My Prelude is running lean it so appears. I totally rebuilt the carbs a couple of months ago (because it failed smog and they said it was running rich), but now it passes smog all w/ flying colors except NOX where it fails miserabley. They said it is now running too lean. I rebuilt the carbs with ALL new Honda parts and did it with tweezers and a magnifying glass (followed the manual to a T, and made everything to spec, right-on), especially the float levels. The EGR was replaced (the old one tested out fine). The engine now runs a little rough, low and high speed, but idles pretty nice - only a missfire every 5 to 10 seconds on idle. The vacuum gauge I attached shows in the " green" (normal) range, but very close to the line that starts the " retarded ignition timing" section on the gauge. I set the ig timing to exactly 12 degree BTDC. I think it is pulling air in from somewhere (there are a lot of vacuum tubes in there, so there are many opportunities). On studying the shop manual for a long time and scanning over the vacuum ICD page and emissions section, I suspect that it may be the " main air jet solenoid valve". This valve lets air into the " primary main air jet" to change the A/F mixture according to certain driving conditions. This is where I am at now - there are other possibilities, What do y'all think?