zenith vs weber

[1QuickS]

Hi Mukilteo911,

Send an email to me at:

info@PerformanceOriented.com

I'd be happy to supply a kit with instructions for installation and a selection of air jets to use for tuning. You will need to downsize your idle jet size (probably) assuming it was increased to overcome lean flat spot.

Paul Abbott
www.PerformanceOriented.com

[Tooth911]

Great topic regarding the transition spot,I have weber 40idas on a 2.8l ecams 9.5cr.The rpm at 1800 to 3000 is inconsistent at best iam running 60 idles 180 air f7 etubes 145 mains on top of pmo tall manifolds .Intakes bored to 37mm.Seems hesitant and sneezes out the stacks at the hit of the gas.

[1QuickS]

Hi Tooth911,

The idle air correctors for the 40IDA3Cs were 110s where the air correctors for 46s were 80s. Perhaps installing a smaller air corrector will enrich your progression.

Also, are you SURE your accelerator pumps squirt INSTANTLY at the SLIGHTEST touch of throttle? If not then check out this posting for more info regarding this device: http://forums.pelicanparts.com/porsche-911-technical-forum/390456-perplexing-full-throttle-issue-67s.html

Send an email for more info regarding the air jets.

Paul Abbott
info@PerformanceOriented.com
Performance Oriented

[nilknarf]

I have my kit on order- thanks Paul!

Curt

[Mukilteo911]

One modification i did to mine was replace the roller bushings with a sealed roller bearing to smooth out the action. The old rollers get worn and get flat spots in them and neew roller bearings are cheap and work better. I just called a bearing shop, told them the DIA i needed and they got them for me, about 10 bucks.

Pcar, i pm,d you i need some 34 venturies for my zeniths.

[James Brown]

BRD, I think you are spot on and the Zenith's are the way to go. Start with a rebuild and recurved distributor to really wake the engine up.

[911st]

I do not know it this is helpfully or encouraging but years ago Dwight Mitchel used to run a 1970 911 with a 2.7 E cammed motor in national SCCA autoX. He became national champ I think for two years against the Corvettes.

He ran modified Solex's. In one article about his car he said they worked better in the corners than the Webber's which I belive he thought leaned out from the G force.

I think this was about a 2300lb car with 225 R1's on 8 & 8.5's with RS flairs.

[VFR750]

I've been running the Zeniths for about two months now. Overall very good, but they could be better. My current set up is 55 idle, 165 mains and 175 air corrections and 34mm vents from pcar911. I'm using a Innovate Motorsports wideband O2 sensor hooked up to a laptop for tuning.

What I have found is the Zeniths run rich at low power and lean out at high power. I biased my jetting to get ~12.7-13.1 at WOT 4000 to redline. Part power cruise is unfortunately at ~12;1 which explains the poor mileage. After doing some calcualtions, I figured out the Zeniths would run perfectly on a 3.0l with 31.5mm vents, but will have this characteristic rich to lean transition with the larger vents.

What we need are tweaked emulsion tubes to lean out the bottom end!

Anyone who has a extra set, I'd be interested in buying them ($30 +S&H). I'll make educated guesses based on Weber tubes. Run them in my car and see with the O2 sensor how they worked out...

[James Brown]

Sounds great Mike, that's what mine are like. (rich idle lean to normal at high rpm) The tube work would really help. I have no spares but will keep an eye out.

[1QuickS]

I have a modification kit to change the idle air correctors so the lean transition may be adjusted in the same fashion as air correctors change main circuit richness. Zeniths and IDTP Webers were emissions compliant and had a lean progression. PMOs and most performance Webers (two barrel IDAs and DCOEs to name a couple of familiar ones) incorporate idle air jet tuning but our Webers and Zeniths came with fixed correctors, probably to keep us from fooling with them. The Weber IDTPs used 1.40mm air correctors, the Zeniths had 1.60mm and the IDAs had 1.10mm for the 40mm versions; a larger air corrector results in a leaner transition.

Send me an email if you wish to find out more about my kit.

[VFR750]

I've run various jets sizes and air correction jets and measure AFR, but they can't change the fundamental behavior. Just decreasing the Air correction jet from #185 to #175 had minimal effect on the high end. Maybe a slight shift to a slightly richer top end, but 2000-4000 was the same: 185 or 175.

Per my simple flow analysis, I found the original 2.2l with the Zeniths and 27.5mm vents had a particular pressure verses fuel flow verses rpm relationship. A 3.0l w/Zeniths would be perfect with 31.5mm vents. All you would need is #55 idles, and ~145 or 150 mains. AFR would be pretty good across the range, because the pressure/flow/rpm would be about the same. Just scaled by the displacement, compression ratio and intake port size.

When we added the larger vents (34mm) we decreased the high rpm pressure signal hence less fuel = lean. We all ended up with either #160 or #165 mains to get a proper AFR at the high end. Low end is too rich.

We really need to change the emulsion tubes to get more air in the low end, so that the system can be re-balanced with the different pressure/flow/rpm signal coming from the larger vents.

[1QuickS]

There are two air correction jets in Zeniths, one that retains the emulsion tubes in their wells and one that is pressed into the top of the throttle body. Emulsion tubes and their air correctors adjust fuel delivery along with the main jets for 2500+ RPM. The progression circuit consists of holes in the side of the throttle body bore which are uncovered by throttle plate movement from idle along with the idle jet and its unique air corrector jet; all of which control mixture from idle through 3500+ RPM.

As the progression circuit loses effectiveness the main circuit gains effectiveness and the balance of these two circuits is called transition. By not having an adjustment other than idle jet size for the progression circuit you are left with enriching your transition by selecting a large idle jet or with a larger main jet/emulsion tube. Unfortunately the main jet/emulsion tube path is less effective than working the top end of the progression which is what the air corrector jet for the progression circuit controls.

Zenith air correctors for the progression are insanely large @ 160 compared to a normal Weber IDA size of 110; an area ratio of over 200% larger. These air jets lean the progression fuel delivery as the RPMs rise through the transition phase of carburation. Again, these jets are pressed into the top of the throttle bodies and are not adjusted as per normal Weber carburetors; I assume the jets were pressed in on the Porsche carburetors to control end users from tweeking them but Webers sold to customers directly had the tuneable air jets for the progression since the end applications wouldn't be known.

[VFR750]

Paul,

As you saying, the idle circuit air correction jets should be decreased, to allow the slightly smaller Idle jets, and that will lean out the transition region? I thought that was the other way around, as the smaller air bleeds would richen the idle transition?

I have the earliest Zeniths that came with 140 idle air bleed (jets) and 185 air correction jets. What size do you think they need to be decreased to?

[1QuickS]

You are right, smaller idle air jets will enrichen the fuel mixture as the RPMs increase. The idle fuel jets don't get much air from the idle air jets at low RPMs. If large idle jets were installed to enrich the upper range of the progression then they might be too large for lower RPM operation once the smaller air correction jets are installed.

Also, in tuning these carbs it is useful to assure the throttle plates are open enough to prevent sticking shut (all the way closed, mechanically) but closed enough to block the first progression hole. Most times the idle speed is set with the idle speed stop screws and minimizing the air bleed screws. In carbs with some miles on them this works out well as there is edge wear of the throttle plates which almost requires closing them all the way to keep idle speed down. In carbs that are not so worn or for applications with engines of larger displacement the throttle plates are opened up to provide the air needed but the first hole of the progression circuit is exposed which activates it. In these later two scenarios you have idle mixture supplied from both the mixture screw and from the first progression hole. Setting idle mixture would then have you closing the idle mixture screw and perhaps lead to a leaner idle jet selection but this contradicts the need for a larger idle jet to overcome a lean progression...it gets complicated.

The "fix" to overcome a large displacement engine's need for more idle air (assuming the air correction screws can't provide enough and still allow for idle air balancing) is to drill the throttle plates with a 1mm hole, opposed to the side where the progression holes are and about 3mm from the edge of the plate. (This is easy to enlarge so don't over size the hole and may be closed with solder so this isn't as risky as it sounds.) This hole allows more air at idle to allow the throttle plates to close enough to seal the first progression hole and thereby makes the mixture and air correction screws work as designed.

To determine if the above "fix" is warranted you should remove your carbs, manually adjust the throttle plates to blank off the first progression hole, open the air corrector screws one turn and reinstall the carbs. If you can start your engine and set idle speed and air balance without using the idle speed stop screws to increase idle speed then everything is good, if not then you might consider the 1mm hole fix.

OK, a bit off topic...For a 3.0 with 34mm venturis; typically Webers are jetted with 145 mains and 180 main air corrector jets to match the 34mm venturis and a 55-60 idle jet for a 3.0 engine. Routinely 110 idle air correctors are in the 40mm Webers used in these applications so this is what I'd start with. However the progression hole sizes and arrangements of the Zeniths don't match that of the IDA Webers so I'm not sure what would be a best fit. Your mains seem a bit large and the 185 air correctors may be trying to take away a bit of the upper RPM richness they provide although there isn't much difference between a 180 and a 185.

Once you open this door it becomes a try it and see effort. Changing idle air corrector jets requires removal of the top cover of the carb which isn't as easy as a main jet change.

I have a simple kit that provides a tap, tap drill, instructions and a selection of idle air corrector jets if you like.

I also have a method to drill the top cover to achieve an anti-percolation fix for the Zeniths. Just ask.

[VFR750]

Paul,

OK, this sounds good. I was conscious during the initial rigging to have the 1st port covered at idle. But thinking about the need for more air at idle with the bigger engine makes me wonder if I should make sure the 1st port is covered. I'll think about my idle set up and use the idle air screws. I like the idea of drilling a small hole if needed to keep the 1st port covered.

Originally I had 60 idle jets, and they were too big. I'm currently running 55s although the car seemed a little snappier with the 57 idle jets. With the 55 idles, I'm at ~13 at idle, drop to 11.7-12 under light throttle, but see a slight pause on tip-in. At a steady cruise, 3000 rpm, I'm stuck at ~12, and can't seem to get a better light throttle cruise ARF. Going smaller than 55 Idles seems to be the wrong thing for throttle response. (I'll likely go back to the 57s.)

I was thinking, prior to your comments, about increasing the Idle Air Correction bleeds (jets) from 140, to lean the cruise AFR a little. But you've go me thinking about being a little more conscious of the tip-in behavior...... I may need an idle bleed port kit, but first I need to do more work.


I agree the 165 mains seem too large. At the low end they are way too rich, but lean out by 4500-5000 rpm. I'm pretty sure the main circuit would work better with a smaller venturi and the vacuum at the throat was larger (like the 27.5mm vents in the 2.2)

If I can get a stronger signal, i.e., change the relationship between air flow and fuel flow (forces me to a smaller venturi or even smaller air correction jet) or add more air at the low end (emulsion tube holes) I can hopefully lean out the 3000-4500 range relative to the high end. I'm leaning towards the emulsion tube change, because the WOT 3000-4500 range is too rich.

BTW, we kind of hijacked this thread, everyone OK with that? I'll probably start a Zenith AFR post with charts and stuff as I do more runs.

[1QuickS]

Mike,

Right about the hijack, I sent a PM.

[patssle]

OK, a bit off topic...For a 3.0 with 34mm venturis; typically Webers are jetted with 145 mains and 180 main air corrector jets to match the 34mm venturis and a 55-60 idle jet for a 3.0 engine. Routinely 110 idle air correctors are in the 40mm Webers used in these applications so this is what I'd start with. However the progression hole sizes and arrangements of the Zeniths don't match that of the IDA Webers so I'm not sure what would be a best fit. Your mains seem a bit large and the 185 air correctors may be trying to take away a bit of the upper RPM richness they provide although there isn't much difference between a 180 and a 185.

OT but thanks for the numbers. Been researching what people have been running on their 3.0L as I am in the early stages of converting my CIS to Weber carbs.

From what I've found...

34mm vents
145/155/160/165 main jets (your number is the lowest here)
175-180 air correction jets
55/60/65 idle jets
F26/F3 emulsion tubes

[viafnoinuaway]

awesome. nice man.