Zenith TIN - Acceleration Jet Flow Visualization

[VFR750]

Here is my experiment measuring the relative flow of the Zenith TIN acceleration jet when you snap the throttle open.

Video:
Zenith TIN Acceleration Jet Flow - YouTube

The timing of the spray is documented by using the time stamp in the video. I plotted the relative spray strength (purely a judgement call) verses time, starting the instant I began to move the throttle. I takes ~.3 seconds from closed to open throttle.



Why do this? To document the acceleration jet flow rate and timing. When I look at my acceleration runs with AFR verses Time, I now know what the acceleration jets are doing. It will help me figure out what part to tweak next.

Notice the strong squirt before reaching full throttle. Then a tail-off to a steady stream, and final decay to zero flow before ~1.4 seconds.

New and fun ways to use your GoPro camera.

[neilca]

Mike,

Great data. I am glad you posted this, I never know when I may need this.

[VFR750]

I have recently recorded AFR for several acceleration runs. Combined with GoPro video and a crude cockpit mounted throttle position sensor, I am now able to accurately map when the throttle opens, how long does it take, and what affects the lean AFR spike on a snap acceleration.



There are three acceleration runs, starting from 2000 rpm. Two starting in 2nd gear and one in 3rd gear. Time = 0.000 second is exactly when I STARTED to open the throttle. Notice it took me almost .3 seconds to completely open the throttle butterflies.

I took the three runs worth of AFR data and used the TPS to correctly align the start of the throttle opening, so that the AFR spike and subsequent AFR behavior are properly syncronized.

Note the AFR spike on the 3rd gear run is significantly lower that the 2nd gear runs. This is new learning. I think it has to do with how open the throttle is to hold 2000 rpm in 3rd going up a sight hill. Maybe the transition circuit is flowing more, and becomes less sensitive to throttle tip in??

Since I also had the acceleration jet flow timing, I was able to overlay the flow of the acceleration jets with the AFR spike. See how the engine goes lean as the transition ports are shut off, and the acceleration jet catches it. Than the mains kick in and the acceleration jet flow tails off.

Also note the jagged little line at the bottom. That represents the timing of the intake valve, for one cylinder. It is not syncronized, but mearly a aid to see just how often the intake valve would cycle open. Since we have individual throttle bores, at 2000 rpm, the pulsing of the intake is happening "slowly". 11 intake pulses in .3 seconds and you have captured the entire lean spike!

What to do with this new data? What's next?

As I try to improve the transition stumble inherent in the Zenith TIN (and weber IDTP) there are many individual contributors, transition port cutoff, main circuit starting, and the ability of the acceleration jet to bridge the gap. This particular effort is focused on the snap acceleration lean spike. It can be smoothed over via big idle jets, big main jets, higher float level, smaller Idle air bleed jets, and/or increased acceleration jet flow.

The Zenith TIN acceleration jet is only 0.3mm. A future modification is to increase the jet size to 0.32-0.34mm to increase the peak flow. If I ratio a 2.2l 911T engine to a 3.0l SC engine, the 0.30mm jets shoudl be ~0.35mm to deliver a proportionate acceleration flow. But, I have learned over the past year that: small changes have many more effects than you think. I don't want to jump up too big and suddenly flood the engine with too much fuel and have a rich stumble. Smaller jumps in hole size will be the next step.

I have the small drills to do this. Since this is a more significant carburetor disassembly, I will do this after I finish my last round of idle jet/idle air bleed jet testing. May get to it this season, maybe not; it was 28F this morning!