Zenith TIN - Idle Air Hole for 3.0L conversion

[VFR750]

Now 1.1mm See later posts. 2/13/11

Per my AFR post, I found the typical 3.0l engine pulls so much air at idle, the butterflies are opened up enough to uncover the first transition port. This means you are setting idle mixture with both the idle screw, and the idle jet.

Paul Abbott suggested I re-synchronized the carburetors with the Idle Air Bleed Screws opened up all the way. I did this and the transition got a little better, and as you can see, the 1st port is covered. the engine idled at 1000 rpm this way.



Paul also suggested I drill a small hole in the butterfly to allow a little more air at idle, and essentially allow me to close the butterfly back down, and close the air bleed screws back in. So tonight I did it. I measured the air bleed hole in the throttle body at .8mm, so an equal .8mm hole in the butterfly made sense.





I used a .75mm drill to get the hole through the butterfly, and finished with a .8mm to make sure the hole to hole variation was very small. Hand drill took a little time, but a .75mm drill is really flimsy. You have to push on the drill to get the bit to cut, yet not too hard, or you'll bend, buckle and snap it. No power needed!

I expect the .8mm hole might be a little small, Paul Suggested a 1mm hole. I only had .75, .8 and 1.1mm, so I went conservative. If it works out that I need a little more I can easily drill it larger. Unfortunately, I have to wait till March 2011 to test this out.

[James Brown]

To get mine to sync well (3.0, Zenith) I had to turn in the air bleed screws all the way. it idles well at 950, no transition problems. With the jets open even, can't get a good idle. would drilling out the plate help? not that it needs help but always want to improve things.

[VFR750]

I think I am running leaner idle jets than you. #57s vs. #60. So I'm a little more sensitive to transition.

I could easily run 950 rpm idle with the air bleed screws turned in all the way, it just required a turn on the idle stops, which opened the butterflies a little. But as I dropped jet sized to get a little leaner cruise ARF, transition suffered a little.

When I opened up the Air Bleed Screws, I had to do two things. First I had to close the butterflies a little to get the rpm back down. Second I had to open the Idle Jet screws a little to restore a smooth idle (and ARF). Hence, before I made the change, I was idling on both the Idle Jet Screw, and the 1st transition port.

What I am trying to do it to get the Air Bleed Screws back in, where they are more secure, and let them manage synchronization only.

BTW, it's not irreversible. A little bit of JB Weld, or solder would plug this hole.

[James Brown]

Sounds good, mine work and thats fine but I really have know idea what circuit is it running at any time below WOT. Understand these carbs (or any carb) is fascinating for me, keep up the great work.

[VFR750]

I changed my mind and went to a 1.1mm diameter hole.


What changed my mind? I was getting ready to reinstalled the carbs, and I did my usual bench synchronization using a vacuum cleaner. I found that the .8mm hole was less effective than the .8mm air bleed hole.

I have an old style synchrometer, the one with the little red cup, that floats on the vacuum signal. I has a rotary valve, that was set just right for my engine at idle. With the cup at the first line, the shop-vac suction matched the engine. So I know what airflow is required for a 3.0l at ~1000 rpm.

BTW, to meet that airflow with Zeniths, the first transition port is definitely uncovered. I unscrewed the plug covering the transition ports, and I could clearly see the butterfly above the first hole. Drilling the air bleed hole is a really good idea for the larger displacement engine.

As I said, I found that the magnitude in airflow change for the .8mm hole was less than opening the Air Bleed Screws turned all the way. The butterfly was still open a lot. Armed with this new information, I decided I really did need to go to ~1mm as recommended. Enlarging the hole from .8mm to 1.1mm works out to be ~2X more bypass air.

It seems to have helped. The first transition port is only partially uncovered when the idle airflow requirements are met. At this point, I could do better, but I really think I need to test this configuration as I think I'll need to balance the Idle Mixture screw and first port to get a good idle AFR.

But not so fast. I need the 4 feet of snow on my yard to melt, and take all the salt and chemicals away. Tell you how it works, in March/April...

[VFR750]

Quick Update. Snow has melted, Road Treatments are washed off, time to drive.

I have put about 200 miles on the 57/165/175 combo with the drilled butterfly plates. One 1.1mm hole, opposite the transition ports, on each plate. Overall working great. I have not done any AFR testing. I just got the car on the road two weeks ago, and it's been very cold and damp. No fun.

Even with the 1.1mm holes (and the Air Bleed Screws turned in) I could get down to a solid 1000 idle rpm when warmed up. ~2.25 turns on the Idle Screws set a reasonable 12.2 AFR at idle. I'd go leaner, but there has been a few 35F starts, and I did not want to get into hard starts. Also snap-throttle only raised the AFR to ~14.5 which is fine.

I'll be going to the 57/160/165 for LRP this week. That will be the test for the holes, and how they might help transition.

[James Brown]

Nice!

[VFR750]

Been a while since I posted here. Many miles driven and the answer is: Not quite done...

The added air-bleed hole in the butterfly helped a lot, and let's me run 57/162/165 on the street. This is about as lean as I can get and have a minimal transition lean spot. I have noticed a leaning out under light load at 3000 rpm in 5th. It appears the butterflies are open just enough where the transition ports are dropping out and the mains haven't entirely kicked in. When I was running 60/165/175, I think this was covered up by the rich idles.

I think this last modification allowed me to get a better sense of the transition behavior of the Zenith. Watching the AFR spike, and observing what is actually happening at the time was made easier, because the holes lowered the position of the butterflies for cruise power, it seemed to allow me to turn the transitional lean spot on and off, with just a very slight increase in throttle position and engine load. Now that it appears to be isolated to this very specific condition, I can make changes and get a direct result.

So, what does this mean? Four options. 1) change the idle air bleed jets to a smaller size, or 2) change the location/number/size of the transition ports. 3) Decrease the top Emulsion tube hole sizes. 4) Run richer jets on the main or idle circuit

Starting with #3, I don't think it is an emulsion tube issue. Because just a little more throttle and the AFR drops from 14-15 to 12-13. I don't think the Emulsion tube can be that sensitive. Of course, decreasing the upper hole size, or lowering them a little might make the mains kick in a little faster.

Changing back to richer jets is a real, near term option, but somewhat unsatisfying, because getting the "best" jetting was the goal. Plus, on a Zenith's with the K&N filter kit, changing the air Correction jets from 165 back to 175 on a routine basis is a PITA because getting to then is tough, with limited visibility. Even with my modification discussed on another thread. Easier would be go back to #60 idle jets, but then I'd be pretty rich at idle and puttering around.

So the real answer is to either tweak the Idle air jets or drill a new set of transition ports. Since these two main option require a full tear-down, I'm going to pass on that until the winter. Plus, in discussion with Paul Abbott, drilling a new set of transition ports requires either a good drill press setup or a mill, as accurate location from one cylinder to another is mandatory.

For now, I can say:
1) Added hole in butterfly helped reduce the transition lean spot/stumble when running reasonable AFR jetting
2) It is now limited to a slight 14-15 AFR spike under light transitional load with 57/162/165 jets.
3) Running a little richer 57/165/175 or 60/162/165 would probably make this go away.

Or, maybe a set of #58 or #59 Idle jets....

[VFR750]

Just came back from a test run with #60 idles. I have to say it runs better that with the #57 idles. Although it is quite rich at low throttle opening. So I'm going to run 60/162/165 this summer.

To recap, I have bee trying to lean out the idle side to get better fuel mileage. I also tried to get a better transition by making sure the engine was running only on the idle circuit at idle, and not on the first transition port. All these things improved the system, but a pesky light throttle hesitation crept in at 3000 rpm. Since I could turn the problem on and off with a fair amount of consistency in a narrow range I felt I could figure out how to solve the problem. Now I'm pretty sure of what to do.

With the #60 idles, car is rich at very low power, and leans out up to the mains cutting in. With the #57 idles car is just about right at low power and leans out too much just before the transition to the mains. This has really narrowed it to the to the Idle Jet circuit.

Easy answer is: #1, change the idle air bleed jet to a smaller size. (This is what Paul Abbott suggested, and I have to give him full credit for being right. Plus thank him for sending me a kit to do the conversion)

What this will do is change the mixing ratio as throttle opening increases. The Idle circuit is now 60/140, and should probably go to something like 57/130. The smaller idle air correction jet will richen the mixture as throttle position (flow) increases. Since the #57 work right in the low throttle range, I needs to make the 57s act richer at larger throttle openings.

But an oddball situation exists. Solex increased the idle air jet from 140 to 165 and simultaneously increased the last transition port from 1.1mm to 1.45mm diameter, while keeping the idle jet constant at 47.5. Hmmmmmm. Check it out.



So I still have two options when rebuilding the Zeniths this coming Winter. More thought required...