Zenith AFR Testing on a 3.0l SC

[VFR750]

This past Friday at Lime Rock was excellent. Jetting for the day was 57/160/165 and for 45-55F it was spot on. Comments included: "that's a stroker, right?" Nope. "it's got a cam?" Nope, 28 yr old stock SC. I concluded my passenger noticed how strong this engine is.

What I predicted, and what I got was pretty much what I wanted. I stole a few glances down the main straight and saw typically 12.8-13.3 WOT. Car pulled 6,400 in 4th down the main straight for an honest 120 mph peak leading up to Big Bend. Best Ever.



All was not perfect. I cheated first thing in the morning and opened up the idle screws 1/4 turn; it was 22F! With the extra fuel it started pretty good considering it was out all night and spent 90 minutes being blasted by very cold air.

Did you notice the 25-30F 1st run? Man was it cold at 9AM! Even after 10 minutes of warn up in the pits, the engine was cold. I had a transition stumble in a few places. Even started to regret keeping the 160 mains. However, one guy blew out his oil cooler line from overpressure, so we were pulled in after only 3 laps. After sitting in the hot pits for 15 minutes, while they cleaned up a LOT of oil, we went back out and the transition stumble was gone. I attribute this to a very cold morning, and fuel not completely atomizing. Once the heads and the manifold got warm, no worries.

The rest of the day was fantastic, clear blue skies, 50-55F, and the jetting making max power. Quite a few people were surprised by the Zenith choice. But I think I might have made a good impression.

Of course, you all know what this means. Now I have to get more jet sizes, probably #155, #158, and jet for each track elevation and expected temps.

[PropellerHead]

Mike,
This is a great thread. I'm so impressed that you're taking things this far. I'm learning a lot just listening. I rebuilt my Zenith's this past winter when I bought my car (it wasn't running, they were a mess with old gas, etc) and I've yet to get them fully dialed in. When I rebuilt them I kept the original idle and main jets as well as air correction but I did remove/disable the aux decel fuel enrichment circuit.
Honestly, I have not spent much time on them yet and I have made massive improvements thus far but there is room to improve. I'm looking forward to diving in again next spring.
My car/engine is a stock '71 2.2L 'T' with SSI's and K&N watershields. I've struggled to get rid of a transition hesitation and pop. It is better when the engine is fully warm, but not gone.

From an airflow standpoint I have the carbs well balanced bank-to-bank and cylinder-to-cylinder. What I'm struggling with is getting the mixtures correct from cylinder to cylinder. What is the best way to set idle mixture? I don't have an AFR meter and the exhaust does not have provisions to mount one at the moment although I could have some bungs welded in but I'd like to avoid spening huge sums of $ on sensors if there is an alternate method that would work sufficiently.
I have the idle mixture screws backed out quite a bit, significantly more than the baseline 2.5 turns from closed. I'm wondering if I need a larger idle jet.

Next question - where in the world does one buy idle/main/air jets for these carbs? Also, the original 27.5mm venturi set is awefully small. Perhaps it is well suited to the 2.2L 'T' motor but the appropriate Weber seems to use a 30mm for this application. That could be the root of the myth that Webers offer a significant improvement over the Zeniths of this era?

Thoughts? Sorry to hijack your thread as you're focused on a 3.0L application. In most of my search attempts I find very little of this level of experience to draw from!
Cheers,
Chet

[VFR750]

Chet,

Easy answer First: Source for new Zenith TIN Jets Tell Gabriele David that I sent you

The 1971 Zeniths were caught in the tightening emissions rules, so I believe they were jetted lean. You should have #47.5 Idle Jets. If you needed more than 2.5 turns on the idle screws you either have an air leak (are the air bleed screws open a lot to get synchronization?) or the idle jets are too small. If I ratio the displacement from 2.2 to 3.0, and then ratio the #47.5 idles to the equivalent 3.0, I get #55.

My experience was the #55 were a little too small. So going up to #49 would equal my #57s, #51 would scale to my #60, so go for the "easy answer" Try a #50 Idle Jet.

I can't yet prove it, but there does seem to be a correlation between Main Jets and transition too. So maybe increasing the #115 Mains to #120 could help. Note they increased the Air Correction Jets from #185 to #195 for the 1971T, which would lean out the top end. The #120 would offset that a little.

Idle screw setting: Unfortunately, without a AFR sensor, you're stuck with the try-it method. I can only measure AFR on one side. I assume the other side is similar. As I said earlier, your Idle jets maybe too small, hence you have to back the Idle screws out too far. I also assume equal turns on the screws results in equal fuel to each cylinder. If you think one cylinder is popping all the time, you could richen that one a little, but I would not think you want to stray more than a 1/8 turn from the others. Otherwise open them all 1/8 turn at a time, and drive.

Here is my matrix of tests so far. I think I have an ideal LRP set of jets. Now I want to fix the rich low end, without hurting transition.



pcar9119 made the 34mm venturis, you could ask him about 30s.

AFR Sensor $220. Comes with a bung that can be welded on. I wouldn't think that would be too expensive. $40?? Case of beer??

[Schmierung]

Mike,
Would your sizes work with 36 mm venturis..all else the same? 57idle/160 main/ 165 air?

[VFR750]

With 36mm venturis you should probably start out with a little bit larger mains. 165 at least since you will be flowing more air at the higher rpms.

Do you have an afr meter? Its a good investment.

[VFR750]

Schmierung, remeber 57/160/165 was ideal for a cool day @500' asl. I am running 57/165/175 at a lower altitude and generally colder weather here in April. So where are you? 165/165 might be a good place to start.

[Schmierung]

Mike,
I am in So Cal, 92592, semi-arid, never freezes, sea level to 4k ft. Sadly without AFR meter. Zeniths are still on the bench waiting for vents and kit from Performance Oriented.

[James Brown]

Great job Mike, My jets are spot on to yours. And I love how the car/engine responds. I have been driving it most of the winter. It needs a few more pumps in the morning but after 20-30 seconds, it runs smooth. Idle is down to 300-500 rpm cold (I have no throttle). When warm, look out! In the year I have had them, O problems. Tuning has held steady. Bring on the warm weather!!

[VFR750]

Sea level is what matters; you'll end up rich at 4k. 160/165 is probably better than 165/165. Just watch out on cold mornings at SL, it may be a little stumbly until warmed up. Plus, I assumed you opened up your Air Bleed Screws to help with transition or you drilled the butterfly.

Edit: I'll venture, that for us 57/162/165 is the best for a 3.0l and a SC cam near SL. Plus ~55/158/165 will be good for WGI in June. To prove it, I'm going to buy a set of 162 and 158 Main Jets.

Plus, I haven't even tested or reported on the butterfly modifications


Testing will follow once the bathroom project is done, someday...

Mike,

I am in So Cal, 92592, semi-arid, never freezes, sea level to 4k ft. Sadly without AFR meter. Zeniths are still on the bench waiting for vents and kit from Performance Oriented.

[VFR750]

The fun part about doing the jet swaps is when your close, you can predict the behavior. I now have three favorite jet combinations:

Windsor CT @ ~Sea Level: 57/162/165
Lime Rock @ 550' 57/160/165
Watkins Glen @1500' 55/158/165

Subtle changes for sure, but you get about 12.5-13.5 ARF WOT, and 12-13 Idle at each altitude condition.

I guessed on the WGI setup, based on my observed sensitivity between SL and LRP. And it all seemed to work out. I started the car at WGI with the #57 idles, and part throttle was piggy at 11 AFR. Popped the #55 in and it rose to a respectable 11.8-12. I should have bought the #54! Same deal going down to the 158 mains. Seriously, the jetting change was required at the higher elevation of 1500'. It's just nice to know straightforward this is when you can measure the AFR and make informed choices.

Overall the car has pulled strongly to 6300-6700 rpm at both tracks. (with an occasional over-pull to 7000) So the Zeniths and the track specific jetting really helped.

Plus the 160 mains were a little too lean at SL, and the 162s fixed that. So 57/162/165 is my favorite street combo.

On a down note: the Innovate O2 sensor is proving to be less and less reliable. It seems to be either breaking down, or the shots of fuel are affecting the stability of the calibration. Many runs the gauge would flash 7.4, and one flashing pulse on the LED about 5 minutes into a run. Even starting has been a hassle, with it randomly failing, then randomly coming back to life. So I have no good data so far this year. All my assessments have been via a quick glance at the gauge when it is working.

Still have a minor transitional hick-up, but it only goes to 14-15 AFR during tip-in. On the track it is not an issue as there is no cruising allowed.

[VFR750]

Transition/Hesitation eliminated: 60/162/165. Tip-in leaning is only 13.5-14. Runs great

But too rich at idle and low power: 11-12. Worth the trade for now.

[VFR750]

Just got back from testing. Car was 60/162/165, ran great but a little hesitation at 3000 under light tip in.

Switched to 60/165/165, rich, but ran without tip in hesitation

Switched to 60/162/165, slight hesitation returned

Switched to 57/162/165, hesitation got worse, and earlier in the tip in.

Reinstalled 60/162/165, and just blipping the throttle showed an improved response. So, 57/162 too lean during transition, 60/165 fat but no stumble, 60/162 perfect high end but cruise w/load too lean. Man I need to tweak something!

After a lot of thought, I will be decreasing the idle air jets from 140 to 130. The transition lean spot is more sensitive to the idle circut. I figure, fix that first, then worry about the main circiut.

[VFR750]

Obviously, I did not try my previous best 57/165/175. I should also try 57/165/165. Changing air corretion jets is a pain.

[Eagledriver]

Keep in mind that a lean cruise condition won't harm your engine, in fact it will keep it cleaner and improve your milage. I would try to get your WOT curve as good as you can get and run a big enough idle jet to keep from having a lean stumble at tip-in. That's about all you can do, just let the part throttle stuff be where it is as long as it still fires and doesn't dilute the oil.

-Andy

[VFR750]

Quick Update.

After messing around solving the hesitation issue (or at least figuring out what works OK verses not OK) I now have adjustable Idle air Correction Jets. (Thanks Paul)

Yesterday at Lime Rock, I saw 12.7-13.1 WOT down the main straight. Perfect.

Idle Jet: 60
Idle Air Correction Jet: 145
Main Jet: 162
Air Correction Jet: 165

I went up from my previous 160 at LRP, because I changed from full rain hats to a cut down version I made from fiberglass. Lets in a little more air. Car pulls 6500+ rpm nicely.

[andytat]

Mike.

I am just fitting a set of Zeniths to a stock SC engine and wondered if you initially tried the carbs with the original chokes fitted and if so how did it run? I don't have easy access to larger chokes hence the question.

Cheers.

Andy

[chris_seven]

I have a couple of sets of 36mm left over and we are using these on a 204BHP engine fitted with a pair of 1.625 dia headers and a 2:2 muffler.

[daniel911T]

aftermarket Zenith vents are works of art. I have a set of custom made 30mm aluminum ones that are beautiful to behold.

To be clear, I'm running a 2.2 using the 27.5mm stock vents at the moment, and they run amazingly well. Someday I'm planning on a 2.2 >2.4 upgrade (using my existing E cams and pistons) and expect those larger 30mm vents will be a perfect match.

Properly sorted Zeniths are the most underrated induction system ever made for a Porsche. Wow they run well when dialed in...

-Dan

[VFR750]

I did not use the 27.5mm venturis. I only used the 34mm ones. I think the 27.5s will work, but the engine will run out of breathe long before redline. You probably will get little benefit vs. cis, or worse loose power.

Mike.



I am just fitting a set of Zeniths to a stock SC engine and wondered if you initially tried the carbs with the original chokes fitted and if so how did it run? I don't have easy access to larger chokes hence the question.



Cheers.



Andy

[andytat]

Thanks for the reply Mike.
chris_seven, how much do you want for a set of venturis & where are you located in UK? I'm coming over next week for a few weeks.

Cheers.

Andy