Zenith Enrichment Circuit

[DanPez]

Hi fellow members,
I have a particular question concerning the Zenith carbs on a 911T-V Sporto 2.4l.
I’m wanting to get the engine to its original performance but having issues keeping the engine on idle.

When I purchased the car it was running.
During the winter months I got it up on stands and checked the carburetor setup and found some fuel pressure issues and also some electrical wiring that was omitted.
In general the car was running with +15 psi of fuel pressure and the 12V power supplying to the Speed Switch was not connected.

For what I’ve gathered, correct me if I’m wrong, but the speed switch relay (contact output) is connected to the micro switch that supplies voltage to the solenoid only when the throttle is closed causing the solenoid to activate. This happens in and around 900 rpm.

Is this correct by saying ….. At this stage the solenoid valve, that’s under #6 intake, opens up to create vacuum to the enriching valves for a Richer combustion???
As for the Flow Rate adjustment screw (Opening sends more fuel right??)

The reason for asking is because the previous carb setup was more pressure and no enrichment now I have it working with less pressure (3-4psi) and the enrichment circuit working. My challenge is adjusting it.

I’ve read a few members that have removed the auxiliary enrichment circuit all together and was wondering how their engine ran after this mod. Were the jets changed?
Some input from you all who worked on this would be great

Thanks in advance

[VFR750]

The enrichment circuit is for deceleration emissions. The purpose was to keep the engine speed up when you lift off the throttle. With it deactivated the engine snaps to idle.

There are no adverse effect removing it. Webers don't have that circuit.

Since you were running too high a fuel pressure, your float level might be off when you dropped down to 3-4. I would start there, reset the floats and then tune the carbs.

[VFR750]

If you rebuild the Zeniths, you can delete the aux circuit. You will likely need to fiddle with jetting anyway, as E10 fuel tends to run lean.

[DanPez]

I neglected to mention that I took the carbs apart, clean, changed orings and added some where they were missing. I was surprised it ran. The floats were checked 17-18mm in height. (It was a static level check)
In fact the pressure issue came about when I removed the air box an noticed fuel overflowing.
The fuel pressure was to a point that the needle values were unable to close properly.
Giving throttle to the engine now revs fine but seems to be missing fuel at low rpm.

[VFR750]

Overflowing can also be due to leaks at the float valve and float valve cover gaskets. I flooded a cylinder due to this. Leak flooded the top of the carb and then drained back into the intake thru the main circuit. Good thing you found the problem. It can lead to a fire.

Lean at idle, low power? What idle jets are you running? For a 2.4 may try a 50-55 idle jet.

Zeniths are notorious for lean stumble, so covering that leanness normally means running a little rich at idle and light throttle.

How are you tuning the carbs? Using a air-fuel gage? CO meter? Sound-feel? Each method works, I use an AFR gage. 12.5:1 idle. 11:1 light throttle results in 14:1 under tip-in. 12.7-13:1 WOT.

[DanPez]

Overflow may have been the wrong term to use. (Fuel Injected is more like it) :-)
The fuel return line was restrictive and caused the pressure to push 15+ psi to the carbs,
Removing the kinks, on the return line, solved this problem. Turning the fuel pump on and checking the fuel pressure I get 3-4 psi.
The idle jets are 47.5.
For now I'm on the preliminary adjustment by feel and sound. (I say lean because its popping like firecrackers)
(BTW are you using an O2 sensor? Where did you put the port?)

This would be the reason I'd like to get the aux enrichment adjusted. Since the circuit was not functioning before I'm asking myself if by adjusting the flow rate screw I can supply more / less fuel to get it stable.
How many initial turns out should I give? (Flow rate and Mixture screws)

Thanks

[VFR750]

Popping can be lean settings or air leaks or both.

I use the O2 sensor with an Innovate LC1 controller hooked up to a laptop. O2 port was in the exhaust. You would have to weld a port to the exhaust pipe before the muffler

[924CarreraGTP]

I'm having the exact same problem in my 2.2 911T with Zeniths. My idle jets are also 47.5. I've spent roughly 8 hours trying to get the car to idle, but it sputters and dies and I can't seem to figure it out. Mine was idling just fine before I decided to try to readjust the carbs. It's strange too because my enrichment circuit was turned completely closed. Also the air corrector screws were all closed. I can't figure how the last mechanic got the car to run, let alone idle. Now I'm just experimenting trying to get it to idle. I've adjusted the throttle stops with the synchrometer numerous times between 1 and 4 to pull around 5 bar, but the car either runs at below 500rpm or I adjust the throttle stops to get an idle, and then when I hit the throttle the engine immediately idles at 2,000rpms and won't come back to the idle that I set. It's as though there is no adjustable idle between 200 and 2000rpms. I cleaned the idle jets, but I'm about to break down and order some. The car runs good throughout the whole throttle range. It just won't idle for more than 6 seconds. The enrichment circuit setup in my workshop manual says two turns out for both flow rate, and mixture control screws.
Let us know if you figure it out. Maybe it will help me too.

[VFR750]

Odd that the car was running fine before you started to make adjustments. You may have broken free some crud that is affecting the idle circuit.

If you search under carburetor tuning, you should find some great step by step instructions. I am not a professional, so my process follows the experts. It is an iterative process.

In essence, you are going to have to start from the beginning.

Assuming the car is already warned up. (get it running at a high idle if needed)

Engine off.
Set float levels
Drop links off
Turn in all idle adjustment screws in all the way, being very careful not to over tighten.
Back out all 2.5 turns
Close all idle air bleed screws.

Start the car and using a syncrometer adjust the throttle screws to set an idle speed (I go with 1000 rpm, higher than standard, but it is just happier there)
Balance #1 and #4 cylinder to the same reading
Adjust the idle air bleed screws to get #2, #3, #5 and #6 to match #1 and #4.

At this point the airflow is balanced (synchronized) Idle speed may have crept up some. Yoo can then adjust the throttle plates to change to you desired idle speed, and then re-balance all over again.

If you don't have an AFR gage or CO sensor, you may or may not be at the right fuel flow for a good idle, and you will have to tune by ear and observation. I don't do this as I use AFR. I set ~12-12.5 at idle and declare success.

Tuning idle by ear is tougher, in that you will slowly turn the #1 idle screw inward looking for the beginning of lean popping. Once you find that point, back the idle screw outward ~1/4 turn. Repeat for all cylinders. Then check idle speed.

If the 47.5 jet is too small, you may need to back the idle screws out to 3 turns before you start. 50s might be better with todays fuel (10% ethanol)

[VFR750]

Unless the carbs have been remanufactured in the recent past, it is also likely you are chasing leaks in the throttle shaft. Mine are being rebuilt for this reason. It made idle a crap shoot.

So I covered up the leak with:
1) large (60 for a 3.0l) idle jets
2) 1000-1100 rpm idle
3) 12 AFR at idle.

With this set up I also had odd idle speed effects occasionally.

I also think the throttle plates were not closing the same each time. With the carbs off, I could see where the plates were closing completely without the throttle bellcrank touching the throttle stop screw. That would also say the airleaks on the shaft were pretty large.

[ddubois]

With respect to the original question regarding the enrichment circuit - abandon and plug it. I did years ago on my 71T with no problems. Assuming all of the enrichment system parts work (RPM transducer, solenoid, throttle switch), I can guarantee the rubber/metal diaphragm in the vacuum port housing on the side of the carb is torn and not functioning. This diaphragm acts as the 'switch' for the air enrichment and is not supplied in a rebuild kit.
With respect to tuning and idle, I as well have spent many, many hours trying to get a good consistent idle. I think I have finally achieved this using an approach similar to VFRs with a few additions. Make sure your timing and valve adjustment are correct before tuning the carbs. I also like to check the float levels with the engine running and have discovered they have to be set perfectly. I also stand by my MSD and Magnecor cables upgrade as helping stabilize the idle, but that's a separate argument.
I use a Innovate AFR meter to check my jetting and adjustments. Once the idle is good I take it for a drive with everything cabled up (RPM and AFR). I can then download it to the laptop and look at AFR over RPM and engine load. Or I can hook up the laptop in the car and see everything real time.
Lastly, I have played around with jetting and have come to a few conclusions.
1.) Buy new jets. The chances are high yours have been reamed. If you insist on using old jets then at least buy jet gauges.
2.) Getting the idle jet correct I believe is critical. It seems in my case that one jet size makes the difference concerning the erratic idle others have described. My theory is that trying to correct for incorrect idle jets using idle mixture (i.e. straying from the seemingly magical 2.5 turns), and throttle plate idle position causes issues.

As a data point here are my jet sizes (all new jets), for my stock, original and a little tired 2.2, but keep in mind I live at 6,800 ft altitude. There car is running perfectly and I told my wife to smack me with a 2x4 if she catches me tinkering with carb adjustments.

Idle - 50, Main - 110, air correction - 185

Note that if a 50 idle jet is working for me at 6,800 ft, then those of you near sea level should be larger than 50 based on my results. Gotta love this ethanol crap.

VFR also makes a good point about worn throttle plates/shafts. I have also suspected this on these 40+ year old carbs. My approach to that problem is to not spend the money on rebuilding zeniths and instead save the money for some PMOs in the future.

[DanPez]

"Assuming all of the enrichment system parts work (RPM transducer, solenoid, throttle switch), I can guarantee the rubber/metal diaphragm in the vacuum port housing on the side of the carb is torn and not functioning. This diaphragm acts as the 'switch' for the air enrichment and is not supplied in a rebuild kit."

So the diaphragm switch on the side of the carbs opens up an Air Passage? (Not fuel ?)
The possibility of a pierced diaphragm causing this issue gotta check it out.... Thanks

[ddubois]

It actually allows a fuel/air mixture to flow during throttle closed/above 1350 rpm conditions. The fuel in the mixture I'm sure is what destroys that diaphragm. Here is a description of the circuit. Just screw in the two adjustment screws, put plugs on all the vacuum ports and disconnect the wire to the solenoid. All this circuit did was reduce emissions during the throttle closed/above 1350 rpm condition. I think it also helps reducing backfiring during those conditions, but I haven't had a problem with that since disconnecting it years ago.

[DanPez]

Great info!
I gather in this position the throttle valve is completely shut, absolutely no opening, the enriching circuit is what supply's the A/F mixture.
Now I know why the previous mechanic adjusted the idle speed adjustment screw to have the throttle ever so slightly open. (Was compensating this way)

Since I want to get it working to its original state ....
I'll check if there are any damaged parts. If I can't get replacements I'd probable have no choice to remove the plumbing.

Naturally without this circuit the idle jets and adjustments will be by ear.
Thanks

[924CarreraGTP]

This has been one of the best posts for these issues that I've seen lately on pelican. Better than my own in fact. As stated by ddubios, the Pelican rebuild kit for the zeniths does not have the correct diaphragm for the enrichment circuits. It's just a flat piece of rubber to act as a gasket, and probably serves to plug the enrichment circuit. Note: you can get the exact same rebuild kit from Allstate Carburetor for a little over $50. The kit is not very good.
I'm going to go ahead and order new Solex jets from either Gene Berg or Aircooled.net because I don't like Ebay transactions. So Alfa1750 is not a choice for me though the prices are comparatively the same and I hear good things about Alfa1750. I'm going to go ahead and try new jets. I don't have a tester for the float settings. I heard you can build one, and if anyone can show a picture on how to do that please do. I'm also about to check my Marelli to make sure the advance is working correctly. I read on here that could also be a problem. My car is in time. That checked out correct. Still haven't verified correct fuel pressure, but it doesn't make sense in my case because my car was idling fine before I changed the carb settings.
Above someone said the idle circuit. What would need to be cleaned in this case besides the idle jets? Please be descriptive because it's hard to understand some of what is explained on here. All of us aren't Karl Ludvigsen, but some of us need explanations as good as Ludvigsen's to understand. Maybe I am just dense.
Also, thanks VFR750. That information might be just what I'm looking for.

[VFR750]

If you disassemble the carbs, there are many small passages. Each one should be cleaned out as best possible. Soaking in a carb cleaner, then flushing with air and repeat with carb/brake cleaner.

The transition ports could be cleaned out carefully with soft wire.

The idle air bleed circuit inlets are the small brass plugs with holes in them they are on top of the main throttle body. Make sure they are also free from crud or restriction.

Since you suspect worn throttle shafts go with larger jets to cover this up. 50-53-55 should give you about a step range of 0.5 points in AFR. 55 will be ~1. Point richer than the 50s.

But your mileage will go down at all part throttle conditions. <-- air leaks really do suck.

I would look carefully at the air bleed screws for leaks. If they are loose, they will also make you efforts difficult. I added a sleeve of silicon tubing over the threads and let it get squished against the housing making a seal. Model airplane fuel line is what I used. I think it was 3/16 outside diameter.

[VFR750]

I bought two of the float gages. You get a whole bunch of thin seals for precision float level setting.

The body of the gage interferes with the Zenith. You have to grind away some material to get clearance. AND the length of the brass knob has to be shortened. Otherwise it sticks in too far and locks the float in place.

I got two so I could adjust the floats with less mess and time.

[924CarreraGTP]

More good information! The idle air bleed circuit inlets are the brass fittings with ball bearings that come in the kit? They are under the plugs on top of the carbs and a total of two for each carb? If that's correct information I have new ones and will install them.
The 3 turns out to start on the idle mixture screws might be what I'm missing as well. At 2.5 turns out I get a lot of popping from #1. I'll start at 3 turns next time and try that. I think that might have been closer to what the setting were when I started messing with the carbs. I'll start over again and see what I can come up with. Thanks again VFR750. Maybe this will help DanPez as well.

[VFR750]

Not sure what you mean with fittings with the balls. Picture would help.

Also search under "zenith TIN". You will find many of my posts on what I did to my zeniths.

[924CarreraGTP]

The idle air bleed circuit inlets that you mentioned. Where exactly are they? I think I'm confusing them with the float level needle valve.