Zenith TIN - Idle Jets + Idle Air Bleed Comparision

[VFR750]

I have now tried three distinct combinations of Idle Jets (IJ) and Idle Air Jets (IAJ) in an effort to minimize the transition flat spot. The differences turned out to be smaller than I was hoping for, but i will leave the judgment to the reader as to the effectiveness.

The idea was based on the 4-port Zenith TIN verses the 3-port Weber IDA. My previous testing showed that transition stumble seemed worse with the #57 IJ verses the #60s. Even when I dropped down to #120 IAJ I could not induce enough fuel flow to help the stumble with the #57s.

My thinking is, if we make the IAJ really big, then proportionally the IAJ to the Zenith TIN transition port area will be similar to the Weber IDA. To run a really big IAJ, you need a much larger IJ to avoid being too lean everywhere. That is what led me to this round of testing.

I decided, if 60/135 was equal or better than 57/120, then 62/150 would be even better. So i bought some #62s, and used my #150 IAJ from prior testing. Here are the results for my typical hill climb.


You can see prior to the hill climb, 57/120, 60/135 and 62/150 are virtually identical under light throttle (The first transition port is fully uncovered) This proves you can mix and match any idle jet with the right IAJ

What looks to be there, is after the initial peak, the 62/150 is running richer than the 57/120, with the 60/135 in the middle. I was hoping this would be a bigger effect. Driving around with the 62/150 feels very nice, but I only have this data to say it is likely "better".

What is clouding this analysis is my ill-fated port balancing test. I opened up the 4th transition port (uppermost) from 1.1mm to 1.15mm. Refered to TP4115. Unfortunately this leaned out the top end more than the bottom end of the transition circuit. I did not run the #57/120 with the larger TP4 hole. This is what lead me to go from 60/140 down to 60/135, to fix the leaning out.

What it does suggest is the 62/150 will be more stable under load, and may be less likely to stumble if additional power is required. It appears to be happier than the 60/135 I was running, and for now, 62/150-162/165 is my new baseline

I will also contemplate a #64/160 combo in the future.