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I've been doing some tuning of my Mikuni 44 set up as of late with the assistance of a wide band o2 sensor, and I have learned a few things that may be helpful to some others. Generally, I recommend determining these in the order listed: Pilot Screws: One bit from Honsowetz in the How to Modify Your Nissan Datsun OHC Engine which is a very specific instruction: run your pilot screws 1.5 turns out. I have mine set at 1.5 dead on, and the engine likes the 57.5 pilots at that setting. AFRs are where they should be for idle and low speed operation. The engine runs well in both of these conditions. Mains (Fuel Jets) and Air Correction (Air Jets): This bit came from TonyD... and that is stay out of the pump nozzles when trying to determine what the engine wants for main and air jets. Accelerator pedal movement may engage the pump circuit and if it does, this will throw fuel into the mix from that circuit. This will "mask" what is happening with the main circuit and you won't know what is what. According to the Mikuni manual, accel pumps are in play from 0 to 30% of throttle. So use inputs like 50% or 100% and ignore where the 0-30% is most likely occurring. Since the pump circuit has a known volume... and the duration of the fuel "injection" from the nozzle is specific to each nozzle, you could time the length of the injection duration if you are so inclined. Then you could ignore that time duration (from throttle open) in your Wideband AFR (Air Fuel Ratio) plots. Pump Nozzles: This one came from my engine builder. Know that sometimes wideband o2 sensors/loggers may show you rich readings when actually the engine is lean. This can occur when there is any engine misfire condition. The rich reading is resulting from a chain event of the flame going out too early, and the subsequent passing of remaining unburnt mixture through to the sensor. When you open the throttle quickly, if the engine is sluggish, but doesn't misfire, and your wideband shows rich, you are rich. However, if the engine misfires, and your wideband shows rich, you could be lean. Regarding this last one, I think that is what I am currently experiencing. I will be testing that theory out soon. As I have changed the pump nozzles from 50 to 45 to 40, I am fairly certain that I have been experiencing a more prevalent misfire when applying large throttle inputs (I haven't been doing much full throttle yet and there has been a fair amount of time involved from when I swapped nozzles one to the next to the last). I was going smaller and smaller because of the rich readings I was getting on the wideband when "throwing large throttle inputs" at the engine. Last tuning session, I was getting a large amount of misfire for the first couple of seconds of full throttle... even when in first gear. I am going to switch back to the 50's next and see if that situation improves.
