driving the engine does not stumble on light to middling throttle application. Then it is the issue of adjusting the progression fuel delivery curve with air correction jets if perfection is desired. However, I support getting "close enough" as plenty good by any fashion that suits you.

[ena]

Hello Paul, Hello Chris,

thanks for letting us all know more about the Webers and how to get them set right on our cars. Many interesting hints and clues. After talking to Paul, I did set up my Weber IDA on my 66 Porsche 911 and my Weber IDS on my 67 911S. He made some very good and interesting points about setting them up and how the whole procedure should go from one part to another.

It was kind of a back and forth but in the end, both cars run smoothly in idle and transition range and go up to the redline and stay there without any hesitation. Since here in Germany we still have parts on the Autobahn without speed limit. It is a pleasure to drive both cars at full throttle in fifth with 50 years in there numbers matching engines and trannys and go 215 KM/H with the 66 and 235 KM/H with the 911 S.

Thanks also to James, who got that Weber discussion started here.


Stefan

65 Coupe
66 Coupe
67S Coupe