Zenith TIN Idle Air Jet Modification

[VFR750]

Thanks to Paul Abbott, I am modifying my Idle Air Jet size from 145 down to 120.

Background: My ongoing saga of trying to get good ARF at cruise has been thwarted by a transition lean spot. #57 Idle Jet a little too small, #60 way too rich. I've thought about many possible fixes. Paul and I have gone back and forth, and his suggestion to decrease the IAJ makes good sense.

This should tip the AFR curve going from a rich to lean as the throttle opens, to hopefully rich to richer. If that is true, I can drop the idle jets down to #57, and maybe get away with #55s for better fuel mileage. Time will tell.

I've got a week of rain coming, so now is the time. Carb torn down, drilled, and tapped. Easy, with a drill press. See the remnant of the old idle air jet sitting on the carb, next to the newly tapped hole. One down, five to go.


Note the old jet was center drilled, which relieved the interference press fit, and it slid right out! How cool is that! The predrilled set screws will go into the tapped holes. I chose #120 as a big change from the stock #145s, so the effect should not be subtle.



Note shameless plug for Paul.

[VFR750]

Conversion done and carbs rebuilt, installed and.... Not quite what I expected.

Sure the cruise circut is definately richer with the #57 idle jets and #120 IAC (Idle Air Correction) jets, but the jury is still out on the improvement to the transition. It seems the effect was to fatten up the whole range, not tip the bottom to top AFR curve. No plot here, yet, but I did not see a richening trend as rpm increased.

Initial impression is the 15-16 AFR spike is still causing a slight stumble, and going to #60 idle jet will actually make the crusie circut richer! I'm going to have to get real data, to plot the next step. But it may involvle going back to the #60 idle jets, and increasing the IAC jets from stock #145 to #155 or #160. All else fails, new transiton ports! Or larger accelerator jet holes....

Good news is the conversion was fairly straighforward. I made three modifications to Paul's instructions:

1) Shortened the set screw
2) Deepened the threads ~.05"
3) Added countersinks to the upper cover

The set screws in the zeniths stuck too far out of the tapped holes, and they interfered with the upper cover. I added a small countersink to the upper cover to make more clearance, but the thin casting wall limited the depth of the countersink. I tapped the hole a little deeper so the set screw was close to flush with the top of the main body, but that resulted in the set screw getting very close to the idle jet, so I filed off the tapered portion of the jet.

Now that this is all done, changing the IAC jets can be done on the engine, with a fair amount of carb disassembly, but at least, the carbs don't have to come off the car. May take less than 2 hours to swap IAC jets, verses the 10-12 hours spent converting the carbs in the first place.

Plus making new jets will be easy, brass set screws are cheap, and the small drills not too expensive. McMaster Carr is a great source for this stuff.

[VFR750]

Just got back from a drive. Yeah, cruise afr dropped to 11, and it spikes up on tip in. More work to come, order set screws and drills. Will make new idle air jets this week and try the other way next.

[piggdekk]

Mike,
are the carburators stock for Your car, or You're using the Zenith's on an engine that dind't have them from the beginning? I'm asking becuase I'm not sure I understand why you're changing the idle air jets.
thanks
luca

[VFR750]

They are designed for a 2.2l engine, and they are on a 3.0l. hence jet changes to match the engine. Plus, I'm trying to make them better.

[daniel911T]

I have the same kit, also sourced from Paul.

I experienced the same problem with them sticking up, but I found a much simpler solution. I simply moved the stop on the tap up about 1/4" I know that sounds huge, but it really just made the full size threads extend down a little bit further. Note that the tap has a very shallow taper, and you don't get to the final tap width until significantly up the tap. It's also important to note that there isn't anything below that hole for a long way. You are tapping into the cavity that the idle jet fits in, and on a Zenith those are pretty large. Thinking back on it, I think I just tapped until I bottomed out... worked great.

By just running the tap a bit deeper I got all of my jets to fit flush with minimal torque required.

It's also important to note, as soon as you change the air correctors, you have to start from scratch on the tuning. Go back and recheck all the air, then redo the "best lean" procedure. If you've done it right, there should be little or no change in idle, a slightly fatter transition, and once you're on the mains it should be just about the same.

If I remember your jetting right, you also were compensating for the lean transition by running a smaller air corrector on the main circuit. If you're getting rich at anything more than like 1/8 throttle, then you're on the mains, and you may need to move up a size or two in the main circuit air jets, to lean it out a bit.

I remember that Paul recommended drilling some compensating holes in your throttle plates so that you would have adequate adjustment for the air correction screws, and to keep the transition circuit out of idle. Did you ever do that? If you haven't done that, then I highly recommend it. You're probably quite rich at idle, and your balance will go all off if you're trying to make idle mixture adjustments with the transition ports uncovered.

I went down to 120s on my 2.2T/E motor, and after retuning, it came out perfect. It's never cruised so well.

Before you give up on the 120s, make sure at closed throttle, that the bores are actually closed. If you have more than 1 turn on your throttle stops, then you're getting into transition. (I mean just touching, then one turn) If those are turned a bunch, then your idle circuit won't adjust right. Get it some more air, that's where the drilled holes come in. They should allow enough air without requiring you to open the throttles far enough to uncover the first transition port.

Good Luck! Stay patient with the Z's, they run amazing when sorted!

-Dan

[1QuickS]

OK, now my 2 cents worth:

• When you open the throttles from steady state cruise do you have a lean hesitation that could be remedied with an accelerator pump squirt adjustment? If the squirt isn't ready to squirt at the opening of the throttles then it may be due to poor check valve operation allowing fuel to drain back down the fuel gallery so all you have is air to squirt.
• I've measured several Webers and one set of Zeniths for progression hole geometry and size based upon throttle shaft location and have determined that the middle two progression holes of the Zenith have approximately 40% more flow area than the equivalent middle hole of the Weber IDA. Also, the top hole for the Zenith is smaller than the Weber by 40% however it is located at the same relative position to throttle plate rotation as the Weber. The inference from this is that the progression hole of the Zenith starts off fatter and leans out compared to the Weber IDA carbs.
• Idle air bleeds for Weber 40IDA carbs (the standard by which I measure) is 110 or 1.10mm, the IDTP's (emission era Webers) were 140 and the 46mm race Webers were 80s. The 110 air bleeds seem to be adequate for universal engine applications where the IDTPs provided a lean transition problem from new.
• It may be that you will want to physically enlarge the top progression hole; it is currently 1.0mm where the IDA top hole is 1.35mm. A change from 1.0 to 1.1mm would be rather safe, incremental step and would enable you to gauge the response and encourage you to either continue the enlarging process or stop while you are.
• The larger idle air bleed lean the mixture AND delay the effect of the progression by weakening the response to the vacuum signal.

If the idle air bleed screws are too deep then they may be hitting the idle jets; the purpose of stopping the tapping short was to have the threads create an air seal so that air is introduced through the hole and no additional air comes through the threads; kinda like pipe threads.

I would prefer to see some methodical (your definite style) data acquisition to determing AFRs for different air bleeds for a given idle jet selection. In this fashion you would have a data base to apply toward any idle jet selection. The idle jets provide an across-the-board fuel delivery change while the air bleeds tend to delay (larger jet sizes) or advance (smaller jet sizes) the delivery response...this is the theory.

One more thing to consider, a lean cruise isn't bad (which you are aware of) so if the accelerator squirt can get your initial throttle response corrected then this is the goal to pursue.

I'm VERY interested in your outcome, best of success!

[daniel911T]

Originally Posted by 1QuickS

OK, now my 2 cents worth:
... If the idle air bleed screws are too deep then they may be hitting the idle jets; the purpose of stopping the tapping short was to have the threads create an air seal so that air is introduced through the hole and no additional air comes through the threads; kinda like pipe threads.

Paul,
FYI, even going significantly deeper on the tap than your stop mine still fit very tight, it just changed the depth at which they get very tight. I've visually confirmed that when flush with the top, the jets do not interfere with the idle jets. I think by some slight difference between the Zenith castings and Weber castings requires them to be tapped just a bit deeper.

Originally Posted by 1QuickS

I would prefer to see some methodical (your definite style) data acquisition to determing AFRs for different air bleeds for a given idle jet selection. In this fashion you would have a data base to apply toward any idle jet selection. The idle jets provide an across-the-board fuel delivery change while the air bleeds tend to delay (larger jet sizes) or advance (smaller jet sizes) the delivery response...this is the theory.

The change of the idle air jets had a very clear affect on my carbs. At idle, all of them ran fine, at WOT all ran fine, where the difference was glaringly obvious was cruise power. The stock 145s caused a significantly uncomfortable bucking and surging at cruise, along with a lot of popping on trailing throttle. As an experiment I tried changing idle jetting to 55s, which resulted in a very rich idle, but still had a lean cruise and trailing throttle. Changing down to the 130s and back to 47.5 idles reduced all those symptoms, and finally going down to 120s, and staying with the 47.5s finally got it "just right"

Moral: it was very clear to me that the idle air jets were having a huge affect on cruise, a minor affect on idle, and little or no affect on WOT.

Originally Posted by 1QuickS

I'm VERY interested in your outcome, best of success!

Me too!

-Dan

[1QuickS]

Updated my previous post with air bleed sizes for various Webers.

[VFR750]

Wow! A lot of good discussion. I will reply later.

[VFR750]

OK, a few moments to answer and comment.

The Idle Air Correction Jets are 140 not 145 as previously discussed.

Accelerator jets working well, but could kick in sooner. Anyone know how to make new profile bellcranks?

Yes, after a change like this, you have to redo the idle adjustments. Reset the needles to get the proper idle mixture, and so forth.

Yes, I did drill a hole in the butterfly, 1.1mm in diameter, and when I pulled the carbs, I confirmed the first hole was covered, with just a hint of "smudge" on the butterfly. Since I am running a high idle, it's OK.

BTW, I added a vacuum gage to my brake booster manifold. I can see how much vacuum a cold engine makes verses a hot engine. ~2" Hg difference! Obviuosly it explains the low idle speed when cold, as the engine is less efficient at pumping, until the pitsons grow enough. Pull a much stronger/consistent vacuum at idle when hot ~12-13" Hg.

Data aqusition will be forth coming. I actually have a lot of data from early on, that is held hostage on the laptop in my son's room. Need to pry it free this weekend!

I did a little area/flow modeling. And it should not have been as much of a surprise that the AFR dropped so much. Ah well.

But good point have been made on the #120s, but here is where I start to think I've got to go the other way: displacement. The transition holes are "sized" for a 2.2l with 8.5:1 compression. Now on a 3.0l with 9.3:1. Fual and AIR flow must go up, with the limitations of the exizsting geometry of transition ports, secondary venturi, and emulsion tubes. I've tweaked the Idle and Main jets, Air Correction Jets, changed from 27.5mm primary venturi to 34+mm, added a bleed hole, and now added adjustable Idel Air Correction jet capability.

OK new code: 57/120-165/165 57 idle jet, 120 idle air corrector, 165 main jets, 165 (main) air correction jet.

With 60/140-165/165 all was good, with a fat cruise, and great, immediate tip in.
With 57/140-162/165, good crusie, noticable transition hesitation,
With 57/140-165/165, a slight transition hesitation, better crusie AFR
With 57/120-165/165, FAT crusie, slight hesitation.

So, observed data suggests the engine likes the 60 idles with the 165 mains. This is where I plan to start over with the jetting, thinking the engine wants MORE fuel and AIR.

Next stop: 60/145-165/165, can I keep the good transition, with a little better crusie?

[1QuickS]

A couple of more details regarding Zenith/Weber differences:

• Zenith main venturi waist is 13mm down from top and Weber is 8mm and the diameter at top of venturi is 46mm for Weber and 50mm for Zenith; the air flow into the waist of the Zenith will be a bit smoother than for the Weber
• Zenith has tall secondary venturi and Weber has short one
• Zenith main venturi throat dia was 27.5mm for a 2.4T, a rather tame engine where the original 2.0 normal 911 used 30mm venturis

Coarse comments on the above are that the small venturis and tall secondary venturis provided a really strong signal to activate the main circuit in a low performance engine. On a 3.0 with CIS(?) cams the air flow will be stronger but the 34mm venturis will weaken it back down; the ratios of areas between venturis and displacements is pretty close to that of the 27.5 venturis in a 2.4; about a 2% difference. You should experience a strong signal to activate the main circuit, much like that on the 2.4T. Since there are no alternate emulsion tubes to try to tailor the beginning ot the main circuit then you are left with adjusting transition.

You could try lowering fuel level, this is a way to manipulate the beginning of the main circuit. Or, a bit more radical, get another set of aux venturis and cut the length down; this would delay the initiation of the main circuit also.

The nice thing about Weber accelerator pumps is that you have three unique cams to select from.

[VFR750]

145 idle air jets have been drilled. Next step is to do a baseline AFR cal, then swap the 120s for the 145s.

Why does it have to get dark so soon?

Anyway, will do the swap on Friday. Need to get this sorted; DE on Monday!

[VFR750]

I got the old file out and extracted the portion of the data that shows what I have been struggling with. This was collected last year, October 11, 2010. The configuration was 57/140-165/175. I since that time I changed to 165 Air correction jets. But this captured the issue nicely.

First picture should be familiar to those following my posts: WOT acceleration from 2,000 to 6,500 rpm, in 2nd gear, uphill. Of note is the clean WOT AFR signal, and small hesitation just after i stepped on the gas.



Now a much messier problem. A slow accel shows the AFR rising at the transition circuit leans out, then, step on the gas, and then a stumble.



So, although one would think decreasing the idle air jet should fix this, I suspecting the #57 is just too small for a clean transition. What i am seeing now is a cruise AFR of 10.8! when it was 11.7-12.3. But the stumble is still there.

So, my therory is: the 3.0l just needs more fuel/air than it's getting. The #57s just aren't enough. (unless you are ok with a little hesitation and better fuel mileage) Go to #60s and you hurt cruise AFR. But I suspect the leaning out at the high end of the transition ports is more fuel-driven than air-driven. There are options such as drilling larger transitions ports, but the first item will be to go back to the #60 idle jets and combine that with #145 idle air correction jets, That will definately lean out the low end, which is good, but may not lean out the top, IF the top is fuel limited. Give me some time, and I'll get the data to prove, or contradict my therory. Tommorow will be 57/120-165/165 baseline data run, conversion to 60/145-165/165, and hopefully get a another data run in before it's dark.

[daniel911T]

I think you should try the next size bigger air jet. That will lean you out right in the spot you're noticing the trouble. Stock was 185, I would try a 175 with the logging equipt. (you did say that the 120s were giving you a rich transition, right?)

Before you tear the tops off and change out the idle air jets, do a data run after only changing the main air jets. I am pretty sure you'll see that rich spot go away. The 120 idle air jets will also help you a lot with the late transition leanness. I wouldn't run away from them so quickly.

You're close!

Dan

[1QuickS]

When you "step on the gas" it goes lean due to lack of accelerator squirt is my guess.

[VFR750]

I do think there could be more fuel sqirted at that point. I also noted the vacuum gage goes from 13" to ~5" the instant you open the butterflies so the whole circuit is affected. More accelerator squirt would be good just at that point. How can that be changed on a Zenith? They are flowing, i checked. Could it be as easy as drilling the hole a little larger? I'm not sure if the pump can flow more. I'm already at the high end ~.5-.6ml.

I also think that this lean spike is normal, it's the duration for one, and second, if the transition circuit is already leaning itself out, any stab on the gas will hurt.

This is precisely what led me to work on this problem. I found that at 3000 rpm in 5th pull a slight incline, it would stumble on a fast accel. Odd. I spent the next few months observing and pondering what to do. I went back to richer idle jets (#60) but that was unsatisfing, as fuel mileage went down.

I agree if it really is fuel starving at the top end of the transition circuit, bigger holes in the transition ports are the next step.

Dan,
I'm note sure what you mean. I has 165 main circuit Air Correction Jets down from 185. That solved the leaning out at WOT/high rpm. It is rich down low because of the #165 main jets. I would like to run #162s, to get a better AFR on the main circuit, but that further amplifies th transition stumble. Once the transition stumble is fixed (improved?) I will go back to the #162 main fuel jets, and have a virtually flat WOT AFR of 12.5-13.

The change from 140 to 120 Idle Air Correction Jets were to richen the transition port circuit, but it did not behave as intended.

[1QuickS]

Weber accel pump design incorporates two fuel drain-back stops: the first is the ball check valve in each squirter (same as for Zenith) and the second is a "flapper valve" which is a rubber gasket that seals against the fuel delivery orifices for each squirter. Once the fuel in the galleries (for Webers) has been charged then it stays full due to the rubber gasket seal, I think the check valve is less dependable which for the Zenith is the only fuel drain-back device. Counteracting gravity's effect to drain the gallery is the low air pressure at the tip of the accelerator nozzle tip. At low air flows this is close to atmospheric so I doubt this effect provides much help to keep the squirter nozzle charged. One thing to check is that the rubber gaskets around the squirted nozzles are making effective seals so at least you aren't losing fuel there.

I'm glad to hear that you are testing in a high gear on a slight incline, this is the recommended process to work out progression fuel delivery issues. I assume you are using a constant throttle opening for each data run, it appears that you have info to correlate RPM to your AFR data. If the testing is performed on alternate inclines or if there is a change in resistance (headwind, variable vehicle weight, tire pressure changes, etc.) then the resulting throttle plate angle will be affected for a constant run time, etc. With different load conditions the throttle plate angle for a given engine RPM is affected which provides an unique curve. If you replaced the carbs with EFI then a huge amount of load/RPM/throttle plate angle/ atmospheric conditions data could be logged and used to adjust spark and fuel delivery...simple carburetors can only do so much. I know this isn't news to you but I needed to state this to help keep in mind that the data logging available today provides a world of info not available when carbs were the only fuel delivery system available; getting them close to ideal for limited driving conditions was all you could reasonably expect to achieve.

Regarding progression hole modification; have a review of my post of 26Sept where I provided comparisons of progression hole sizes/areas with Weber IDA3C carbs, the top hole in the Zenith is 40% smaller by area and the other holes provide earlier/larger fuel flow.

Also, you may want to try adjusting fuel level in the float bowls, a higher fuel level will initiate activation of the mains earlier which may be helpful to bridge the transition issue. Besides, it is easier to correct this adjustment than drilling enlarged progression holes smaller after enlarging them.

Enlarging the holes on the tips of the squirters will provide more fuel but for a shorter duration, total fuel delivery is limited to the .5-.6ml you already have. Weber accel squirters have a .5mm dia tip hole where Zeniths use .3mm.) The only way to increase squirt would be to modify the cam that the accel jump jack-shaft is driven by. This mod could be as easy (and as course as) grinding the area where the roller rides to change the local radius of the cam to be smaller. Upon rapid accel you would have an initial "step function" for fuel squirt with a greater total amount delivered. This grinding would need to extend to include your normal throttle angle position at cruise conditions.

Are we having fun yet?

[VFR750]

Originally Posted by 1QuickS

Are we having fun yet?

Yes. We are having fun.

Ran a baseline this morning, swapped out the idle jets in about 1 hour, retuned the carbs and getting ready to try this set up 60/145-165/165.

Yes, I always run the identical WOT pull, and entrance ramp where it is straight up a hill, long enough that I can pull redline in 2nd and comfortable lift to make the turn at the end of the straight. Always start in 2nd, 2000 rpm.

Obviously air temp, humidity and barometric pressure affect the results, so I am looking more at comparable trends, verses absolutes. But when the AFR hits 16, precision really doesn't matter.

The O-rings are seating well. In fact they were really tight when I pulled out the squirters to get the gasket off during the conversion. I also run near the top of the float level, so there might not be much more I want to go. But it's a thought.

I did see your part about the progression holes. Yes I think the Zeniths, set for a smaller engine with a tame cam, are not optimal for a 3.0. I agree opening them up a little would help. We'll see.


EFI! This would be 1000x easier with EFI. Record AFR data, change map, record data, change map. Piece of cake compared to guessing what is happening inside a multi-passage hydro mechanical fuel control! I know that carbs are not even close to perfect, and I am reaching a point of diminishing returns, but they are so much cheaper than EFI. ITBs; they might be in my future!

[1QuickS]

WOT does not activate the progression circuit. Progression circuit is activated by high speed air passing between edge of partially opened throttle plate and the exposed progression hole(s); when you are operating at WOT it is nearly atmospheric pressure at the wall of the progression ports and the main circuit is activated by high speed air passing through the aux venturi which draws fuel from the emulsion tube well.

With EFI you can stomp on the gas at ANY throttle position and get correct fuel delivery; classic carb operation is to resist the urge to stomp on it and to progressively open the throttles to avoid the excessive lean stumble; the less demand placed upon the accelerator pump squirt to overcome this leaness the better.

Also, the lines on the fuel level vial are just that: lines. If setting fuel level plus/minus 1mm beyond the lines helps running or helps in determinimg what course of action is warranted then do it. You can install a uniform shim washer under all needles or remove a uniform washer and very easily get a performance response. Returning to original shimming is easy. The lines are the reference fuel level to allow tuners to achieve nominal results during periodic maintenance. You have the freedom to modify YOUR settings as you want, within reason.