Zenith TIN - AFR Testing 2, Method and Results

[VFR750]

It has been a while since I updated my Zenith TIN AFR testing. This post will be about Testing Methods, Tools and Results for my current 60/135-162/165 set of jetting.

My Test Method:

1. 45 MPH Cruise with Roll-on in 3rd Gear
2. Snap Accelerations on a Hill - 2nd Gear
3. Snap Acceleration on a Hill - 3rd Gear
4. Highway Hill Climb - 5th Gear 65 mph

Over the past two years I have used AFR logging with my Innovate wideband O2 Sensor and a Laptop. I added video from my GoPro camera. And recently added a crude throttle position sensor (TPS) to aide in timing of the video to AFR data, and to observe AFR as a function of Transition port activation.

I have created my test protocol to insure that I always get data in a consistent manner. I warm up the car, and go out for a 30 minute drive, where I can continuously record AFR & Video. I record the Outside Air Temperature to correct the AFR data to a 59F standard day. (Very important in NE where it can be 80F one day, and 40F the next!) I basically drive around a prescribed route, and do certain things at the same location.

Summary of Acceleration Testing shows the Main Jet #162 is the best for cooler weather, and Main Jet #160 best for the summer.

[VFR750]

This is the infamous Transition Stumble as a function of throttle position and which transition ports are exposed:


If you are cruising along, and gently accelerate, you will often find a little hole where you need more throttle, proportionally, to keep the acceleration increase going. You have hit the hole.

The chart shows how going from a very light throttle, to just at the point where the throttle uncovers the last (upper) port, causes a huge AFR rise. This is the hole. If I use #57 idle jets, the AFR spike is higher, and a noticeable stumble occurs. That is why I keep going back to #60 idles. They make it tolerable. An AFR spike of ~15 will not cause a noticeable stumble, but >15 will.

This is not the Main Circuit cutting in, either. this is purely the Transition port holes combined with the throttle position changing the fuel flow rate as a function of RPM and vacuum.

This is at ~2800-3000 rpm. Drop the RPM down to 2000-2200 rpm and the AFR spike is far worse.

This points to the biggest issue with the Zenith TIN. With the 4-port hole configuration, you need to run rich idle jets to cover this up. (And there lies a future post on transition port modifications)

[VFR750]

Here are the 3 recent acceleration runs. RPM is included.







Overlaying the three, you see the initial 2 second behavior is identical:


With RPM pulled from the video (looking at the tachometer) I can also compare AFR verses RPM for the three runs, even though they occurred over different time frames:

This shows a change in AFR at a particular RPM. This has always been in the data, but now, with RPM, and two different gears, I believe I am seeing the effect of an Emulsion Tube hole being uncovered and the corresponding rise in AFR at 3700-3900 rpm. The engine does not care about gear selection or the speed in which you pass through 3700-3900 rpm. it only knows the vacuum pulling air and fuel into the engine.

On a separate note, look how the time to rpm decreases as the engine gets closer to redline. This engine really pulls hard above 5000 rpm. This is The Reason for carbs.

[VFR750]

Last for today, The Highway Hill Climb. You are cruising flat and level, at 65mph in 5th. AFR is reading 11 or so. Boy you'd really like to lean out the idle jets and get better fuel mileage. Then a slight rise, and you gently roll on the throttle to maintain speed. Suddenly you see an AFR of 13-14. Hmmmmm. Can I really lean this thing out?

You did drop your idle jet to #57 and get a cruising AFR of 12. Not bad for mpg, still pretty rich though. But, hit that same hill and... The AFR goes over 15 and the engine would sit in the flat spot, or stumble.

You stay with #60 idles, and make it tolerable, but only get ~14 city and ~20 highway.... This is THE issue with the Zenith TIN.



BTW, my crude TPS made all of this possible. I know where the throttle is, and what part of the carburetor circuit is involved. Future tuning will be paying particular attention to this region of operation.

TP4 means the throttle is open to the point where the 4th transition port is fully uncovered.

[Schmierung]

Fantastic research Mike!!

[304065]

Ok, how did you hook up the TPS?

[VFR750]

I expect a little bit of ribbing on this. Laugh all you want....

This is my throttle position sensor. It is a cockpit mounted gage used to correlate throttle pedal motion to the actual rotation of the carburetor bellcrank. I made it from wood (Porsche uses wood too), a little bit of thin steel cable, a pointer, and a rubber band to keep it preloaded. Simple but very effective, and accurate enough for what i wanted to do.



I had previously measured the rotation of the bellcrank verses the opening of the transition ports. I plotted the angles on a piece of paper, and with the aide of my son, and a pointer, we were able to create the black hash-marks indicating when the ports were open. I also marked when full throttle was reached.

The key is to realize I was not looking for great precision. I was looking for a way to know where the throttle was, as I accelerated, and more importantly gain understanding of where during the opening of the throttle was the flat spot. Plus, when I did snap accelerations I could very accurately synchronize video to AFR logging.

Laugh now.

[304065]

That is great.

It reminds me of the airspeed indicator in a New Standard biplane.



The faster you fly, the more the stick moves against spring tension.

Do you have any means to electronically index the data trace? I mean, a simple pushbutton to create a 5V pulse when you push the throttle down would work. It's easy enough to spot the pulls by looking at the RPM trace, but something to indicate WOT would be easy enough. I think it would be possible to glue a small pushbutton into the white plastic throttle stop and run the wires through the hole in it, behind the floorboard, over to the data logger. This would tell you WOT. Maybe even a linear pot would work.

[VFR750]

Without EFI, i would have to add sensors with a/d signal converters to get better data.

I think i could rig up a on/off switch for wot. But with the visual and the video, it is pretty easy to see whats happening. I record at 30fps. So it can capture things i do not catch in real time.

The airspeed indicator is classy. Simple and effective.