I have a 335i BMW with a 046 jet dryshot. It used to work fine, but lately, I keep getting this flutter, lean spikes in the middle of a pull. There isn't much info out there on the BMW boards, so I figured I would try here to see if someone has insight.

So my car would have a misfiring/flutter at times, with corresponding spikes in the AFR to around 20;1. Normally no CEL, no codes stored. It used to do it normally if I rolled into the throttle and hit the nitrous before the AFRs stabilized to the WOT/Rich level. But lately, It is difficult to even get one clean run in without this misfiring.

I figured it was the fuel system somehow failiing to keep up with the demands. On the other hand, I made dozens of runs on the same setup with no problem before. Last few times at Famoso, I didn't have any problems.
So something has changed.

FYI: After this video, I ran the C63 on the 35-155 run, and went to Sacramento, etc. So off the bottle, the car is performing respectably well.

I also thought that maybe a bad spark plug could cause this. A misfire may possibly result in lean AFRs of unburned gases coming out of the exhaust. But on the other hand, I don't hear any popping, etc. And again, no misfire faults.

I showed the shop owner this video where I refill my bottle, he said it is a lean flutter. The fuel system isn't keeping up with the nitrous. So again, what in the world could make the fueling adequate on one run, and then collapse in the next. Or make it occur only during part of the run?

I am going to pull the plugs and check them. However, they are only 5 months old, with about 4000 miles on them. And as you all know, the car runs plenty smooth and strong off the nitrous. On the other hand, the plugs, and combustion probably have to be a little more perfect when we are talking about much higher combustion pressures.

Anyone else experience the same thing? And ideas? Here is the video, and the corresponding log to the run.
The mustard color line is the AFRs. This is 2nd, 3rd, 4th gear...Car usually traps right about 130mph at the end of the qtr mile, top of 4th gear.

http://www.youtube.com/watch?v=qtH1C...layer_embedded

 

Ill take a stab. One thing to remeber is its an oxygen sensor not a fuel sensor so the missfire will cause a "lean" reading since the oxygen was unburnt. so without any data you will need to figure out was it lean then back fired or was the lean reading from the back fire. is it popping thru the intake or exhaust? I have experianced on dry shots when it is extremly lean and low timming you will pop thru both. its not voilent or break any thing but it will pop off. it then burns the oygen in the intake which results in a rich condition but not from burning it in the cyl. how often do you change plugs? have you changed the type of plug ?

If nothing has changed im going to guess its fuel related inj or pump. log fp or watch it or something.