
Originally Posted by
jmortensen
Actually it is a rich condition, but it will show up on an O2 sensor as a lean condition, which makes it kinda confusing. When the throttle is closed and the rpms are low, there is a strong vacuum behind the throttle plates and the air that leaks past the plates is moving very fast. When you slap open all 6 44 mm butterflies the vacuum goes down to almost zero and the air in the intake slows down as well. Right at the same time you dump a bunch of fuel in. You'll find that this problem doesn't happen if you roll onto the throttle. Only when you punch it from a low rpm. At rpms above 2500 or 3000 the velocity is already high enough that the engine won't stumble.
It is this combination of low vacuum and rich fuel condition that causes the stumble. The O2 sensor will read lean here because suddenly the air velocity drops. No O2 into the chamber, no O2 into the exhaust. You can see this effect on dyno sheets done with wideband O2s. Air/fuel will go from 14:1 to 10:1 until 2500 rpm or so, then jumps back into a reasonable range.
What's interesting to me is that the car will pull better through this spot with the bigger pilot. I spent a lot of time on this in my Z, because I had a lot of situations where I would come out of a turn at a low rpm at autox and the thing would stumble. I was reading the O2 sensor and it was showing lean when the throttle was opened, so I kept going to a bigger and bigger pump jet nozzle thinking that would help. It did at first, but at a certain point it wouldn't make any difference how much larger I went. Then I read online that the O2 would show lean in this spot despite the fact that the car was actually super rich, and it also recommended using a larger pilot and not the acc pump nozzle to fix. So I tried that. MUCH better. I was surprised at how big the pilot had to be. I kept making it bigger and bigger thinking that it couldn't possibly run better with a bigger pilot, and it just kept running better and better.
I think the pilot works better than the acc pump because the pilot only puts fuel in when there is air to burn it, where the accelerator pump will pump a big stream of fuel with no air to burn it.
Mine still has a tiny bit of a hesistation there, but I just don't think it can completely be alleviated on the 44's.
That's my understanding of the problem and its causes anyway.
texasz--thank you for the .pdf. The info there combined with the info in the How to Hotrod Your Nissan/Datsun OHC Engine book should be enough to give anyone a good understanding of whats going on, but those Mikuni manuals are few and far between.
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